Dobereiner’s triads
A German chemist Johann Wolfgang Dobereiner identified that elements with similar properties could be grouped together in groups of three which were referred to as triads. The elements in a particular triad had similar chemical and physical properties.
Law of Triads: According to Dobereiner, when elements are arranged in increasing order of their atomic masses, the arithmetic mean of the atomic masses of the first and third element in a triad is approximately equal to the atomic mass of the second element in that triad. He also proposed that this law was valid for other properties of elements too. One such property was density.
Various Dobereiner’s Triads
The first Dobereiner’s triad was discovered in 1817. It is comprised of alkaline earth metals i.e. calcium, strontium and barium. Later three more triads were discovered. Let us look at these triads in detail. There are five of Dobereiner’s triads which are as follows:
- Triad 1: This triad was constituted by the alkali metals lithium, sodium and potassium.
| Element | Atomic Mass |
| Lithium | 7 |
| Sodium | 23 |
| Potassium | 39 |
The law of triads can be verified for this triad as follows:
The atomic mass of Sodium = (Atomic Mass of Lithium + Atomic mass of Potassium) / 2
= (7 + 39)/2
= 46/2
= 23
Thus the law of triads is verified for this triad.
- Triad 2: This triad is comprised of alkaline earth metals i.e. calcium, strontium and barium.
| Element | Atomic Mass |
| Calcium | 40 |
| Strontium | 87.6 |
| Barium | 137 |
Let us verify the law of triads for this triad too.
(Atomic Mass of Calcium + Atomic mass of Barium) / 2 = (40 + 137) / 2
= 177/2
= 87.5
87.5 is very close to the atomic mass of Strontium. Thus the law of triads is verified.
- Triad 3: This triad is comprised of halogens, Chlorine, Bromine, Iodine.
| Element | Atomic Mass |
| Chlorine | 35.5 |
| Bromine | 80 |
| Iodine | 127 |
The Law of triads can again be verified for this triad too.
Atomic Mass of Bromine = (Atomic Mass of Chlorine + Atomic mass of Iodine) / 2
= (35.5 + 127)/2
= 162.5/2
= 81.25
which is very close to the atomic mass of Bromine. Hence the law of triads is verified.
- Triad 4: This triad consisted of sulphur, selenium, and tellurium.
| Element | Atomic Mass |
| Sulphur | 32 |
| Selenium | 79 |
| Tellurium | 128 |
The arithmetic mean of atomic masses of Sulphur and Tellurium is 80 which is very close to the atomic mass of Selenium.
- Triad 5: This triad was constituted of Iron, cobalt and nickel.
| Element | Atomic Mass |
| Iron | 55.8 |
| Cobalt | 58.9 |
| Nickel | 58.7 |
The atomic mass of Cobalt is roughly equal to the arithmetic mean of the atomic mass of Iron and Nickel that comes out to be 57.25.
Limitations of Dobereiner’s triads:
Although Dobereiner’s triads made an attempt to classify the elements according to their properties into groups of three still had various limitations which are discussed below:
- Many new elements were discovered in the 18th and 19th century which made the classification of elements in Dobereiner’s triads difficult and also impossible in some cases.
- Only 5 Dobereiner’s triads could be identified and new elements could not be grouped in Dobereiner’s triads.
- There also existed several elements at the time of Dobereiner’s triads formation that did not fit into Dobereiner’s triads.
- It resulted in disordered element collection.
Although Dobereiner’s triads could not group all elements according to their properties it said the foundation for the formation of the modern periodic table.
Sample Questions
Question 1: What is the law of triads?
Answer:
According to the law of triads, when elements are arranged in increasing order of their atomic masses, the arithmetic mean of the atomic masses of the first and third element in a triad is approximately equal to the atomic mass of the second element in that triad.
Question 2: What do you mean by triad in reference to Dobereiner triads?
Answer:
In reference to Dobereiner’s triads, triad refers to a group of 3 elements. According to Dobereiner, the elements could be made into groups of three having similar physical and chemical properties.
Question 3: State any two limitations of Dobereiner’s triads.
Answer:
Limitations of Dobereiner’s triads were as follows:
- Many new elements were discovered in 18th and 19th century which made the classification of elements in Dobereiner’s triads difficult and also impossible in some cases.
- Only 5 Dobereiner’s triads could be identified and new elements could not be grouped in Dobereiner’s triads.
Question 4: Which halogens were a part of Dobereiner’s triads?
Answer:
Dobereiner triad that consisted of halogens was formed by Chlorine, Bromine and Iodine.
Question 5: Verify if Carbon, Nitrogen and Oxygen with an atomic mass of 12, 14 and 16 form Dobereiner’s triads.
Answer:
To form a Dobereiner triad, the arithmetic mean of the atomic mass of first and third element must be approximately equal to the atomic mass of the second element.
Here arithmetic mean of the atomic mass of Carbon and Oxygen is 14 which is equal to the atomic mass of Nitrogen. Hence they form Dobereiner’s triad.
Question 6: State the alkaline earth metal Dobereiner’s triad.
Answer:
The alkaline earth metals that are a part of the Dobereiner triads are calcium, strontium and barium.
Question 7: Verify the law of triads for the alkali metals triad.
Answer:
The alkali metals triad consists of Lithium. Sodium and Potassium. The atomic masses of these elements are 7, 23 and 39 respectively. The arithmetic mean of the masses of lithium and potassium is equal to 23 which is the atomic mass of Sodium. Hence the law of triads is verified.
Newland’s Law of Octaves
A British chemist named John Newlands attempted to combine the 62 elements known at the time in 1864. He arranged them in ascending order according to their atomic weights and discovered that the properties of every eighth element were the same. As a result of this discovery, Newland’s law of octaves was born.
The law of octaves states that when the elements are arranged in ascending order of atomic mass, every eighth element has comparable properties.
Newlands contrasted the components’ proximity to musical octaves, in which every eighth note is comparable to the first. This was the first time an atomic number was assigned to each element. However, this method of classifying elements was met with skepticism in the scientific community.
Examples of Law of Octaves:
- Sodium is one of lithium’s eight elements. Potassium, lithium, sodium, and potassium are the chemical properties of the eight sodium elements.
- Chlorine is the eighth element after fluorine. The chemical properties of fluorine and chlorine are similar.
- When elements are arranged in increasing atomic mass order, Newland’s law of octaves states that the properties of the eighth element are the same as the first.
Advantages of Newland Law of Octaves
- This law establishes a framework for classifying items with comparable features into groups.
- The statute gave the government extensive authority to organise all known elements into a tabular format.
- The Newlands law of octave was the first to be based on atomic weight, linking element properties to atomic masses.
- For the lighter sections, this method performed significantly better. Lithium, sodium, and potassium, for example, were combined.
Limitations of Newland’s Law of Octaves
- In Newland’s periodic classification, some elements were grouped together. Nickel and cobalt were both placed in the same slot.
- Element qualities that were distinct were grouped together. Metals such as cobalt, nickel, and platinum, for example, were classified with halogens.
- Only up to calcium did Newland’s law of octaves hold true. Elements with higher atomic masses had atomic masses that were too large to fit within octaves.
- The octave layout was unable to accommodate later discovered components. As a result, new elements could not be discovered using this classification scheme. Elements that were discovered later could not be incorporated into the octave pattern. As a result, this method of classifying elements left no room for the discovery of new elements.
Sample Questions
Question 1: Why are the Noble Gases Placed in a Separate Group?
Answer:
Noble gases such as helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe) are the most inert (non-reactive) of all known elements and are present in very low concentrations in our atmosphere. As a result, in Mendeleev’s periodic table, they are grouped together in a separate group called zero groups.
Question 2: Give a brief history of periodic classification of elements.
Answer:
The periodic table has a history that spans over a century of progress in chemical property understanding. Dmitri Mendeleev published the table in 1869. He based his work on previous discoveries made by scientists such as Antoine-Laurent de Lavoisier and John Newlands.
- The triads of Dobereiner
- The Law of Octaves of Newland
- Periodic Law of Mendeleev
Question 3: What were the drawbacks of Dobereiner’s triads?
Answer:
The drawbacks of Dobereiner’s triads are:
- Dobereiner was only able to identify three triads. He couldn’t make triads of all the known elements.
- All of the known elements could not be arranged in triads.
- The law did not hold true for elements with extremely low or extremely high masses. Consider F, Cl, and Br.
Question 4: What are the advantages of the Newland Law of Octaves?
Answer:
Advantages of the Newland Law of Octaves are:
- This law establishes a framework for classifying items with comparable features into groups.
- The statute gave the government extensive authority to organise all known elements into a tabular format.
- The Newlands law of octave was the first to be based on atomic weight, linking element properties to atomic masses.
- For the lighter sections, this method performed significantly better. Lithium, sodium, and potassium, for example, were combined.
Question 5: What is Mendeleev’s Periodic Table? Which elements were left by him in his periodic table that was discovered later?
Answer:
Dmitri Mendeleev, a Russian chemist, published a periodic table in 1869, just five years after John Newlands proposed his Law of Octaves. Mendeleev also arranged the elements known at the time in order of relative atomic mass, but he also did a few other things that contributed to the success of his table. Mendeleev recognized that the physical and chemical properties of elements were ‘periodically’ related to their atomic mass, and arranged them in his table so that groups of elements with similar properties fell into vertical columns.
Germanium, Scandium, and Gallium are the elements discovered but not included in Mendeleev’s periodic table.
Who is called the Father of Periodic Table?
The famous Russian scientist Dmitri Ivanovich Mendeleev also called Mendeleev is called the Father of the Periodic Table. He was born on 8 February 1834 in the small town of Tobolsk in Siberia. He was an alumnus University of St. Petersburg. and received and received his Master’s degree in chemistry in 1856 and his doctoral degree in 1865.
Introduction to Mendeleev Periodic Table
In 1869, Dmitri Mendeleev came up with a periodic table of chemical elements based on the properties that appeared in the elements. He placed the elements from lightest to heaviest. Before Mendeleev’s Periodic Table, Dobereiner’s triads and Newland’s Law of Octaves were mostly considered for the classification of elements in the form of tables.
Mendeleev’s Periodic Table came into the picture after the rejection of Newland’s Law of Octaves. In Mendeleev’s Periodic Table, elements are distributed according to their atomic masses, fundamental property, and chemical properties.
Mendeleev’s Periodic Law states that “The properties of elements are the periodic function of their atomic masses.”
At the time when Mendeleev formulated this periodic table, only 63 chemical elements had been discovered. After observing all the properties of the elements, Mendeleev concluded that the properties of the elements periodically related to their atomic masses. Other than atomic masses, Mendeleev also used chemical properties to categorize. Formulae of hydrides and oxides of the elements were one of the primary criteria for categorization. He arranged the elements in the periodic table such that all the elements with similar properties would fall in the same vertical columns of the periodic table. He named the vertical columns “groups” and the horizontal rows “periods“.
| Group | I | II | III | IV | V | VI | VII | VIII | |||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Oxide | R2O | RO | R2O3 | RO2 | R2O5 | RO3 | R2O7 | RO4 | |||
| Hydride | RH | RH2 | RH3 | RH4 | RH3 | RH2 | RH | ||||
| Periods ⇓ | A B | A B | A B | A B | A B | A B | A B | Transition Series | |||
| 1 | H1.008 | ||||||||||
| 2 | Li6.939 | Be9.012 | B10.81 | C12.011 | N14.007 | O15.999 | F18.998 | ||||
| 3 | Na22.99 | Mg24.31 | Al29.98 | Si28.09 | P30.974 | S32.06 | Cl35.453 | ||||
| 4 | First series: | K39.102 | Ca40.08 | Sc44.96 | Ti47.9 | V50.94 | Cr50.2 | Mn54.94 | Fe55.85 | Co58.93 | Ni58.71 |
| Second series: | Cu63.54 | Zn65.37 | Ga69.72 | Ge72.59 | As74.92 | Se78.96 | Br79.909 | ||||
| 5 | First series: | Rb85.47 | Sr87.62 | Y88.91 | Zr91.22 | Nb92.91 | Mo95.94 | Tc99 | Ru101.07 | Rh102.91 | Pd106.4 |
| Second series: | Ag107.87 | Cd112.4 | In114.82 | Sn118.69 | Sb121.75 | Te127.6 | I126.9 | ||||
| 6 | First series: | Ce132.9 | Ba137.34 | La138.91 | Hf178.49 | Ta180.95 | W183.85 | Os190.2 | Ir192.2 | Pt195.09 | |
| Second Series: | Au196.97 | Hg200.59 | Ti204.37 | Pb207.19 | Bi208.98 | ||||||
Mendeleev recognized the significance of periodicity. He used a broader range of physical and chemical properties in order to classify the chemical elements. Particularly, he relied on the similarities in the given empirical formulas and properties of the compounds such as oxides and hydrides formed by the elements.
Mendeleev noticed that some of the elements didn’t fit according to the periodic law if the order of atomic masses was followed strictly. He thus ignored the order of atomic masses, assuming that the atomic measurements might be incorrect, and placed the elements with similar properties together. For example, the atomic mass of Tellurium is more than that of Iodine. Yet, he placed Iodine with Group VII and Tellurium in Group VI. This was because Iodine has more similar properties to Chlorine, Fluorine, and Bromine. At the same time, keeping his primary aim of arranging the elements of similar properties in the same group, he proposed that some of the elements were still undiscovered and, therefore, left several gaps in the table.
Merits of Mendeleev’s Periodic Table
Merits of Mendeleev’s Periodic Table are,
- It was one of the first periodic tables that successfully accommodated all known elements at that time.
- It gave the periodic law based on which the modern periodic law was formed.
- The table had gaps for undiscovered elements, which proved to be beneficial in the discovery of new elements. The periodic table was not disturbed in any way.
- Mendeleev’s Periodic Table in 1869 had only seven groups, but Sir William Ramsay suspected new elements belonging to a hitherto unknown eighth group.
- Ramsey and Rayleigh discovered noble gases in 1895. These noble gases were added to Mendeleev’s Periodic Table as a separate group.
Demerits of Mendeleev’s Periodic Table
Demerits of Mendeleev’s Periodic Table are,
- Hydrogen, the first element to begin with didn’t have a definitive place on the table. It was placed differently from the rest of the elements.
- Isotopes are variants of a particular element that have the same atomic number but differ in atomic mass. These isotopes violated Mendeleev’s Periodic Table as they couldn’t be placed.
- The increase in the atomic mass of the elements is irregular and not in a fixed pattern. Thus, it was not possible to exactly determine how many elements were yet to be discovered.
Predictions of Mendeleev’s Periodic Table
Mendeleev has made various predictions on the elements that are not known till that time on the basis of the atomic mass of the other elements present.
- Mendeleev predicted several elements after observing his periodic table. He estimated that some unfilled spots would be similar to elements existing in the given group.
- He named some elements as (eka)-(similar element), where Eka means ‘one’ in Sanskrit.
- Mendeleev predicted eka-boron (Eb), eka-aluminium (Ea), eka-manganese (Em), and eka-silicon (Es).
- These elements were later discovered to be Scandium, Gallium, Technetium, and Germanium respectively.
- Mendeleev also predicted an element between Thorium and Uranium amongst others.
Mendeleev’s Predictions and Actual Properties on Discovery:
| Property | Eka-silicon (predicted) | Germanium (found) | Eka-aluminium (predicted) | Gallium (found) |
|---|---|---|---|---|
| Atomic Weight | 72 | 72.6 | 68 | 70 |
| Density | 5.5 | 5.36 | 5.9 | 5.94 |
| Melting Point | High | 1231 | Low | 302.93 |
| Formula of oxide | EO2 | GeO2 | E2O3 | Ga2O3 |
| Formula of chloride | ECl4 | GeCl4 | ECl3 | GaCl3 |
To conclude, in his time, Dmitri Mendeleev made a very insightful Periodic Table based on the Atomic Masses of Elements. It was the foundation of the Modern Periodic Table and successfully predicted several unknown elements of that time.
Difference between Mendeleev’s Periodic Table and Modern Periodic Table
The difference between Mendeleev’s Periodic Table and Modern Periodic Table are discussed below in the table,
| Mendeleev’s Periodic Table | Modern Periodic Table |
|---|---|
| This table is based on the atomic masses of the given elements. | This table is based on the atomic number of the given elements. |
| The chemical elements are arranged in the increasing order of their atomic masses. | The chemical elements are arranged in the increasing order of their atomic numbers. |
| Hydrogen doesn’t have a fixed place in this table. | Hydrogen has a fixed place in this table. |
| This table has 7 groups and 6 periods. | This table has 18 groups and 7 periods. |
| Isotopes violate this table and have no allotted place. | Isotopes are placed in the same element as the atomic number is constant throughout. |
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FAQs on Mendeleev’s Periodic Table
Q1: What is basis of Mendeleev’s Periodic Table?
Answer:
The basis of Mendeleev’s periodic table is the arranging of elements on the basis of their increasing order of their atomic weight. Mendeleev had arranged all the 63 elements known at that time in the form of a table with 7 horizontal columns (period) and 8 vertical columns (groups known as Mendeleev’s periodic table.
Q2: What is the Law of the Mendeleev Periodic Table?
Answer:
Mendeleev gave the famous Mendeleev Periodic law which explains that ” The periodic properties of the elements are the periodic function of their atomic mass.”
Q3: How did Mendeleev calculate the Atomic Masses of the unknown Chemical Elements?
Answer:
Mendeleev did not calculate atomic masses on his own. He used the masses of existing elements to place them in the table and predict the masses of undiscovered elements. Mendeleev predicted eka-boron (Eb), eka-aluminium (Ea), eka-manganese (Em), and eka-silicon (Es), which were later discovered to be scandium, gallium, technetium and germanium respectively.
Q4: Did Mendeleev violate the Periodic Law in the table? Explain.
Answer:
Mendeleev arranged the elements according to the atomic masses. Yet, there were instances where he had to place elements not in sequence to ensure that elements with similar properties were in the same group. Therefore, he had to violate the periodic law and ignore the order of atomic masses in some cases.
For example, the atomic mass of Tellurium is more than that of Iodine. Yet, he placed Iodine with Group VII and Tellurium in Group VI. This was because Iodine has more similar properties to Chlorine, Fluorine and Bromine.
Q5: Why couldn’t Mendeleev place Hydrogen in the periodic table?
Answer:
Hydrogen reacts with metals to form ionic compounds called hydrides and also with non-metals to form covalent compounds. Mendeleev’s periodic law could not assign a fixed position to hydrogen in the periodic table because hydrogen resembled both alkali metals (Group 1) and halogens (Group 17) in some of its properties.
Q6: How did isotopes violate Mendeleev’s Periodic Table?
Answer:
Isotopes are variants of a particular element that have the same atomic number but differ in atomic mass. As Mendeleev’s Periodic Table is according to the atomic masses, he couldn’t place one element twice in the table. Thus, isotopes violate the table and can’t be placed.
Q7: What is the difference between Mendeleev and Modern periodic table?
Answer:
The basic difference between Mendeleev and Modern periodic table is that Mendeleev’s periodic table arranged elements on the basis of their atomic weight whereas Modern periodic table arranged elements on the basis of their atomic number.
Q8: What is Mendeleev famous for?
Answer:
Mendeleev the famous Russian scientist is known for his work in arranging all the known element till that time in the form of a table which make it easier for us to study their properties.
Merits of Mendeleev’s classification of elements
Mendeleev’s periodic table is the first systematic attempt to classify elements based on their fundamental properties. The following are the merits of Mendeleev’s element classification.
- It was able to anticipate the properties of several elements based on their periodic table positions.
This method was used to predict the properties of then-undiscovered elements including gallium, scandium, and germanium. Mendeleev predicted the properties of the unknown element eka-aluminium based on its position in the periodic table in 1871. The actual properties of this element (called gallium) and eka-aluminium are listed below.
| Property | Eka-aluminium (predicted) | Gallium (actual) |
| Atomic Mass | 68 | 69.7 |
| Density | 5.9 g/cm3 | 5.94 g/cm3 |
| Melting point | Low | 30.20C (Low) |
The properties of eka-aluminium predicted by Mendeleev are almost identical to the actual properties of the gallium element, as seen in the table. In 1875, four years after Mendeleev’s table was published, the element gallium was discovered, and its properties matched up amazingly well with eka-aluminium, fitting into the table exactly where he had predicted.
- When noble gases were identified, Mendeleev’s periodic table could accommodate them.
When a new group of elements known as noble gases was discovered, it was given its own spot in the periodic table as a separate group. It did not change Mendeleev’s periodic table’s original arrangement. Since noble gases are chemically inert, they are classified as a separate group. Noble gases were discovered much later than expected because they are chemically inert and exist in extremely low concentrations in the atmosphere.
- It predicted the existence of a number of elements that had not yet been discovered.
Mendeleev’s periodic table had gaps for elements like gallium (Ga), scandium (Sc), and germanium (Ge) that had not yet been discovered. When these elements were discovered later, they were inserted in the gaps between the existing elements without disturbing them. It was able to predict the existence and properties of three elements with similar properties to boron, aluminium, and silicon. Eka-boron, Eka-aluminum, and Eka-silicon are the names of these elements. He gave them names by prefixing the name of the preceding element in the same group with the Sanskrit numeral Eka (one). Eka-boron is the name given to the gap he left for the undiscovered element that comes after boron, and Eka-aluminium is the name given to the gap he left for the undiscovered element that comes after aluminium. Eka-silicon is the name given to the gap he has left for the undiscovered element that comes after silicon. They were isolated and given the names scandium, gallium, and germanium. Their oxides and halides have experimentally determined atomic weights, physical properties, and chemical formulas that were identical to those anticipated by Mendeleev. The missing periods indicated the presence of elements that were yet to be discovered. Since he expected the presence of elements that had not yet been identified, Mendeleev’s periodic table had certain gaps.
Sample Questions
Question 1: How did Mendeleev predict the existence of some elements in his periodic table that had yet to be discovered?
Answer:
When Mendeleev proposed his periodic table, he identified gaps in the table and predicted that undiscovered elements with properties suitable for filling those gaps existed.
Question 2: Mendeleev predicted the properties of two elements based on their positions in the periodic table. Name them.
Answer:
Since some elements were not discovered at the time, their properties were expected based on their positions in Mendeleev’s periodic table. The properties of Gallium(eka-Aluminium) and Scandium(eka-Boron) were predicted based on their positions in Mendeleev’s periodic table.
Question 3: In Mendeleev’s original periodic table why was the noble group of elements missing?
Answer:
The noble gases were not included in Mendeleev’s original periodic table because they were not discovered at the time.
Question 4: Write a few demerits of Mendeleev’s periodic table.
Answer:
Some demerits of Mendeleev’s periodic table are:
- It was unable to fix the position of isotopes.
- It was unable to explain why the atomic masses of some elements were in the wrong order.
- In the periodic table, hydrogen could not be assigned to a correct position.
Question 5: What factors guided Mendeleev to classify the elements in the periodic table?
Answer:
Mendeleev was guided by two factors:
- The increasing atomic masses of the elements.
- Elements with similar properties are grouped together.
Anomalies of Mendeleev’s classification of elements
Mendeleev’s periodic table was extremely useful in the study of elements, but it had some shortcomings that could not be explained using Mendeleev’s periodic law. The three major anomalies or limitations in Mendeleev’s element classification are given below:
- Position of Isotopes could not be explained
The atoms of the same element having similar chemical properties but different atomic masses are called isotopes. Since isotopes have different atomic masses, they should be placed in different groups of the periodic table if the elements are arranged according to atomic masses. In Mendeleev’s periodic table, the isotopes were not given their column. In Mendeleev’s periodic table, the isotopes are arranged in the same place. Cl-35 and Cl-37, for example, are two isotopes of chlorine with atomic weights of 35 and 37, respectively. Mendeleev’s periodic law could not explain why these two chlorine isotopes having different atomic masses were placed in the same group of the periodic table.
- Wrong Order of Atomic masses of some elements could not be explained
The elements are arranged in order of increasing atomic masses, according to Mendeleev’s periodic law. As a result, the element with the lower atomic mass should come first, followed by the element with the higher atomic mass. When some elements were grouped, it was discovered that the element with the greater atomic mass came first, followed by the element with the lower atomic mass. When placed in the correct group based on chemical properties, the element cobalt, which has a higher atomic mass of 58.9, comes first, followed by the element nickel, which has a slightly lower atomic mass of 58.7. Mendeleev’s periodic law was unable to explain this unusual situation of atomic masses in the wrong order.
- Correct Position could not be Assigned to Hydrogen in the Periodic table
Hydrogen is in group I alongside the alkali metals in Mendeleev’s periodic table. This is because, like alkali metals (such as sodium), hydrogen reacts with halogens, oxygen, and sulphur to form compounds with similar formulas. The table below shows the similar compounds formed by sodium and hydrogen.
| Compounds of hydrogen (H) | Compounds of alkali metal sodium (Na) |
| HCl | NaCl |
| H2O | Na2O |
| H2S | Na2S |
This indicates that hydrogen exhibits some properties like alkali metals. Some of the properties of hydrogen are similar to those of halogens (fluorine, chlorine, and bromine). Hydrogen, like halogens (F2, Cl2, and Br2), can be found in the form of diatomic molecules (H2). Furthermore, hydrogen, like halogens, forms ionic compounds termed hydrides when combined with certain metals and reacts with non-metals to form covalent compounds. All of these characteristics indicate that hydrogen belongs to group VII of the halogen element. As a result, it is possible to conclude that hydrogen belongs to both the alkali metal group and the halogen group based on its properties. Thus, Mendeleev’s periodic law was unable to place hydrogen in the correct position in the periodic table.
The inability of Mendeleev’s periodic law to explain the position of isotopes, the incorrect order of the atomic masses of some elements, and the position of hydrogen led to the conclusion that atomic mass could not be used to classify elements. There was a theory that there must be a more fundamental property of elements that could explain periodicity in element properties better. The atomic number of elements was discovered to be this property. All of Mendeleev’s classification anomalies vanished when the elements were arranged according to increasing atomic numbers.
Solved Questions
Question 1: What was Mendeleev’s reasoning for leaving a gap in his periodic table?
Answer:
Mendeleev left some gaps in his table to ensure that items with similar properties were placed together in the same vertical column or group.
Question 2: In Mendeleev’s original periodic table which group of elements was missing?
Answer:
Since noble gases were not known at that time, so there was no group of noble gases in Mendeleev’s original periodic table.
Question 3: Give some merits of Mendeleev’s periodic table.
Answer:
Some merits of Mendeleev’s periodic table are.
- When noble gases were discovered, the periodic table could accommodate them.
- Mendeleev’s periodic table was able to anticipate the properties of several elements based on their position in the periodic table.
- Mendeleev’s periodic table anticipated the existence of several elements that were unknown at the time such as gallium, scandium, and germanium.
Question 4: In the modern periodic table, how was the position of cobalt and nickel resolved?
Answer:
In Mendeleev’s periodic table, the wrong order of atomic masses of some elements like cobalt with atomic mass 58.9 and nickel with atomic mass 58.7 could not be explained. To maintain similarity in properties, copper with higher atomic mass had to be placed before nickel. But in the modern periodic table, the elements are arranged in the order of increasing atomic number. So, the problem was resolved because cobalt has a lower atomic number (27) than nickel (28). Therefore nickel with atomic number 28 is placed after cobalt with atomic number 27.
Question 5: In Mendeleev’s original periodic table, how many groups and periods are there?
Answer:
Mendeleev’s original periodic table has eight groups and seven periods.
Question 6: Why was hydrogen not placed in the correct position in Mendeleev’s periodic table?
Answer:
Hydrogen exhibited similar properties to both alkali metals and halogens. So, it was concluded that hydrogen belongs to both the alkali metal group and the halogen group based on its properties. Therefore, hydrogen was not placed in the correct position in Mendeleev’s periodic table.
What is Modern Periodic Law?
Dmitri Mendeleev, a Russian chemist, observed a pattern among the elements: as their atomic masses increased, their chemical and physical properties repeated with prior elements. He named this property Mendeleev’s Periodic Law. However, this law was not accurate for all elements, as only 63 were known at the time.
In an effort to improve upon Mendeleev’s Periodic Law, Henry Moseley proposed the Modern Periodic Law, which states that
“The physical and chemical properties of the elements are periodic functions of their atomic numbers.”
The atomic number is considered the number of protons or electrons in a neutral element. Through extensive advancements and research in the field of science, we now have a wealth of knowledge about atoms, their behaviours, and their properties. With this knowledge, scientists can now classify and pair elements with similar properties with ease that Mendeleev could never have imagined. These advancements have resulted in the form of the Modern Periodic Table.
Moseley Law
Modern periodic law is sometimes also called Moseley Law as it was given by Henry Moseley hence the name Moseley law, which states that,
“Various properties of the chemical elements whether Physical or Chemical are periodic functions of its atomic mass.”
Mendeleev Periodic Table
Dimitri Mendeleev arranged all the known elements to him in the order of increasing atomic mass and saw the periodic pattern in their properties. As there were only 63 elements known to Mendeleev and some of the smaller nucleus atoms are also not discovered yet, he left some gaps in the table so that the other elements can be collected with their similar property elements. Mendeleev’s version of the periodic table is as follows:
Modern Periodic Table
The Modern Periodic Table, also known as the long form of the Periodic Table, is a continuation of Mendeleev’s work. However, in the Modern Periodic Table, Neil Bohr used the atomic number of elements as the basis for periodicity. Bohr divided all the elements into 18 groups, labelled 1 to 18, and 7 periods, named 1 to 7. The groups are made up of elements that have atoms with similar outer shell electronic configurations, while the periods are made up of elements with the same number of shells in total.
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Classification of the Elements in the Modern Periodic Table
We classify elements in the periodic table in rows, columns, and blocks that are discussed below,
In Peiods and Groups
Elements are classified into 7 periods and 18 groups where elements in each group are placed using the electrons in their outermost shell and elements in each period are placed using the number of shells of each element.
The number of elements in each period:
- First period has 2 elements.
- Second period has 8 elements.
- Third period has 8 elements.
- Fourth period has 18 elements.
- Ffifth period has 18 elements.
- Sixth period has 32 elements.
- Seventh period has the rest of the elements.
In Blocks
The Modern Periodic Table is divided into four blocks that are as follows:
- s-Block
- p-Block
- d-Block
- f-Block
Now let’s learn about each block in brief.
s-Block Elements
Groups 1 and 2 are included in the s-Block as elements these blocks have their valence electron in the s-orbital. Also, these groups are also called alkali and alkali-earth metals respectively and their electronic configuration is ns1 and ns2 for group 1 and group 2 elements, where n is the period of that element.
p-Block Elements
Group 13 to Group 18 are included in the p-Block as elements in these blocks have their electronic configuration like ns2np1-6. Some of the most useful elements to mankind are found in this block such as Carbon, Silicon, Aluminium, Phosphorus, Sulfur, etc.
Learn more about, p-block
d-Block Elements
All the elements from group 3 to group 12 are collectively called d-Block elements. These elements are also called transition elements or transition metals as these elements show unstable transitional behaviour between s and p block elements.
f-Block Elements
Only Lanthanide and Actinide series are part of the f-Block. These are called inner transition elements and mark their presence between the elements lanthanum and hafnium and between actinium and rutherfordium.
Read More,
FAQs on Modern Periodic Law
Q1: State Modern Periodic Law.
Answer:
“The physical and chemical properties of the elements are periodic functions of their atomic numbers.”
Q2: What is the difference between Mendeleev’s Periodic Law and Modern Periodic Law?
Answer.
In Mendeleev’s period law atomic mass is considered as base for periodicity property but in modern periodic law atomic number is considered as the base for the periodicity property.
Q3: Who Gave the Modern Periodic Law?
Answer:
Henry Moseley proposed the modern periodic law in 1913.
Q4: Who invented Modern Periodic Table?
Answer:
Neil Bohr invented the Modern Periodic Table using the Modern Periodic Law.
Q5: How many Periods and Groups are in Modern Periodic Table?
Answer:
There are 18 groups and 7 periods in the Modern Periodic Table.
Properties of the Modern Periodic Table.
- The atomic number of elements, which is the most fundamental property of elements, are used to create the modern periodic table.
- The isotopes of an element are placed along with the parent element.
- The reason for the periodicity in properties of elements is explained in the modern periodic table. It relates the periodicity in the properties of the elements to the periodicity in their electronic configuration. It says that the properties of elements are repeated at regular intervals since the electronic configurations of the elements are repeated at regular intervals.
- The reason why elements in a group show similar properties but elements in different groups show different properties is explained by the modern periodic table. The elements in the table are organised according to their electronic arrangements. All elements with comparable electronic configurations are grouped together and exhibit similar characteristics. Elements with varying electronic configurations are grouped together and have varying characteristics.
- There are no abnormalities in the arrangement of elements in the current periodic table.
- The modern periodic table explains why the characteristics of elements are repeated after two, eight, twenty-eight, and thirty-two elements. Since 2, 8, 18 and 32 is the maximum number of electrons that can be accommodated in K, L, M and N shells of the atoms of the elements, so the electronic configurations of the elements are repeated after 2, 8, 18 and 32 elements. Now, since the electronic configurations of the elements are repeated after 2, 8, 18 and 32 elements, therefore the properties of the elements are also repeated after 2, 8, 18 and 32 elements. This fixes the number of elements in a period of the table.
Advantages of the Modern Periodic Table.
- The periodic table chart is used as a teaching-aid in chemistry in schools and colleges.
- If the position of an element in the periodic table is known, then it is easier to remember the properties of an element. For example, the element rubidium with atomic number 37, occurs in group 1. We know that the common elements of group 1 are sodium and potassium. So the chemical properties of rubidium will be similar to the properties of sodium and potassium.
- If the position of an element in the periodic table is known, then it can be predicted which type of compounds can be formed by the element. For example, if an element is on the left side of the table, it will be metal and hence form only ionic compounds. If an element is on the right side of the periodic table, then it will be a non-metal and can form ionic as well as covalent bonds.
- The lanthanides and actinides, which differ from other groups in their properties, are put at the bottom of the periodic table separately.
- Chemistry can now be studied in a systematic and simple way due to the periodic table. It serves as a memory aid. All of the elements have been classified into a few groups in the periodic table. Each group is made up of elements having similar characteristics. It is far more convenient to study the properties of a few elements out of each group than to study all of the elements individually.
Sample Questions.
Question 1: Which of the following elements belong to the same group? Element A has an atomic number is 5. Element B has an atomic number is 10. Element C has the atomic number 13.
Answer:
The elements which have same number of valence electrons (outermost electrons), belong to the same group of the periodic table.
The electronic configurations of the given elements are:
Element Atomic Number Electronic configuration A 5 2, 3 B 10 2, 8 C 13 2, 8, 3 Since element A and C have the same number of valence electrons, so elements A and C belong to the same group.
Question 2: The elements which belong to the left side of the periodic table form ionic compounds or covalent compounds?
Answer:
Since metals form ionic compounds only and if an element is on the left side of the periodic table, it will be a metal and hence the element will form only ionic compounds.
Question 3: Which of the following elements show similar chemical properties? Element A has an atomic number is 3. Element B has an atomic number is 11. Element C has the atomic number 9.
Answer:
The elements which have same number of valence electrons (outermost electrons), show similar chemical properties.
The electronic configurations of the given elements are.
Elements Atomic Number Electronic Configuration A 3 2,1 B 11 2, 8,1 C 9 2, 7 Since element A and B have the same number of valence electrons, so elements A and B show similar chemical properties.
Question 4: Do magnesium and potassium show similar chemical properties or not?
Answer:
The elements which belongs to the same group, i.e., the elements which have same number of valence electrons (outermost electrons) shows similar chemical properties.
The atomic number of magnesium is 12 and atomic number of potassium is 19. The electronic configuration of these elements is.
Elements Atomic Number Electronic Configuration Magnesium 12 2, 8, 2 Potassium 19 2, 8, 8, 1 Since magnesium has 2 valence electrons and potassium has 1 valence electron, which are not same. So, magnesium and potassium do not show similar chemical properties.
Question 5: How does the modern periodic table help as a memory aid?
Answer:
The modern periodic table helps as a memory aid, as all of the elements in the modern periodic table have been classified into a few groups. Each group is made up of elements having similar properties. It is far more convenient to study the properties of a few elements out of each group than to study all of the elements individually.
Characteristics of Periods in a Periodic Table
Moving from left to right in a period of the periodic table, that is, moving from left to right in a horizontal row of the periodic table, we will discuss the variation of some of the important properties of elements including the number of valence electrons, valency, atom size, and metallic character. These variations will be explained further below.
Valence Electrons
The number of valence electrons in elements increases from 1 to 8 as a period progresses from left to right, while it increases from 1 to 2 in the first period.
The element sodium (Na) contains one valence electron in the third period, whereas the element argon (Ar) has eight valence electrons. Every period’s first element has one valence electron and every period’s last element has eight valence electrons, with the exception of the first period, where the last element, helium (He), has only two valence electrons.
The number of electrons in the outermost shell of an atom grows from 1 to 8 as the electronic configurations of elements vary over time. Along the period, the number of valence electrons increases from 1 to 8. The atomic numbers of the elements in a period are also consecutive. For instance, elements of the third period, from sodium to argon, have atomic numbers ranging from 11 to 18.
| Elements of the third period | Na | Mg | Al | Si | P | S | Cl | Ar |
| Electronic configurations | 2, 8, 1 | 2, 8, 2 | 2, 8, 3 | 2, 8, 4 | 2, 8, 5 | 2, 8, 6 | 2, 8, 7 | 2, 8, 8 |
| Number of valence electrons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
Valency
The valency of elements increases from 1 to 4 and eventually decreases to zero as you move from left to right in a period.
Sodium has a valency of 1, magnesium has a valency of 2, aluminium has a valency of 3, silicon has a valency of 4, phosphorous has a valency of 3, sulphur has a valency of 2, chlorine has a valency of 1, and argon has a valency of 0 in the third period. Valency increases from 1 to 4 and then decreases to zero over a period from left to right. The valency increases from 1 in sodium to 4 in silicon in the third period of the periodic table, then decreases to zero in argon. As a result, the valencies of elements from the same period differ. The valency of an element is determined by the number of electrons lost, gained, or shared by one atom to attain the nearest inert gas electron configuration.
| Elements of the third period | Na | Mg | Al | Si | P | S | Cl | Ar |
| Electronic configurations | 2, 8, 1 | 2, 8, 2 | 2, 8, 3 | 2, 8, 4 | 2, 8, 5 | 2, 8, 6 | 2, 8, 7 | 2, 8, 8 |
| Valency | 1 | 2 | 3 | 4 | 3 | 2 | 1 | 0 |
Size of atoms
In a period, the atomic size decreases from left to right. The number of protons and electrons increases over time as the atomic number increases, therefore the extra electrons are added to the same shell. The electrons are drawn in closer to the nucleus due to the strong positive charge on the nucleus, and the atom’s size shrinks. As a result, there is a stronger attraction to the nucleus. Hence, the atomic size shrinks.
Thus, in any period, the alkali metal atoms (lithium, sodium, potassium, etc.) at the far left of the periodic table are the largest, while the halogen atoms (fluorine, chlorine, bromine, etc.) at the far right of the periodic table, excluding inert gas, are the smallest. However, inert gas’s atom, on the other hand, is larger than the halogen atom preceding it.
Metallic Character
The metallic character of elements decreases as you move from left to right, while the non-metallic character increases. Metals in the third period include sodium, magnesium, and aluminium. Silicon has properties that fall in between metals and non-metals, making it a metalloid. Phosphorus, sulphur, and chlorine are non-metallic elements.
The components on the extreme left side of a period have the most metallic character, whereas the elements on the extreme right side of a period have the most non-metallic character. Metals are known as electropositive elements because they lose electrons and generate positive ions.
Non-metals, on the other hand, receive electrons and create negative ions, giving them the name electronegative elements. The atomic size shrinks from left to right over time as the nuclear force of attraction increases. As a result, losing valence electrons is difficult. Hence, metals’ electropositivity decreases over time. A non-metal atom can also easily gain electrons.
As a result, the electronegativity of non-metals increases over time. As a result, sodium is the most electropositive element in the third period, whereas chlorine is the most electronegative.
| Elements of the third period | Na | Mg | Al | Si | P | S | Cl |
| Nature of elements | Metals | Metalloid | Non-metals | ||||
Characteristics of Groups in a Periodic Table
Moving from top to bottom in a group of the periodic table, that is, moving from top to bottom in a vertical column of the periodic table, we will discuss the variation of some of the important properties of elements including the number of valence electrons, valency, atom size, and metallic character. These variances will be explained further below.
Valence Electrons
Each element in a periodic table group has the same number of valence electrons. Lithium, sodium, and potassium, for example, all contain one valence electron in their atoms and belong to group 1 of the periodic table. Lithium, sodium, and potassium atoms can easily lose their one valence electron to create potassium ions with one unit positive charge, Li, Na, and K, respectively.
As a result, group 1 elements are monovalent with a valency of 1. In their atoms, all elements in group 2 have two valence electrons. Except for helium, which has only two valence electrons in its atom, group 13 elements have three valence elements, group 14 elements have four valence electrons, group 15 elements have five valence electrons, group 16 elements have six valence electrons, group 17 elements have seven valence electrons, and group 18 elements have eight valence electrons. As a result, as you move down the periodic table, the number of valence electrons in the elements stays the same.
Valency
All elements in a group have the same valency because the number of valence electrons that determine valency is the same. Lithium, sodium, and potassium, for example, all have one valence electron, hence all the elements in group 1 have the same valency of one.
Group 1 elements have a valency of 1, group 2 elements have a valency of 2, group 13 elements have a valency of 3, group 14 elements have a valency of 4, group 15 elements have a valency of 3, group 16 elements have a valency of 2, group 17 elements have a valency of 1, and group 18 elements have a valency of 0. As a result, each group’s valency is the same.
Size of atoms
The size of atoms or atomic size grows as one moves down a group of the periodic table. When we proceed down in group 1 from top to bottom, the size of the atoms gradually increases from lithium to francium. Every time we proceed from the top to the bottom of a group, a new shell of electrons is added to the atoms.
As a result, the number of electron shells in the atoms steadily increases, causing the atoms’ size to increase as well. So, the lowest atomic size may be found at the top of the group, while the biggest atomic size can be found at the bottom. For example, in group 1, lithium (Li) is the smallest element, whereas francium (Fr) is the largest element.
| Li | Smallest atom |
| Na | Atomic size increases on going down the group |
| K | |
| Rb | |
| Cs | |
| Fr | Biggest atom |
Metallic Character
The metallic character of elements increases as you move from top to bottom, while the non-metallic character decreases. The elements in the bottom half of the group have the most metallic character. For instance, the metallic nature of group 1 increases from lithium to francium. Every time we move down a group of the periodic table, one more electron shell is added, and the size of the atoms grows.
The valence electrons move further away from the nucleus, and the nucleus’ hold on valence electrons weakens. As a result, the atom can lose valence electrons more quickly and create positive ions, increasing its electropositivity. Furthermore, as the size of atoms grows larger as they progress through the group, the nucleus becomes more embedded in the atom. The nucleus’ attraction to the incoming electron decreases, making it difficult for the atom to create negative ions and reducing electronegative characteristics.
Sample Questions
Question 1: Which element has the largest size in the third period?
Answer:
In a period, the atomic size decreases from left to right. This indicates that the element to the left of the period is the largest, while the element to the right of the period is the smallest. The element on the left in the third period is sodium, hence sodium (Na) has the largest size in the third period.
Question 2: What is the tendency to lose electrons over a period?
Answer:
The nuclear charge grows as the number of protons increases over a period, and the valence electrons are drawn in more strongly by the nucleus, making it more difficult for the atoms to lose electrons. As a result, as a period progresses from left to right, the tendency of atoms to lose electrons decreases.
Question 3: What are the usual valence electrons and valency of the elements of group 2?
Answer:
The elements of group 2 are beryllium, magnesium, calcium, strontium, barium, radium. All these elements have electrons in their outermost shell, so the valence electrons of the elements of group 2 are 2. Since there are two valence electrons,m so these elements can easily lose 2 electrons, hence the valency of the elements of group 2 is 2.
Question 4: An element belongs to group 2 of the periodic table, is this element metal or non-metal?
Answer:
The metallic character of elements decreases as you move from left to right in a period. The components on the extreme left side of a period have the most metallic character. Since the elements of group 2 are on the left side of the table, so the given element is a metal.
Question 5: Element A has atomic number 4, and element B has atomic number 8, and element C has atomic number 12. Which of these elements is a member of the same group?
Answer:
The number of valence electrons of elements in the group are the same.
Elements Atomic Number Electronic configuration A 4 2, 2 B 8 2, 6 C 12 2, 8, 2 Clearly, elements A and C have 2 valence electrons. Hence they belong to the same group.