Periodic Classification of Elements Revision Notes

The process by which the chemical elements are classified based on their characteristics is called the periodic classification of elements. Earlier, the elements were classified mainly into metals and non-metals by the scientists, but then some elements showed properties that could neither be considered metal or non-metal, they came to be known as metalloids. Therefore, this classification became insufficient for scientific studies. This led to the improvement of study by the chemists, which ultimately resulted in a systematic classification based on the physical and chemical properties of the elements in a tabular form called the periodic table. This table shows the arrangement of the known elements in such a way that elements with similar properties are located in the same vertical columns, while the remaining different elements are segregated.

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EXPLANATION

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EARLY MODELS OF PERIODIC TABLES:

• Dobereiner’s Triads
• Newland’s Law of Octaves
• Mendeleev’s Periodic Table and Law

DOBEREINER’S TRIADS:

In 1829, the German scientist Johann Wolfgang Dobereiner grouped the elements that showed similar properties in order of their increasing atomic masses into clusters of three called ‘Triads’. According to this arrangement, the atomic mass of the middle element is the arithmetic mean of the other two elements in the triad. Unfortunately, at that time Dobereiner could identify only the following three triads from the known elements. The examples that are mentioned below shows how the arithmetic mean of these triads is quite similar to the atomic mass of the middle element: 

Limitations of the Triad Classification:

• There are similar elements that cannot be grouped into triads. For example, cobalt, zinc, copper and iron, manganese, nickel.
• Some dissimilar elements can be grouped into triads.
• Only 3 triads were successfully classified by Dobereiner.

NEWLAND’S LAW OF OCTAVES:

According to Newland, when we arrange the elements based on their increasing atomic masses, the chemical and physical properties of every eighth element are similar to that of the first element, just like the Indian and Western notes of music.

Limitations of Newland’s Law of Octaves:

• Only applicable for elements of atomic mass till 40.
• The newly discovered elements did not fit the law of the octaves.
• When arranged in increasing order of the atomic mass, the properties of the 8th element did not match and was there unsuccessful for the heavier elements.
• Two elements are sometimes placed in the same slot to fit elements in the table.
• Dissimilar elements were grouped under the same slot.

MENDELEEV’S PERIODIC TABLE:

In this table, Mendeleev arranges the elements based on the increasing order of the atomic masses. It is observed that after that there is a periodic reappearance of the chemical and physical properties of the elements. It not only corrected the atomic masses of certain elements, but it also accommodates noble gases and left space for the undiscovered elements (like Boron, etc.). There are 12 horizontal rows condensed into 7 which are called periods, and 8 vertical columns which are called groups. The Groups I-VII are further subdivided into A and B subgroups. The only exception is Group VIII which does not have any subgroups, and it also contains three elements in each row. Therefore, the elements that belong to the same group show similar properties.

Limitations of Mendeleev’s Periodic Table:

• The position of isotopes is unexplainable.
• Hydrogen’s position is not rigid. Though it is placed in group 1A, it resembles some properties of the halogens.
• Irregular pairs of elements, such as Co (Cobalt) which has a higher atomic mass is placed before Ni (Nickel).
• Elements such as Pt (Platinum) and Au (Gold) exhibits similar properties but still, they are placed in different groups.

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MODERN PERIODIC TABLE:

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Henry Moseley in 1913 discovered that more than the atomic mass, the atomic number of an element is a greater fundamental property. The atomic number is the periodic function of the chemical and physical properties of the elements. Therefore, this table deals with the atomic number of the elements, which is equal to the number of protons present in the nucleus of an atom of the element and it is represented by Z. The table consists of 18 vertical columns which are called groups and 7 horizontal rows called periods. The number of valence electrons of the elements increases from 1-8 as we move left to right in a period, but the number of shells remains the same. The elements belonging to the same group have the number of valence electrons, but as we go down the group the number of shells increases. The number of bonds formed by an element is determined by the valence electron.

Formula to find the number of electrons in a shell: 2n2

• n is the shell number.
• For example,

K shell n = 1

2n² = 2(1)² = 2

TRENDS IN THE MODERN PERIODIC TABLE:

• Valency and Valence Electrons: 

As we move left to right in a period, the valency increases but then it decreases. It remains the same as we move down in a group. Left to right in a period, the valence electron increases but it remains the same when we go down the group.

• Atomic Size:

As we move left to right in a period the atomic size decreases, which is due to the nuclear charge increase that happens because of the large positive charges on the nucleus. But as we go down in a group atomic size increases, due to the decrease in nuclear charges and the addition of new shell.

• Metallic Character:

The ability of an atom to lose the electron is defined as the Metallic Character. It decreases from left to right in a period, which happens due to the increase in nuclear charge. On the other hand, the non-metallic character increases left to right in a period. But down the group, the metallic character increases due to the increase in size it becomes easier to lose electron.

• Ionization Energy:

To remove an electron from an isolated gaseous atom an energy is required, this energy is known as ionization energy. As we move left to right in a period, the ionization energy increases, which happens due to the increase in nuclear charge. Down the group ionization energy decreases because of the decrease in the nuclear charge, besides some instances.

• Electropositive Character:

As we move left to right in a periodic table the electropositive character decreases, which is caused due to the decrease in metallic character. But down the group, the electropositive character increases.

• Nature of Oxides:

As we move down the group, the basic character of oxides increases because the atomic radius increases and the ionization energy decreases, this happens due to an increase in metallic character or electro positivity of elements. From top to bottom, the acidic character of oxides decreases because of the decrease of the non-metallic character of elements.