Periodic Classification of Elements 

Lavoisier first classified elements into metals and non-metals.

Dobryne's Triad Law:- In 1817, German chemist, Wolfgang Dobereiner tried to arrange elements with similar properties into groups. He chose some groups as three elements each and called them triads. According to this rule: The atomic mass of the middle element is equal to the average value of the atomic masses of the remaining two elements. Dobereiner could find only three triads among the elements known till that time. Due to this, his triad method failed.

Important Facts:- Dobereiner was the first to identify platinum as a catalyst.

Newlands' Octave Law:- John Alexander Newlands gave the Octave Law in 1865. He arranged the elements in the order of their increasing atomic weight and found that the properties of the eighth element were similar to those of the first element. For example, in music, after seven notes Sa, Re, Ga, Ma, Pa, Dha, Ni, Sa, the eighth note is like the first note.

Flaws of Newlands' Law of Octaves :-

[1] It was found that this principle was applicable only up to calcium. Because after this, the properties of every eighth element did not match with the first element.

[2] This principle could be properly applied only on light elements.

[3] According to this principle, there are a total of 56 elements in nature. And no new element will be found in future. But when new elements were discovered, their properties were not according to the rule of octaves!

Mendeleev's Periodic Table :- The main credit for classification of elements goes to "Dmitry Ivanovich Mendeleev". He arranged the elements in his table on the basis of their basic properties, atomic mass and similarity in chemical properties. The properties of elements are periodic functions of their atomic mass.

Mendeleev's original periodic table: - This table has two main parts.

(1) Period (horizontal rows)

(2) Group (vertical columns)

(i) Period: - There are seven horizontal rows in Mendeleev's periodic table. These are called periods.

(1) The first row or period has only two elements hydrogen and helium. It is called the shortest period.

(2) The second and third periods have eight elements each. And each is called a "short period".

(3) The fourth and fifth periods have 18 elements each. And each is called a "long period".

(4) Sixth period has 32 elements. It is called the "longest period". (5) Seventh period is incomplete. It has a total of 19 elements.

[ Note: Elements of second period are from "lithium to fluorine". Elements of third period are from "sodium to chlorine" are called "representative" or "typical elements". ]

(ii) Group: There are 2 groups in Mendeleev's periodic table. These are represented by Roman numerals I, II, III, IV, V, VI, VII, VIII and zero respectively. Each group from I to VII is divided into two sub-groups A and B. Group VIII has a total of nine elements in groups of three each. "Inert gases" were placed in the zero group. Utility of Mendeleev's periodic table:-

(i) Systematic study of elements: Mendeleev arranged the elements for the first time in periods and groups. He arranged the elements in such a way that only elements with similar properties were arranged in a particular group.

(ii) Study of properties of elements:- By studying a particular element in Mendeleev's periodic table, all other elements of that group can be studied.

(ii) Discovery of new elements:- Mendeleev left blank spaces for unknown elements in his periodic table. And also predicted their properties which later proved to be true.

(iv) Correction of doubtful atomic masses:- The valency of any element can be known from its position in the periodic table. And by knowing its equivalent weight, atomic weight can also be determined.

Limitations of Mendeleev's periodic table:- Mendeleev's periodic table is helpful in classifying many elements in groups and periods. But still it has many shortcomings.

(i) Place of hydrogen:- The place of hydrogen in the periodic table is not fixed. It has been placed with alkali metals in group I-A. Whereas it also shows similarity with halogens present in group VIIA.

(ii) Place of rare earth and trans-uranic elements:- There are similarities in the chemical properties of rare earth elements. But their atomic weights are different, yet these 14-14 elements have been placed together in the third subgroup B (sixth period). Which is not appropriate.

(III) Place of inert gases:- There was no proper place for inert gases in Mendeleev's periodic table.

Inert gases (Noble gases) Noble gases Helium, Neon, Argon, Xenon

(iv) Position of isotopes:- The discovery of isotopes and isobars made it clear that the basic characteristic of elements is not their atomic weight. The atomic weights of isotopes are different and the atomic weights of isobars are the same. Hence, the position of isotopes in Mendeleev's original periodic table is not fixed.

Modern periodic table:- In 1913, Henry Moseley stated that the atomic number of an element is more important than its atomic mass.

The physical and chemical properties of elements are periodic functions of their atomic number.

The modern periodic table has 18 groups and 7 periods. 

The maximum number of electrons in a shell

It is represented by the 2n² formula.

K shell = 2 electrons, L shell = 8 electrons,

M shell = 18 electrons, N shell = 32 electrons.

Advantages of Modern Periodic Table:-

[1] Each group has only one group. There is no place for sub-groups.

[2] Electronic configuration of elements can be easily understood.

[3] Division of elements into S, P, d and f - blocks helps in understanding their properties.

[4] A place was fixed for all isotopes of any element. Because isotopes have the same atomic number.

Disadvantages of Modern Periodic Table:-

[i] The position of hydrogen is the same as in Mendeleev's table. It is kept with alkali metals. But some of its properties are also similar to halogens.

[2] Lanthanoids and actinoids should have been placed in the main frame of the periodic table. But they are placed separately at the bottom of the periodic table.

Trend of Modern Periodic Table :-

[1] Valency :- The combining capacity of an element is related to the number of electrons present in the outermost energy shell of its atom. This is called valency. Valency increases from 1 to 4 as we move from left to right in the periodic table. Then it decreases to 1 and becomes zero in the case of noble gases.

[2] Atomic size :- The size of the atom indicates the radius of the atom. The distance of the outermost shell containing electrons from the centre of the nucleus is called "atomic radius".

Change in Period :- The value of atomic radius decreases as we move from left to right in a period.

Change in Group :- The atomic radius of the elements increases as we move down a group in the periodic table.

[3] Metallic and Non-metallic Properties :- Metallic properties increase as we move from top to bottom in a group. Because atomic size increases as we move from top to bottom.

As we move from left to right in a period, the effective nuclear charge acting on the electrons of the valence shell increases. The tendency to lose electrons decreases.

In the modern periodic table, a zig-zag line separates metals from non-metals. The elements falling on this line viz. boron, silicon, germanium, arsenic, tellurium, polonium show the properties of both metals and non-metals. Hence, they are called metalloids/semi-metals.