Classification of Elements
To simplify the study of chemistry of elements and their compounds, systematic classification is necessary. The first attempt was made by Dobereiner.
Dobereiner's Law of Triads
According to Dobereiner's law of triads, every 3 elements with similar properties is considered as one triad. Dobereiner's triads are
- Li, Na, K
- Ca, Sr, Ba
- S, Se, Te
- CI, Br, I
The atomic weight of the middle element is the average of the two neighbouring elements of a given triad.
Mendeleev's Periodic Law
The physical and chemical properties of elements are the periodic functions of their atomic weights. In the Mendeleev periodic table, elements are arranged in the increasing order of their atomic weights.
It consists 7 horizontal rows or 7 periods and 8 vertical columns or 8 groups. Each group is sub divided into two sub groups A and B.
He introduced the concept of valency and observed that valency of elements belonging to a group will be equal to its group numbers.
Modern Periodic Table
Modern periodic table was constructed on the basis of atomic numbers. Properties of elements also depend on the electronic configuration.
The physical and chemical properties of elements are the periodic functions of their atomic numbers or electronic configuration. The properties are repeated after regular intervals of time when the elements are arranged in the increasing order of their atomic numbers.
Neil's Bohr constructed the modern periodic table based on the electronic configuration of the elements. It is a graphical representation of Aufbau principle. The vertical columns are called groups and the horizontal rows are called periods. There are altogether 18 groups and 7 periods in the long form or extended form of periodic table.
From left to right the atomic number increases by one unit. The electron which differentiates an element from the preceding element is called the differentiating electron. The differentiating electron is the last coming electron of that element.
First period contains only two elements H (1s1) and He (1s2) and it is called very short period. Second period contains 8 elements and it is called 1st short period. Third period also contains 8 elements and it is called 2nd short period. The first 3 periods are discontinuous periods.
Fourth period contains 18 elements and it is called 1st long period. Fifth period also contains 18 elements and it is called 2nd long period. Sixth period is the longest period containing 32 elements.
Elements belonging to group will exhibit similar properties due to similar valence shell configuration. The elements which exhibit both vertical and horizontal similarities are transition elements. The number of electrons in valence shell is equal to the group number.
Classification of Elements into Blocks
The elements are classified into four blocks as s-block, p-block, d-block and f-block based on the orbital into which differentiating electron enters. This classification is based on electronic configuration.
s-block contains 2 groups, p-block contains 6 groups, d-block contains 10 groups and f-block contains 14 groups. s-block is at the extreme left and p-block is at the extreme right of the periodic table. ¾ d-block is kept in between s-block and p-block. f-block is separately placed below the main body of the table.