(a) Define the following terms:
(i) Valency, (ii) Atomic size.
(b) How do the valency and the atomic size of the elements vary while going from left to right along a period in the modern periodic table ?

Explanation

(a) (i) Valency : The combining power or the combining capacity of an atom is called its valency.
(ii) Atomic size: Atomic size or atomic radius is the distance between the centre of the nucleus and the outermost shell of an isolated atom.
(b) On moving from left to right in the periodic table, valency increases up to 4 and then decreases. The electrons present in the last shell determine the valency of a particular element.
If the number of valence electrons is less than or equal to 4, valency = number of valence electrons If the number of valence electrons is more than 4, valency = 8 - number of valence electrons
Atomic size decreases along a period. This is because on moving across a period, the number of valence shells remains the same and the electrons increase by one unit. As a result, the nuclear charge increases and thus, the atomic radius decreases.