(a) (i) Valency is the combining capactity of an element and is determined by number of electrons lost or regained to achieve the nearest stable configuration.
(ii) Atomic size is defined as the distance between the nuclei centre and the outermost electron shell of the atom.
(b) When moving from left to the right in a period, the valency of the elements increases from 0 to 4 and then it decreases to 0.
As we move from left to right in a period, the nuclear charge on an element increases but the number of energy shells remains the same. Due to increase in the nuclear charge, the force of attraction between nucleus and electrons increases. As a result, the size of the atom decreases on moving from left to right in a period.
Rate this question :