MODERN PERIODIC TABLE
Valency may be defined as “the combining capacity of the atom of an element with atoms of other elements in order to acquire the stable configuration (i.e. 8 electron in valence shell. In some special cases it is 2 electrons).”
It refers to the distance between the centre of nucleus of an isolated atom to its outermost shell containing electrons.
The atomic radius decreases on moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
In a group atomic size increases from top to bottom due to increase in number of shells.
METALLIC AND NON-METALLIC PROPERTIES
In a period from left to right metallic nature decreases while non-metallic character increases.
In a group metallic character increases from top to bottom while non-metallic character decrease.