Question

explain how the variation of atomic radii in group and in period

Answer 1

The atomic radii decreases in moving from left to right along a period. This is due to the increase in nuclear charge which pulls the electrons closer and reduces the shells and thereby the size of an atom.
The atomic radii increases as we go down the group. This is because new shells gets added we go down the group. This increases the distance between the outermost shell and nucleus.

Answer 2

Answer:

Atomic radius declines as we move from left to right in a period. It happens because in a period, the external electrons are available in a similar valence shell so, the atomic number increments from left to right in a period, which results in increase in the effective nuclear charge. Therefore, the attraction of electrons towards the nucleus is increased.

Also, atomic radius declines as we move from top to bottom in the group. It happens because as we move down in a group then there is increase in  principal quantum number(n) which brings about increase in the  distance between nucleus and the valence electrons.