Question

How does atomic radius vary in a period and in a group ? How yoy explain the variation ?

Answer 1

Atomic masses increase from left to right and from top to bottom of the Periodic Table.
As you go from left to right in the Periodic Table, you are adding more protons and neutrons to the nuclei. The atoms in the rows further down have even more protons and nucleons.
Therefore, atomic mass increases from left to right and from top to bottom of the Periodic Table.

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.