Question -
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 an 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 an increase in principal quantum number(n) which brings about an increase in the distance between the nucleus and the valence electrons.