(solved)Question 3.10 of NCERT Class XI Chemistry Chapter 3

$\displaystyle \underline{\underline{\text{Variation in a Period}}}$

within a period from left to right (metallic and covalent radii):

Point 1: Electrons get added to the same shell. So the electron shielding remains constant.

Point 2: The increasing nuclear charge makes the nucleus more positively charged. There is a greater nuclear attraction. The nucleus pulls the atom's shell closer to the nucleus. So the valence electrons are held closer towards the nucleus. This factor tends to decrease the atomic radius.

Point 3: The electrons that get added to the later p-orbitals experience repulsions that tend to increase the atomic radius.

Conclusion 1: The increasing nuclear charge dominates over the electron-electron repulsions. Therefore, the size of the atom decreases from left to right in a period.

Conclusion 2: The effect decreases towards the right because of the electron-electron repulsions.

Exceptions 1: Noble gases are monatomic so their atomic radii are large. They cannot be compared to covalent/metallic radii.

Exceptions 2: [according to some literature]Radius of oxygen atom is larger than that of nitrogen because of the electron-electron repulsions as explained in Point 3 above.
(Please watch video for better explanation)

$\displaystyle \underline{\underline{\text{Variation in a Group}}}$

Point 1: As we move down a group, the principal energy levels get added. That tends to increase the atomic radius.

Point 2: As we move down a group, the nuclear charge increases. That tends to decrease the atomic radius.

But the Point 1 outweighs the effect of Point 2. Hence the atomic radii increase as we move down a group, even for noble gases.