Question

give reason why oxygen atom is smaller than boron atom​

Answer 1

When moving along a period, the atomic radius is supposed to decrease. However oxygen has slightly higher atomic radius than nitrogen. This is because nitrogen has an electronic configuration 2p3 having its orbittal half filled and hence there is less repulsion between the electrons.

Answer 2

Answer:

However oxygen has slightly higher atomic radius than nitrogen. This is because nitrogen has an electronic configuration 2p3 having its orbittal half filled and hence there is less repulsion between the electrons. ... This leads to inter-electronic repulsion and thus increases the atomic radius slightly.