From a neutral atom where a electron is removed then it become cation and when a electron is added to the atom then it becomes anion. For a atom x when it loose a electron then it becomes cation x−e−→x+ Let atomic no. of x is z then no. of proton in x+=z and no. of electron in x+=z−1 So the ratio ‌
z
e
as no. of electrons decreases. And when no of electron decreases then per electron attraction increases from nucleus and radius of cation deceases. Similarly when atom x becomes x−by adding a electron (x+e−→x−)then no. of proton in x−=z and no. of electron in x−=z+1. Now the ratio of ‌
z
e
in x−decreases as no. of electron increases. As electron increases in x− then attraction from nucleus decreases on per electron and the distance between electron and nucleus increases so the radius also increase in anion. All 4 ions are from second period and for O2− and F−both have 1s,2s and 2p shell and for Li+and B3+ both have 1s shell. So, size of O2− and F−are more then Li+ and B3+ and among O2− and F− For O2−,‌
z
e
=‌
8
10
=0.8 For F−,‌
z
e
=‌
9
10
=0.9 as for O2−,‌
z
e
is less so per electron attraction from nucleus decreases and radius increases more than F−.