Ionic Radius - The Trend Across the Table

The easiest way to understand the ionic radius trend is to see it.

Within the positive ions, the trend matches the trend for atomic radius -- decreasing to the right -- as does the reason (more positive nucleus pulling the electrons in tighter). The same is true within the negative ions. The discrepancy is between the positive ions and the negative ions.

To understand the change in size from the small positive ions to the larger negative ions we need to look at the electron configurations of the ions. The electron configurations for the ions above are listed below.

Li⁺¹: 1s²

Be⁺²: 1s²

B⁺³:  1s²

C⁺⁴:  1s²

N^-3:  1s²2s²2p⁶

O^-2:  1s²2s²2p⁶

F^-1:  1s²2s²2p⁶

What you need to notice is that the electron configuration for all of the positive ions is the same and that all of them are isoelectronic with He.

All of the negative ions are also identical with each other, but they are isoelectronic with Ne.

Since Ne has an entire extra energy level, we would expect anything with the electron structure of Ne to be larger than anything that has the electron structure of He.