There are large uncertainties in the measurement of ionic radii.
Ionic radius depends on the nature of the ions and the co-ordination number.
As you go down the group the ions have more electron shells therefore the ions get larger.
Across a period, for ions with the same number of electrons (such as Na
+, Mg
2+ and Al
3+), the ionic radius decreases as the number of protons increases. The electrons are attracted more strongly and pulled closer to the nucleus. The negative ions (P
3-, S
2-, Cl
-) have an extra shell, so they are much larger.
Period 3:
| group 1 | group 2 | group 3 |
| Na+ | Mg2+ | Al3+ |
| protons | 11 | 12 | 13 |
| electrons | 10 | 10 | 10 |