Question

what causes the trend in ionic radius?

Answer 1

number of electrons present around a given atom

Answer 2

The ionic radius is affected by the positive pull of the nucleus and the interelectronic repulsion of the valence electrons. When you have a positive ion, there is an increase in positive pull leading to smaller size. When you have a negative ion, the interelectronic repulsion causes the size to increase, also the positive pull is a little less because you have more e than protons.