Question

how can you tell which ion is bigger

Answer 1

what????

Answer 2

what do u mean by bigger?

Answer 3

like electronegativity?

Answer 4

Atoms increase or decrease in size because the number of electrons (é) changes. If an é is gained, it increases the size of the atom, in contrast when lost, the atom shrinks.

This is rationalized by taking into account the repulsive/attractive behaviour of these particles. Since é repel one another, the volume which they must occupy increases when more present (i.e. gained é). And when an é is lost, not only do they occupy less volume from a mere reduction in numbers but the effective nuclear (positive) charge increases (because é mask this charge) and are drawn more strongly towards the nucleus.