Use z*(effective nuclear charge) to explain why Ca2+ and S2- ions are stable.?

Answer 1

I don't think that this is a particularly good question - but it probably isn't your fault! The concept of effective nuclear charge is not accepted by examiners in the UK, who prefer (as I do) to keep the ideas of actual nuclear charge, screening and radius separate.

When Ca atoms lose two electrons, there is one less level, and the screening drops. This leads to a drop in radius, and greater attraction for the reamining electrons. Yes, ENC has gone up. Don't forget, though, that the process is endothermic, and without any further processes like the formation of an ionic bond, Ca2+ ions are less stable than Ca atoms!

When S atoms gain electrons, this, too, is an endothermic process. Screening does not increase, but repulsion does. Thus radius increases. Has ENC changed? I don't think so.