Question

which of the following ions is larger in size?
a)Na+
b)Mg2+
c)Al3+
d)H+

Answer 1

Na+ is larger in size as size is inversely proportional to atomic number in a group of isoelectronic species and as electrostatic forces of attraction increases within electron size decreases and ESFA increases when number of electrons decreases so we come to a conclusion that in a group of isoelectronic species,element with least number of protons will have largest size(technique to judge size of isoelectronic species)

Answer 2

Na+ is the largest ion in this group because Na+ has only one electron less than the number of protons it has whereas Mg has 2 and Al has 3 less electrons than their respective proton numbers,so the shielding effect is more in Na+,thus larger size.

Again we know that radius of isoelectronic species is inversely propositional to atomic no which also explains that Na is larger in size.

Also H+ does not contain any electron but only protons due to which it can be easily assumed that H+ cannot be the largest.