Question

which is larger in size Al3+ ,Na+,Mg2+. why?

Answer 1

Na+>Mg2+>Al3+
Because
In Na+ no. Of protons=11 , electrons=10
In Mg2+ no. Of protons=12
Electrons=10
In Al3+ no. Of protons=13, electrons=10
So as no of electrons for all 3 are same then the atom with more number of protons will provide more attractive force so size small
Hence, Na+ is the largest amongst the 3.