Question

Which of the following order of radii is correct :

• A. Li < Be < Mg
• B. H+< Li+< H–
• C. O < F < Ne
• D. Li < Na < K < Cs <Rb

Answer 1

Answer: (B)

H+< Li+< H–

Answer 2

Atomic radius increases on moving top to bottom in a group due to increasing number of shells. However, it decreasing on moving left to right in a period due to increasing Zeff and addition of electrons to the same shell.

For H; cation is smaller than parent atom while anion is bigger than parent atom. H– and Li+ are isoelectronic species. So, ionic size $\propto$ $\frac{1}{nuclearch\arg e}$. Hence the correct order is H+ < Li+ < H–.