Question

The species $Ar,K^{+}$ and $C{a}^{2+}$ contain the same number of electrons. In which order do their ionic radii increase?

• A. $C{a}^{2+}<K^{+}<Ar$
• B. $K^{+}<Ar<C{a}^{2+}$
• C. $Ar<K^{+}<C{a}^{2+}$
• D. $C{a}^{2+}<Ar<K^{+}$

Answer 1

The correct option is A $C{a}^{2+}<K^{+}<Ar$

The species $Ar,K^{+}$ and $C{a}^{2+}$ are isoelectronic species which means they contain the same number of electrons.

The number of protons present in $Ar,K^{+}$ and $C{a}^{2+}$ are 18, 19, and 20 respectively.

The number of electrons is the same but the number of protons (and the effective nuclear charge) increases in the order $Ar<K^{+}<C{a}^{2+}$. Since argon has fully filled shells, it will have high electron-electron repulsions in its shell.

Thus, it has the largest radius. Among, K and Ca, K has a larger size and on moving from left to right, the atomic size decreases.

Thus, the radii increases in the order $C{a}^{2+}<K^{+}<Ar$.

Therefore, the correct option is A.