Question

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

The number of protons present in $Ar,K^{+},C{a}^{2+}$ 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 inter electronic repulsions and they do not form molecules. Thus, in case of noble gases atomic radius is determined by Van der Waals radius which is larger than the covalent radii as well as the Ionic radii.

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
$C{a}^{2+}<K^{+}<Ar$