Question

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

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

Answer 1

The correct option is C

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

$C{a}^{2+}<K^{+}<Ar$
$Ar,K^{+}andC{a}^{2+}$ are isoelectronic i.e. with same number of electrons, 18. For isoelectronic species ionic radii decreases with increase in effective (relative) positive charge. Also Ar, K and Ca belong to the same period (3rd period).