Question

The first ionisation enthalpies of the alkaline earth metals are higher than that of alkali metals but second ionisation enthalpies are smaller, why ?

• A. In alkali metals, second ionisation enthalpy involves removal of electron from noble gas electronic configuration while in alkaline earth metals, second electron is removed from $ns^{-1}$ configuration
• B. Alkaline earth metals have very high melting point as compared to alkali metals
• C. Electrons in $s$-orbital are more closely packed in alkaline earth metals than alkali metals
• D. Due to smaller size alkaline earth metals do not form divalent ions very easily

Answer 1

Answer: (A)

Alkaline earth metals -
$\underset{ns^2}{M}-1e^- \xrightarrow{IE_1} \underset{ns^1}{M^+} \xrightarrow[-1e^-]{IE_2}$ $\underset{\text{(Noble gas configuration)}}{M^{2+}}$
Alkali metals -
$\underset{ns^1}{M}-1e^-\xrightarrow{IE_1} \underset{\text{(Noble gas configuration)}}{M^+}\xrightarrow{IE_2} M^{2+}$ ;
$IE_2 >>IE_1$