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 $n{s}^1$ configuration .
• B. Alkaline earth metals have very high melting points as compared to alkali metals.
• C. Electronic 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

The correct option is 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 $n{s}^1$ configuration .

In alkaline earth metals, after the loss of two electrons, they achieve noble gas configuration.

$M_{\left(g\right)}\left(n{s}^2\right)\underset{I{E}_1}{\overset{-e^{-}}{\to }}{M}_{\left(g\right)}^{+}\left(n{s}^1\right)\underset{I{E}_2}{\overset{-e^{-}}{\to }}{M}_{\left(g\right)}^{2+}\left(n{s}^o\right)noblegasconfiguration$

while in alkali metals, after the loss of one electrons, they achieve noble gas configuration.

$M_{\left(g\right)}\left(n{s}^1\right)\underset{I{E}_1}{\overset{-e^{-}}{\to }}{M}_{\left(g\right)}^{+}\left(n{s}^0\right)noblegasconfiguration$

In alkali metals after the loss of one electron, they achieve noble gas configuration and removal of electron from noble gas configuration requires very high ionisation enthalpy. Hence the second ionisation enthalpy of alkali metals is very high as compared to alkaline earth metals.