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 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 :
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.
Solution :
$${\text{Alkaline earth metals}} - $$
\[M-1{{e}^{-}}\xrightarrow{I{{E}_{1}}}\underset{n{{s}^{1}}}{\mathop{{{M}^{+}}}}\,\xrightarrow[-1{{e}^{-}}]{I{{E}_{2}}}\] \[\underset{\left( \text{Noble gas configuration} \right)}{\mathop{{{M}^{2+}}}}\,\]
$${\text{Alkali metals}}{\text{.}}$$
\[\underset{n{{s}^{1}}}{\mathop{M}}\,-1{{e}^{-}}\xrightarrow{I{{E}_{1}}}\underset{\left( \text{Noble gas configuration} \right)}{\mathop{{{M}^{+}}}}\,\] \[\xrightarrow[-1{{e}^{-}}]{I{{E}_{2}}}{{M}^{2+}};I{{E}_{2}}>>I{{E}_{1}}\]