Question

Why is the Ionisation enthalpy of hydrogen higher than that of sodium?

Answer 1

The electronic configuration of Hydrogen is $1s^1$.

The electronic configuration of sodium is $\left[Ne\right]3s^1$.

1s orbital is more closer to the nucleus as compared to 3s orbital, so the electrons are more strongly held by nucleus and thus it requires high energy to remove an electron from 1s orbital than 3s orbital.

Also, the size of hydrogen is smaller than that of sodium and we know the ionisation energy decreases with increase in size, therefore, the ionization enthalpy of H is higher$\left(1312kJmo{l}^{-1}\right)$ than that Na ($496kJmo{l}^{-1}$). As removing an electron from small atoms will be more difficuit as compared to the larger atom due to their strong effective nuclear charge.

Hence, due to above reasons ionisation enthaipy of hydrogen is higher than that of sodium.