The first ionisation energy of potassium, $\mathrm{K}$, is $418\,\text{kJ mol}^{-1}$. The first ionisation energy of strontium, $\mathrm{Sr}$, is $548\,\text{kJ mol}^{-1}$. Which statement assists in explaining why $\mathrm{Sr}$ has a higher first ionisation energy than $\mathrm{K}$?
- AThe charge on an $\mathrm{Sr}$ nucleus is greater than the charge on a $\mathrm{K}$ nucleus.
- BThe outer electron in a $\mathrm{Sr}$ atom experiences greater shielding than the outer electron in a $\mathrm{K}$ atom.
- CThe outer electron in a $\mathrm{Sr}$ atom experiences spin-pair repulsion.
- DThe outer electron in a $\mathrm{Sr}$ atom is further from the nucleus than the outer electron in a $\mathrm{K}$ atom.