Question

Why is the second ionisation energy of alkali metals higher than alkaline earth metals?

Answer 1

In simple terms, ionization energy is a measure of how difficult it is to remove an electron from an atom or ion, or how likely an atom or ion is to surrender an electron. The loss of one electron usually occurs in the chemical species' ground state.

Complete answer:
Alkali metals contain only one valence electron, whereas alkaline earth metals have two. When one electron is removed from alkali metals, $M^+$ is formed, which is a stable noble gas configuration. To remove another electron from an alkali metal's unipositive ion, a huge quantity of enthalpy would be required.
Alkaline earth metals, on the other hand, do not develop a stable noble gas structure after losing one electron and still have a tendency to lose another. As a result, alkali metals' second ionisation enthalpies are significantly larger than those of alkaline earth metals.
The attraction between the nucleus and the electrons causes the second energy of ionisation to be larger than the first. Due to the extra proton, there is more attraction between the electrons and the nucleus while attempting to detach a second electron from a positive ion.

Note:
When there are two electrons in the same orbital, they are said to experience some form of repulsion. Now, this creates disturbances in the attraction of the nucleus. Hence, ionization energy will be less in paired electrons as they can be removed easily.