First ionization energy of group 1 and group 2 elements

[ianc1339]

TL;DR Summary

Why does the first ionization energy between group 1 and group 2 elements increase?

The first ionization energy decreases between group 5 and group 6 due to the repulsion between the electrons in the p orbital.

Although I understand that the effective nuclear charge increases between group 1 and group 2 elements, why isn't this the case between group 1 and group 2 elements? Shouldn't the first ionization energy decrease between group 1 and group 2 due to the repulsion between the electrons?

 

[DrClaude]

s orbitals extend to the nucleus, so there is a strong increase in potential energy when the nuclear charge increases. This more than compensates for the added electron-electron repulsion.