Question

Which of the following concepts underlies all the others: electron affinity, ionization energy, effective nuclear charge, atomic size?

Answer 1

Effective nuclear charge Or Z eff.
Effective nuclear charge Or Z eff controls all of the others i.e; electron affinity, ionization energy and atomic size.

Answer 2

the concept that underlies the other would be :
effective nuclear charge zeff
It control Ip and electron affinity by determining attractive force between atom and the electron
it underlies atomic size because it draws the valence electron closer

hope this helps