Question

What do you mean by ionization enthalpy? How does it vary across a
period and down a group?

Answer 1

As you go down a group, the ionization energy decreases. This is due to the "shielding effect" which is basically when there is repulsion between the electrons in the inner shell and the valence electrons. As you go across a period, ionization energy increases. This occurs because as the number of protons increase in the nucleus, the attraction to the valence electrons also increase