[gctso]

It is said that, "In general, small atoms require more energy to ionize. Notice that all of these elements are in group 1A. Since atomic radius increases within a group, the ionization energy decreases." And my professor said this is due to the attractive force between protons and electrons (shielding effect). But since the numbers of protons and electrons are always the same in a neutral atom, in a neutral atom, the protons to electrons ratio is always 1 to 1. So how does the size of atom affect amount of ionization energy?

[Darryl1]

Just a guess, but larger atoms have larger radii and the attractive force between the nucleus and electrons probably decreases more than linearly the further out the electrons are.

So like pulling apart two magnets takes much more energy than pulling apart two identical magnets that are already slightly apart.

-d