The first ionization energy of gallium (Ga, 578.8 kJ/mol) is greater than that of aluminum (Al, 577.5 kJ/mol). Which is the best explanation for this?
1) Gallium has a larger atomic radius than aluminum.
2) Gallium has a higher effective nuclear charge than aluminum.
3) Gallium has a higher electron affinity than aluminum.
4) Gallium has a higher ionization energy than aluminum.