Why is the first ionization energy of aluminum lower than the first ionization energy of gallium.if the increased proton no.is the case then why the i.enthalpy is increasing everywhere down the group?

Answer 1

It has something to do with the effective nuclear charge experienced by the valence electrons in Al and Ga. While aluminum's valence electrons are in the 3p shell, and there are 13 protons in the nucleus. Gallium's valence electrons are in the 4p shell, and there are 31 protons in the nucleus. Additionally, the atomic radius of gallium is not too much larger than that of aluminum. Thus, the electrons in gallium are pulled more strongly to the nucleus and they are more difficult to remove. A similar but lesser effect can also be seen in group IV/14, where Germanium has an almost identical first ionization energy to Silicon, and after the F shell is filled--thallium has a higher first ionization energy than Indium, and Lead has a higher first ionization energy than tin.