Question

Explain why gallium has higher ionization enthalpy than aluminum?

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 shells 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.

Answer 2

as we go down the group the size increases but when it comes to gallium the size decreases because 'd' orbital comes into the picture which produces a poor screening effect due to which the size decreases 
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as the size decreases the ionization enthalpy will increase as the nucleus will have a greater hold on its outermost electrons