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Why Gallium has small atomic size than aluminum ?

Generally we know that atomic size increases down the group from top to bottom due to increase in the number of valance shell . But then why the atomic size of gallium is less than aluminum ? 

Because , in case of gallium due to presence of d-orbitals , size of gallium  decreases to greater extend . D orbitals have poor shielding effect . It means that they cannot shield the nuclear force on outer most electrons. Due to decrease in shielding effect and increase in nuclear force , size of Gallium becomes  smaller than Aluminum .

But remember that , this case only for Gallium , remaining 4th period elements follow the same trend , that is atomic size increase down the group .

Effects of decrease in size of Gallium 

●Increase in ionization enthalpy 
Due to decrease in size of Gallium , it's Ionization enthalpy increases . Smaller the size , more will be nuclear force on valance electrons . Therefore more energy requires to remove electron from the outermost shell . That why Ionization enthalpy of Gallium becomes more than Aluminum.

●Increase in Electron gain enthalpy
Electron gain enthalpy of Gallium is more than Aluminum . Due to small size of Gallium and larger nuclear force , incoming electron experience more and more attraction towards this atom . Therefore it becomes easy for Gallium atom to accept electron . 
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