If size of Aluminium and gallium is nearly same, then why not of silicon and germanium?
Please clear my doubt!

Dear Student

In case of aluminium valence electrons go in the 3 shell, 3s2 3p​1 but in gallium valence electrons go in 4s2 4p1 before which 10 electrons go in 3d-orbitals and due to poor shielding effect of d-orbitals, there is strong nuclear charge effect on valence elctrons of gallium resulting in small size of Ga than Al.

Now, when valence electron configuration in germanium is ​4s2 4p2 , thus the second electron goes in p-orbital again and there is electron-electron repulsion which outweighs the poor shielding of d-orbitals and thus, there is increase in the radii of the germanium from silicon.

Regards
 

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