Atomic Radii

Atomic Radii

Going down any group, an extra electron shell is added at each step, so atomic radius is expected to grow steadily. Group 13 mostly follows this rule — except at gallium.

Gallium's atomic radius (135 pm) is actually smaller than aluminium's (143 pm), even though gallium sits one period below aluminium and has an extra electron shell. The reason is exactly the core-shielding issue described above: gallium carries 10 additional d-electrons in its inner core that aluminium does not have. These d-electrons shield the increasing nuclear charge very poorly, so gallium's valence electrons experience an unusually high effective nuclear charge, which pulls the outer shell inward and shrinks the atom below the size a naive down-the-group trend would predict.

| Element | B | Al | Ga | In | Tl | …