The electrons in the outermost shell are the valence electronsthe electrons on an atom that can be gained or lost in a chemical reaction. Since filled d or f subshells are seldom disturbed in a chemical reaction, we can define valence electrons as follows: The electrons on an atom that are not present in the previous rare gas, ignoring filled d or f subshells.

Gallium has the following electron configuration.

Ga: [Ar] 4s² 3d¹⁰ 4p¹

The 4s and 4p electrons can be lost in a chemical reaction, but not the electrons in the filled 3d subshell. Gallium therefore has three valence electrons.

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