"Extra electrons" refers to electrons that are present in an atom or object beyond what is typically needed or expected. These extra electrons can affect the behavior or properties of the atom or object they belong to. Full definition
The amount of oxygen inside your cells is in direct correlation with the number of extra electrons your cells possess. (oceansalive.com)
It still had its own extra electrons, however, making it a base. (newscientist.com)
In a recent paradigm - changing advance, Los Alamos researchers demonstrated that by adding extra electrons into their specially designed quantum dots, they can reduce this threshold to virtually zero. (sciencedaily.com)