Anions

This acquires stability by gaining one or more electrons into the outermost shell to make it stable. They have an outermost electron of 4, 5, 6, 7. Atoms of such number are unstable; they gain electrons to their outermost shell to make them stable.

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NB: Chlorine has gained an electron to its outermost shell to make it stable. Chlorine (Cl) has been represented as Chlorine ion (Cl-). The charge on the chlorine (Cl) shows it has gained an electron to its outermost shell. Gaining or losing of electrons can only occur at the outermost shell.