The gaining of electron by an atom results in the formation of anion shown by the negative charge on the atom whereas lose of electron results in the formation of cation shown by positive charge on the atom. The atom lose or gain electron to complete their octet and get stable in nature.
The chlorine atom will gain an electron and form chloride anion with one negative charge on it. The chloride ion is more stable in nature compared to the chlorine atom due to complete octet of chloride ion by gaining of electron.
Electronic configuration of chlorine atom is:
By gaining of one electron, electronic configuration of chloride ion is:
Thus, the equation that shows the formation of the chloride ion from a neutral chlorine atom is: