The chloride ion has 18 electrons in total. This includes two in its innermost shell, eight in its second shell and eight more in its third valence shell. This is one more than the 17 found in a neutral chlorine atom, which has only seven electrons in its outermost valence shell.
The chloride ion is by far the most common form of chlorine encountered in nature, much more so than neutral chlorine gas. This is because of chlorine's immense reactivity. Chlorine gas, which is composed of two chlorine atoms, is tremendously toxic and corrosive. This is because of chlorine's ability to attract electrons, or electronegativity, which allows it to steal electrons from most other elements. Only nitrogen, oxygen and fluorine exceed chlorine's electronegativity.
The reason nitrogen and oxygen gases are not corrosive, despite their greater electronegativity, is the nature of the bonds within their gas molecules. Like chlorine, nitrogen and oxygen gases are composed of molecules made from two atoms of their respective elements. However, while chlorine molecules have a single bond, sharing only two electrons, oxygen molecules have a double bond, sharing four electrons, and nitrogen has a triple bond, sharing six. The more bonds that connect two atoms, the harder they are to break.To react with other materials, that bond must be completely broken, so oxygen and nitrogen gases are generally less reactive than chlorine, despite equal or greater electronegativity.