The goal of an element is to obtain a noble gas configuration (obtaining a full valence shell). Therefore, certain elements will either gain or lose electrons in order to obtain this full valence shell. In the case of sodium chloride, it is easy for sodium (having one valence electron) to give up its one electron. Chlorine (having seven valence electrons) wants sodium’s electron because it completes its outer shell and chlorine obtains this noble gas configuration. Ionic bonds always involve a transfer of electrons.
