A sodium and a chlorine atom sit side by side. The sodium atom has one valence electron, and the chlorine atom has seven. Six of chlorines electrons form pairs at the top, bottom and right sides of the valence shell. The seventh electron sits alone on the left side. The sodium atom transfers its valence electron to chlorines valence shell, where it pairs with the unpaired left electron. An arrow indicates a reaction takes place. After the reaction takes place, the sodium becomes a cation with a charge of plus one and an empty valence shell, while the chlorine becomes an anion with a charge of minus one and a full valence shell containing eight electrons.