Sodium is in group 1 so it has 1 valence electron (one electron in its outer shell). Sodium will be looking to lose its one valence electron in order to become more stable. Chlorine is in group 17 so it has 7 valence electrons, and therefore only needs to gain one valence electron to attain noble gas electron configuration (become stable with 8 valence electrons, just like the noble gases in group 18 have 8). Because the chlorine atom is trying to gain one electron, and the sodium atom is trying to lose one, sodium will give up its one valence electron to chlorine and the two atoms will form an ionic bond. Because chlorine is looking to gain just one electron and sodium is looking to lose the same number, the ratio of chlorine atoms to sodium atoms will be 1:1, one chlorine atom per one sodium atom.
