The lowest whole number ratio of ions in an ionic compound. Formula units take into account that the overall charge must be neutral. In table salt, or Sodium Chloride, these combine in a 1:1 relationship so their Chemical Formula is NaCl (one Sodium with one Chlorine). For Aluminum Bromide, the Aluminum has a charge of 3+ and the Bromine has a charge of 1-. Therefore, in order to keep the balance at 0, you would need three Bromines with one Aluminum or AlBr3.