The growth and the (electronic) structure of a single layer of aluminum on the Si(100) surface are examined by means of first principles calculations. The calculated local density of states is used to interprete scanning tunneling microscopy data. We identify the adsorption sites that give rise to the polymerization-like one-dimensional growth of Al. The Al-induced surface states of the p (2 x 2) covered Si(100) surface are calculated. The surface bandgap is increased, as compared to the clean Si(100) surface, which means that the first adsorbed Al layer passivates the surface electrically.