A cluster-model study of Cl and F chemisorbed at a fourfold site of Cu(100) shows that the halogen ionicity is essential -1. The interaction and bonding arise, almost entirely, from the Coulomb attraction between the charged halogen and metal and from the polarization of the electrons associated with these units; covalent bonding makes a very small contribution. The polarization dramatically reduces the dipole moment from that given by the unpolarized ionic units. Thus, changes in the work function are not a measure of the adsorbate ionicity. © 1986 The American Physical Society.