The heat of formation, Delta E, for silicon (111) surfaces terminated with increasing densities of the alkyl groups CH3-(methyl), C2H5-(ethyl), (CH3)(2)CH-(isopropyl), (CH3)(3)C-(tert-butyl), CH3(CH2)(5)-(hexyl), CH3(CH2)(7)-(octyl), and C6H5-(phenyl) was calculated using quantum mechanics (QM) methods, with unalkylated sites being H-terminated. The free energy, Delta G, for the formation of both Si-C and Si-H bonds from Si-Cl model componds was also calculated using QM, with four separate Si-H formation mechanisms proposed, to give overall Delta G(s) values for the formation of alkylated Si(111) surfaces through a two step chlorination/ alkylation method. The data are in good agreement with measurements of the packing densities for alkylated surfaces formed through this technique, for Si-H free energies of formation, Delta G(H), corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a C=C double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent.