The Dipole Moment and Nature of the Chemisorption Bond

Experimental results are given for the change of work function of three sp metals, copper, silver, gold, and of three transition metals, iron, cobalt, nickel when hydrogen and carbon monoxide are separately chemisorbed at — 192 °C on these metals in the form of an evaporated film, using the space‐charge limited diode method. With the sp metals a predominantly electrostatic bond is formed, the dipole moment of which is largely controlled by the first ionization potential of the adsorbate and that of the atom of the adsorbent. This bonding is additional to the normal dispersion forces which usually are the most significant contribution to the forces operative in the ordinary van der Waals or physical adsorption. With the transition metals, definite covalent bonding involving empty d‐atomic orbitals takes place ‐ these are hybrid bonds, the character of which changes with increasing coverage of the adsorbent surface; these conclusions are consistent with those drawn from infra‐red spectrometric studies of other authors. The contribution of the dipole component to the heat of adsorption and to the variation of this heat with coverage is not large for the transition metals but is a substantial one with the sp metals.