It has been generally assumed that in the transition elements (Fe, Co, Ni, Cu, etc.) the $3d$ shell is filled with ten electrons or is nearly filled, and that the $d$ electrons make no significant contribution to the cohesive forces in metals. Evidence is presented here to show that about half of the $d$ orbitals (2.56 of the total of 5) are involved in bond formation, through hybridization with the $4s$ and $4p$ orbitals, and that the number of covalent bonds resonating among the available interatomic positions increases from one to nearly six in the sequence K, Ca, Sc, Ti, V, Cr, remains nearly constant from Cr to Ni, and begins to decrease with Cu. The remaining 2.44 $d$ orbitals, with very small interatomic overlapping, are occupied by nonbonding electrons which are mainly responsible for the ferromagnetic and paramagnetic properties of the metals. This point of view provides a qualitative explanation of many properties of the transition metals (including those of the palladium and platinum groups), such as interatomic distance, characteristic, temperature, hardness, compressibility, and coefficient of thermal expansion, and it accounts satisfactorily for the observed values of the atomic saturation magnetic moments of the ferromagnetic elements iron, cobalt, and nickel and their alloys. It is also shown to provide a reason for the occurrence of the positive exchange integrals which give rise to ferromagnetism.

• Received 10 October 1938

DOI:https://doi.org/10.1103/PhysRev.54.899