24 II. METHODS OF INVESTIGATION 



in the case of manganic ion there are actually three 3f/ orbitals avail- 

 able, the optimum strength is attained by bonds of a d'-sp^ type. 



Since there can be no resonance between structures involving dif- 

 ferent numbers of unpaired electrons, there is a sharp discontinuity 

 between the fully ionic and fully covalent structures represented in 

 Table II. On the other hand, although hybridization of d, s, and p 

 orbitals usually occurs, it is possible to assume the formation of up 

 to four weak covalent bonds involving only the s and p orbitals, 

 leaving the d orbitals unaffected. In this case the number of unpaired 

 electrons is characteristic of ionic bonds, although some covalent 

 bonding has also occurred. Consequently bond types determined on 

 the basis of the number of unpaired electrons are usually spoken of 

 as "essentially ionic" or "essentially covalent" to allow for such a 

 possibility. 



6.2. Magnetic Properties of Molecules 



When placed in a magnetic field, all substances exhibit a magnetic 

 polarization due to the accelerating effect of the field on the elec- 

 trons of the substance, the polarization induced being opposed to the 

 applied field. Substances in which this type of polarization occurs 

 are repelled by a magnet and are said to be diamagnetic. Little 

 information about molecular structure can be gained from diamag- 

 netism. Since diamagnetism contributes to the resultant magnetic 

 susceptibility of all substances, a correction for it must be applied 

 when paramagnetic susceptibility alone is required to be measured. 



Paramagnetic polarization, which is in the same direction as the 

 external field, results from the presence in the substance of atoms, 

 ions, or molecules with permanent magnetic dipole moments. These 

 are due partly to the spin magnetic moments of unpaired electrons, 

 and in part to the magnetic moments due to orbital motion of elec- 

 trons. The latter factor is usually negligible in the substances with 

 which this book is concerned. We shall, therefore, in the following 

 assume that the paramagnetic susceptibility is due entirely to the 

 spin of the unpaired electrons, with the proviso that in some cases 

 a correction may have to be applied. 



The molar magnetic susceptibility (xmot) of a system containing only 

 one type of dipole, is given by: 



V 4- ^'"^ 



