MAGNETISM AND THE STRUCTURE OF MATTER From the investigation of the magnetic properties of or dinary matter, solid, liquid or gaseous, conclusions can be drawn as to the magnetic properties of the constituent atoms and molecules. It becomes of interest to consider the relation between the magnetic properties of atoms and molecules and their struc ture, and also how the bulk magnetic characteristics depend on the way in which the elementary particles are associated. Accord ing to the conception of atomic structure which correlates many of the observed facts, atoms consist of a small massive positively charged nucleus round which circulate negatively charged elec trons. In the neutral atom the negative electrons balance the positive charge. The nuclear charge (measured in units numeri cally equal to the charge of the electron) varies from element to element from I for hydrogen to 92 for uranium, being equal to the atomic number (see ATOM). On the Bohr theory of atomic structure, the electrons circulate round the nucleus in closed orbits, the possible orbits being restricted by quantum conditions. The orbits may be classified primarily by a total quantum number n which, to a first approximation, fixes the energy of the orbit. Orbits with the same n may be divided into sub-groups according to the value of the azimuthal quantum number k, which takes values from I up to n and which is related to the angular momen tum. In the neutral atom of sodium, for example, which has an atomic number of 11, the II electrons are divided into groups in the following way:— The number of electrons in a given nk group cannot exceed a cer tain maximum value, equal to 2(2k — 1), and a group which has this maximum number is said to be closed or completed ; the con figuration corresponding to the group is then symmetrical, and the group has no resultant angular momentum or magnetic mo ment. It has been mentioned that the electron itself must be supposed to have an intrinsic spin and magnetic moment. In a closed group the intrinsic magnetic moments of the electrons balance in pairs, and the resultant magnetic moment due to the orbital motion of the electrons also vanishes.
In the neon atom, which contains one less electron than sodium, all the groups are completed. The atom possesses no magnetic moment, and is diamagnetic. All the inert gases are diamag netic for a similar reason. Ions which possess a completed elec tronic configuration (such as Na+, see fig. 26) are also diamagnetic, so that the great majority of polar salts and their solutions are diamagnetic.
Salts and solutions are paramagnetic when there are ions pres ent in which some of the electron groups are incomplete. It is
one of the triumphs of the Bohr theory of the system of the elements that it accounts in a natural and simple manner for the peculiar properties, including the ionic paramagnetism, of the various transition groups of elements in the periodic table. Those ions of elements between potassium and copper, for example, which contain from 19 to 27 electrons (see fig. 3o), are para magnetic, for in them the group of electrons n=3, k = 3, which is being built up, is incomplete and contains less than the maxi mum number, ten, of electrons. The rare earth ions, in which the n=4, k =4 group is incomplete are similarly paramagnetic (fig. 31). The magnetic properties of complex co-ordination corn pounds can also be related to the electronic configuration in a fairly satisfactory manner depending mainly on the number of electrons associated with the central atom.
When atoms unite to form single molecules, the total number of electrons is even usually, and the resultant electronic configuration is such that the molecule as a whole has no magnetic moment and so is diamagnetic. Organic compounds, for example, are prac tically all diamagnetic, and from the value of the molecular susceptibility an estimate of the "size" of the molecule can be made ; also, from the Jusceptibility constant of a particular group of atoms, the size of the electronic configuration associated with the group can be deduced. A nitrogen molecule (N2 with electrons) is diamagnetic, but nitric oxide (NO with 15 electrons) is paramagnetic, owing to the magnetic moment of the "odd" electron. Oxygen, with two more electrons than nitrogen, is also paramagnetic, the two electrons, in this particular case, not balanc ing each other. Just as the magnetic properties of atoms can be predicted from an analysis of the atomic line spectra, so can the properties of molecules from a study of the molecular band spectra, though this has not yet been carried so far. It does not seem possible to extend the astronomical conception of the atom, with electrons rotating about a nucleus, to the molecule, with two or more nuclei. Indeed, helpful though the atomic model has been, it cannot be regarded as more than a conceptual thought model. Later theoretical developments do not lead readily to so definite a conceptual atomic model, but roughly, on the basis of the Schrodinger wave mechanics (see QUANTUM THEORY), the electronic configuration of the atom may be regarded rather as a continuous distribution of electric charge, than as a system of rotating point charges ; and such a conception can easily be extended to molecules.