th electrons conditions which are analogous to those we met in the binding of the
th,
th and
th electrons. In the elements of the third periods, however, we possess a far more detailed knowledge of the series spectra. Too little is known about the beryllium spectrum to draw conclusions about the binding of the fourth electron, but we may infer directly from the well-known arc spectrum of magnesium that the
th electron in the atom of this element is bound in a