The Atoms Be-Ne Hunds Rule

The next atom in the periodic table, beryllium, has four electrons and a nuclear charge +4e as one might expect, its ground state can be represented in the orbital approximation as (Is) (2s). Likewise the next atom, boron, is represented by (ls) (2s) (2p). Note that there is no distinction between the three 2p orbitals in heavier atoms since these are identical in shape, differing only in orientation (Fig. 1.6), they remain degenerate. 

An atom in which all the electrons are paired has of course no net magnetic moment due to electron spin, because the magnetic moments of an a-spin electron and a jS-spin electron are equal and opposite. In atoms where there are unpaired electrons, this need not be the case if the numbers of a- and jS-spin electrons differ by n, the atom will have a net spin magnetic moment of n times that of a electron. The magnitude of the unpaired spin in a given state of an atom is denoted by its multiplicity, the multiplicity of a state being n + 1. States with no unpaired electrons (e.g.. He, Be), and so a multiplicity of one, are called singlet states states with a multiplicity of two always have an odd number of electrons (e.g., H, Li) and are called doublet states states with a multiplicity of three (two unpaired electrons with parallel 

TABLE 1.1. Ground States of Atoms H Ne in the Orbital Representation 

We are now in a position to deduce the ground states of the atoms H-Ne these are shown in Table 1.1. Arrows pointing up or down indicate electrons of a or spin, and the horizontal bars represent the AOs. 

---