Physical Chemistry , 1st ed.

and 1, so that the possible subshells are 2sand 2p. Because of the slightly
higher energy of the 2psubshell, the electrons occupy the 2ssubshell, which
can hold two electrons if they have different spin functions. The occupation of
orbitals in an atom is listed as an electron configuration,using superscripts to
indicate the number of electrons in each individual subshell. It is assumed that
for ground states, the spins of the electrons are appropriate and satisfy the
Pauli exclusion principle. The electron configuration for Be is written as

1 s^22 s^2

This is an obvious electron configuration, since the 2psubshell is higher in
energy than the 2s, as shown in Figure 12.4. However, as we will see shortly, it
is not always so straightforward to assign an electron configuration.

Example 12.6

Electron configurations are rather abbreviated when compared to the more

complete Slater-determinant form of the antisymmetric wavefunction.

Compare the electron configuration of Be with the Slater-determinant form

offor Be.

Solution

The electron configuration for Be, given above, is simply 1s^22 s^2. Using the rule

from above for constructing Slater determinants, the more complete is



1 s 1 1 s 1 2 s 1 2s 1



^1 s^21 s^22 s^2 2s^2



1

24 

 1 s

3 1 s 3 2 s 3 2s 3

^1 s 4 1 s 4 2 s 4 2s 4 

12.5 Other Atoms and the Aufbau Principle 383

H- like Not H- like

3 s, p, d

2 s, p

1 s

2 s

2 p

3 s

Energy

3 p

4 p

4 s, p, d, f 3 d

1 s

4 s

5 s

Figure 12.4 The effect of more than one electron on the electronic energy levels of an atom.
For hydrogen-like atoms, all of the energy levels with the same principal quantum number nare
degenerate. For atoms having more than one electron, the shells are separated by the quantum
number as well. (Energy axis is not to scale.)