Barrons SAT Subject Test Chemistry, 13th Edition

Figure 9. Approximate Relative Energy Level of Subshells

19 K

1 s^2 2 s^2 2 p^6 3 s^2 3 p^6 4 s^1

20 Ca

1 s^2 2 s^2 2 p^6 3 s^2 3 p^6 4 s^2

21 Sc

1 s^2 2 s^2 2 p^6 3 s^2 3 p^6 4 s^2 3 d^1 (note 4s filled before 3d)

There is a more stable configuration to a half-filled or filled sublevel, so at
atomic number 24 the 3d sublevel becomes half-filled by taking a 4s electron;

24 Cr

1 s^2 2 s^2 2 p^6 3 s^2 3 p^6 3 d^5 4 s^1

and at atomic number 29 the 3d becomes filled by taking a 4s electron:

29 Cu

1 s^2 2 s^2 2 p^6 3 s^2 3 p^6 3 d^10 4 s^1

Table 3 shows the electron configurations of the elements. A triangular mark
indicates an outer-level electron dropping back to a lower unfilled orbital. These
phenomena are exceptions to the Aufbau Principle. By following the atomic
numbers throughout this chart, you will get the same order of filling as shown in
Figure 9.