5 Steps to a 5 AP Chemistry 2019

Basics ‹ 51

Be sure to fill the lowest energy levels first (Aufbau principle) when using the diagram in
Figure 5.1. In filling orbitals having equal energy, electrons are added to the orbitals to half fill
them all before any pairing occurs (Hund’s rule). Sometimes it is difficult to remember the
relative energy position of the orbitals. Notice that the 4s fills before the 3d. Figure 5.2 may
help you remember the pattern in filling. Study the pattern and be able to reproduce it during
the exam.

1 s

2 s

3 s

4 s

5 s

6 s

7 s

2 p

3 p

4 p

5 p

6 p

7 p

3 d

4 d

5 d

4 f

5 f

6 d

Figure 5.2 Orbital filling pattern.

5p

4p

3p

2p

4d

3d

5s

4s

3s

2s

1s

↑

↑↓

↑↓

↑↓

↑↓

↑↓ ↑↓

↑

Figure 5.3 Energy-level diagram for silicon.

Following these rules, the energy-level diagram for silicon (Z = 14) can be written as shown
in Figure 5.3.