Untitled - Kelly Walsh High School

112 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
electrons has all the electrons with the lowest possible energy. This lowest
energy arrangement is the ground state. Less stable (higher energy) arrangements
are the excited states. An atom may have any number of excited state
arrangements, but there is only one ground state.
When following the Aufbau principle, the orbitals begin filling at the lowest
energy and continue to fill until we account for all the electrons in an atom.
Filling begins with the n 1 level followed by the n 2 level, and then the n
3 level. However, there are exceptions in this sequence. In addition, Hund’s rule
states that the sublevels within a particular orbital will half fill before the electrons
pair up in a sublevel.
The exceptions begin with the fourth energy level. The fourth energy level
begins to fill before all the sublevels in the third shell are complete. More complications
in the sequence appear as the value of the principle quantum number
increases. The sequence of orbital filling, with complications, is: 1s, 2s, 2p, 3s,
3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and so on.
Figure 7-1 illustrates the Aufbau principle diagrammatically. The orbitals begin
filling from the bottom of the diagram (lowest energy) with two electrons maximum
per individual sublevel (line on the diagram).
The sublevels of a particular orbital half fill before electrons pair up in the sublevel.
___ ___ ___ ___ ___ ___
4d
___ ___ ___
4p
___ ___ ___ ___ ___ ___
4s 3d
___
3s
___
2s
___ ___ ___
3p
___ ___ ___
2p
Figure 7-1 The atomic
orbitals in order of increasing
energy