1.4: Atomic Structure - Electron Configurations
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The lowest-energy arrangement, or ground-state electron configuration, of an atom is a list of the orbitals occupied by its electrons. We can predict this arrangement by following three rules.
RULE 1
The lowest-energy orbitals fill up first, , according to the following graphic, a statement called the Aufbau principle. Note that the 4s orbital lies between the 3p and 3d orbitals in energy.
RULE 2
Electrons act in some ways as if they were spinning around an axis, somewhat as the earth spins. This spin can have two orientations, denoted as up (↑) and down (↓). Only two electrons can occupy an orbital, and they must have opposite spins, a statement called the Pauli exclusion principle.
RULE 3
If two or more empty orbitals of equal energy are available, one electron occupies each with spins parallel until all orbitals are half-full, a statement called Hund’s rule.
Some examples of how these rules apply are shown in Table 1.1. Hydrogen, for instance, has only one electron, which must occupy the lowest-energy orbital. Thus, hydrogen has a 1s ground-state configuration. Carbon has six electrons and the ground-state configuration 1s22s22px12py1, and so forth. Note that a superscript is used to represent the number of electrons in a particular orbital.
| Element | Atomic number | Configuration |
|---|---|---|
| Hydrogen | 1 | |
| Carbon | 6 | |
| Phosphorus | 15 |