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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, 1s→2s→2p→3s→3p→4s→3d1s→2s→2p→3s→3p→4s→3d, 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 1s²2s²2p_x¹2p_y¹, and so forth. Note that a superscript is used to represent the number of electrons in a particular orbital.

Table 1.1 Ground-State Electron Configurations of Some Elements

Element	Atomic number	Configuration
Hydrogen	1
Carbon	6
Phosphorus	15