Atomic Structure: Electron Configurations

The lowest-energy arrangement, or ground-state electron configuration, of an atom is a listing 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 order 1s→2s → 2p → 3s → 3p → 4s → 3d, a statement called the Aufbau principle. Note that the 4s orbital lies between the 3p and 3d orbitals.

Rule 2

   Electrons act in some ways as if they were spinning around an axis, somewhat like how the earth spins. This spin can have two orientations, denoted as up (↑) and down (↓). Only two electrons can occupy an orbital, and they must be of opposite spin, 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 1s2 2s2 2px 1 2py 1, and so forth. Note that a superscript is used to represent the number of electrons in a particular orbital.

Problem 1 - 1

Give the ground-state electron configuration for each of the following elements:

(a) Oxygen (b) Nitrogen (c) Sulfur

Problem 1 - 2

How many electrons does each of the following elements have in its outermost electron shell?

(a) Magnesium (b) Cobalt (c) Selenium