Review of Electron Configurations

Brief Instructions:

An electron configuration is a method of indicating the arrangement of electrons around a nucleus. A typical electron configuration consists of numbers, letters, and superscripts with the following format:

1. The coefficient is the first number that indicates the energy level.

2. The lowercase letter indicates the type of orbital (s, p, d, or f).

3. A superscript (the number to the right of each orbital) indicates the number of electrons in the orbital.

Example: 1s² means two electrons are in the ‘s’ orbital of the first energy level. The element is helium (He).

To write a longhand electron configuration:

1. Begin on the upper-left-most position of the periodic table (1st energy level, s orbital) and move across and down the table.

2. Use the Aufbau principle to fill in electrons in each orbital. The Aufbau process requires that electrons fill the lowest energy orbitals first. In other words, atoms are built from the ground upwards.

3. The sum of the superscripts should equal the total number of electrons.

Example: Mg 1s² 2s² 2p⁶ 3s²

Noble Gas Electron Configuration

A "shorthand" version of electron configurations. It uses the group 18 (noble gases) elements as "place markers" followed by the last energy level for the atom in question.

Noble Gases - group 18 elements (the far-right-most column) - He, Ne, Ar, Kr, Xe, and Rn.

Brief Instructions:

1. Locate the atom on the periodic table.

2. Identify the prior noble gas (the one that is just prior the atom in question; up one row and to the far-right).

3. Record that noble gas in brackets - example [He].

4. Record the electron configuration in the atom's last energy level or the row that it is in on the periodic table (just after the noble gas).

Example - Iron (Fe)

• Longhand Configuration for Iron (Fe): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

• Noble Gas Configuration for Iron (Fe): [Ar] 4s² 3d⁶