Online Introductory Chemistry

How to predict electron configurations


Dr. Walt Volland revised July 5, 2010

Predicting electron configurations and summary of order of filling of subshells: method 1

There is no simple rule that applies to all of the elements.

There are at least three methods to predict electron configurations that work most of the time. Memorizing the sequence is attractive to some people. I prefer to use a method based on the periodic table.

The subshells in atoms "generally" follow this order for filling:

Roll mouse over one of the labels below to see the Electron configuration and energy level populations

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d

It is helpful to remember the limits on each type subshell. See the diagram below for the maximum populations each subshell can hold. The interaction between electrons and the nucleus changes when the atomic number changes and count for electrons changes. An alternate method for predicting electron subshells filling order is shown with the diagram below.


Predicting electron configurations diagram: method 2

This triangle can be built and used to predict the electron configuration for most any atom.

Each successive row has an n value larger by one unit. You should note that there is one more subshell added when the "n" value increases by one. The top of the diagram matches the top of the periodic table.

Every column has the same subshell type.

Population limits for subshells

The maximum population for each subshell type is listed at the bottom of the diagram.
Each of the s subshells can only hold 2 electrons. Each of the p subshells can only hold 6 electrons. Each of the d sublevels can hold 10 electrons.
NOTE the pattern of an increase of 4 additional electrons for each succeeding subshell.

Electron configurations using the diagram method EXAMPLE

This triangle can be built and used to predict the electron configuration for most any atom. Here the electron configuration for Li is figured out.

The electron configuration for lithium uses only the 1s and 2s subshells. The third electron in lithium has to go into the second level because the 1s is completely filled after two electrons occupy the level.

You try doing the electron configuration for boron, B, with five electrons.Click here to see the answer

What is the electron configuration for magnesium atomic number 12? Click here to see answer.