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How to Write an Electron Configuration

electron configuration periodic table

Concepts

In this tutorial, you will learn how to find and write the electron configuration and orbital diagram for various elements using the periodic table. You will learn Aufbau’s principle, Hund’s rule and the Pauli exclusion principle. If you enjoy this tutorial, be sure to check out our others linked below!

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The Electron Configuration

The electron configuration is a description of where electrons are in a molecule or atom. Electrons occupy orbitals that have characteristic levels of energy. Systems with a greater number of electrons will occupy a greater amount of energy levels, meaning that they also will utilize higher energy levels. Electron configurations are represented by standard written notation, or by using orbital diagrams.

Writing Electron Configurations

Electron configurations have a standard notation that tells you the principle energy levels and sublevels that electrons occupy. Here is the electron configuration for Helium: 

1s2

The first integer, 1, gives us the principle energy level, the letter s represents the type of orbital (sublevel), and the superscript 2 gives us the electron occupancy. In this case, there are two electrons in an s orbital with the principle energy level of one. 

Systems with a greater number of electrons will occupy a greater amount of energy levels. The electron configuration for Lithium is:

 1s2 2s1 

Lithium, containing three electrons, has two electrons occupying an s orbital at the first energy level, and one electron occupying an s orbital at the second energy level.

The periodic table is a helpful tool in writing these configurations. The principle energy level is indicated by an integer (1, 2, 3, …7) that corresponds with the periods on the periodic table. Each successive integer generally represents a higher energy level than the last. Sublevels are indicated by letters s, p, d, and f. Groups or blocks of the periodic table share the same sublevel, and are divided as seen in the following diagram.

writing electron configurations

Writing Electron Configurations – Examples

To find the electron configuration of an element, start at hydrogen and trace across each period until your target element is reached. At each preceding element, pay attention to the energy level and block it represents. Let’s practice in this section below.

Lithium

One again we will use the example of Lithium. We start with our attention on hydrogen (1s1), move to Helium (1s2), and then to Lithium (2s1). The electron configuration for Lithium is therefore 1s2 2s1.

A Note: Lithium’s configuration is written using only “1s2” and not “1s11s2” because between Hydrogen and Helium, the energy level and orbital do not change. Only the electron occupancy changes, which we denote by changing the superscript from 1 to 2. 

 Carbon

Follow the periodic table starting at hydrogen just as the example for Lithium. Carbon is located in the second period and in the p-block, so it’s highest energy electrons will occupy the 2p orbital. The electron shell configuration is 1s22s22p2. We can also reaffirm this answer by noticing that carbon is number six on the periodic table, and therefore has six electrons. If we count the electrons in each orbital for Carbon’s configuration, we get 2+2+2= 6!

Phosphorus

Now, let’s find phosphorus on the periodic table. It is located in the third period and within the p-block. Moreover, it has fifteen electrons. The electron shell configuration for phosphorous would be 1s22s22p63s23p3.

Writing Electron Configurations – Shorthand Method

You can still write out every single subshell if you would like, but to save time it is good to know the shorthand method of electron configurations. The shorthand method uses the group 18 elements, the noble gases, as a bookmark.

Bromine

Bromine is located in period four of the p-block. Remember, for electron configurations you work left to right and down the periods until you get to the element you’re focusing on. The last noble gas that was passed for bromine was argon (Ar). Using the short-hand method, we will place argon in brackets like this [Ar] and then continue the electron configuration after argon. It would look like this: [Ar] 4s23d104p5.

Chlorine

Chlorine is located in period three of the p-block. The last noble gas that was passed for chlorine was neon (Ne). Using the short-hand method, we will place neon in brackets then continue the electron configuration. It would look like this: [Ne] 3s2 3p5.

What is an orbital diagram?

Another way to represent an electron configuration is through an orbital diagram. In an orbital diagram, orbitals are represented as boxes and electrons are represented by arrows (↑ or ↓), with two electrons occupying each orbital/box. Orbitals are labeled according to their principle energy levels and sublevels (1s, 2p, etc..). Helium, with two electrons in the 1s orbital has the following orbital diagram. 

orbital diagrams

To successfully draw an orbital diagram, you must be aware of a few principles that dictate how these orbitals are filled. 

Aufbau’s Principle

Aufbau is German for “building up,” so this rule dictates how orbitals are filled based on their energy states. The principle states that the lower-energy electron orbitals will fill up before higher-energy orbitals. So, the 1s orbital will fill before the 2s orbital, and 2s orbital will fill before 2p orbital, and so on.  However, the 4s shell will be filled before the 3d shell because the 4s shell is lower energy than the 3d shell.

In the example below, configuration A shows a fully occupied 1s orbital, and a half occupied 2s orbital. Configuration B shows a half occupied 1s orbital, and a fully occupied 2s orbital. Based on Aufbau’s Principle, which do you think is the electron configuration of Lithium?

aufbau principle electron configurations

Since the 1s orbital is of lower energy than the 2s orbital, the 1s orbital should be filled first, and any remaining electron should be used to fill the 2s orbital, making configuration A the correct orbital diagram for Lithium.

Next, how do we fill orbitals if they are of the same energy?

Hund’s Rule

Hund’s rule dictates how orbitals of the same energy should be filled. One electron is given to each of these orbitals before two electrons can occupy the same orbital. Single electrons will also have the same spin (indicated by the direction of arrows in the orbital diagrams).

hund's rule

In the correct configuration, one electron fills each orbital, and each electron has the same spin. The first incorrect configuration shows not all orbitals were half-filled before adding two electrons to an orbital. And in the second correct example, not all single electrons have the same spin. 

How should three 2p orbitals for Oxygen be occupied?

Option A is the correct configuration, because all orbitals were singly occupied before two electrons occupied the orbital, and all single electrons have the same spin. 

Pauli Exclusion Principle

The Pauli exclusion principle states that no two electrons in an atom or molecule can have the same four quantum numbers. For our purposes, this means that two electrons occupying the same orbitals cannot have the same spin. One must be spin up (↑) and one must be spin down (↓). 

Which shows the correct orbital diagram for a 1s orbital containing 2 electrons?

pauli exclusion principle

Configuration A is correct, because the electrons have opposite spins, as indicated by the direction of the arrows.

Note: It is also convention to draw the first arrow pointing up.

Electron Configuration – Further Reading

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