Predicting Electron Configurations

The electronic configuration describes how electrons are arranged within an atom. Electrons occupy different electron ‘shells’, and the total capacity of these shells correlates with the element’s row on the periodic table.

Identifying Electronic Configurations

To identify the electronic configuration of an atom of an element:

1. Determine the total number of electrons the atom has. The number of electrons equals the number of protons. Therefore, the atomic number found on the periodic table will tell us how many electrons an atom has.

2. Identify the electron capacity for each shell (how many electrons it can hold). Electrons fill shells from the innermost to the outermost. Here’s the capacity for the first three shells:

First ShellSecond ShellThird Shell
Electron capacity2 electrons8 electrons8 electrons
 diagram of an atom with the first, second, and third "energy levels" labelled. The first level has 2 electrons, the second has 8, and the third has 8.

You can draw the shells and distribute the electrons according to the rules above to help visualise the atom.

3. Allocate the electrons across the shells, starting with the innermost shell:

  • Number of electrons occupying the first shell – Let’s call this x
  • Number of electrons occupying the second shell – Let’s call this y
  • Number of electrons occupying the third shell – Let’s call this z

4. Write the electronic configuration like this: x.y.z. If one or two shells are empty, don’t write the final two or three digits as zero – just don’t write them at all.

Example: Beryllium’s Electronic Configuration

If we want to find the electronic configuration of Beryllium:

Step 1. Determine the number of electronsAtomic number = 4, so there are 4 electrons.
Step 2. Identify the number of electrons in each shell.First shell = 2 electrons
Second shell = 2 electrons
Step 3. Make a drawing to visualise this. This image has an empty alt attribute; its file name is image-5.png
Step 4. Write the electronic configuration.2.2.

Using the Periodic Table

You can also use the information contained in the periodic table to find this configuration for the first 20 elements:

Periodic table informationInformation for electronic configuration
Period number (row)Indicates the number of shells.
Group number (column)Indicates the number of electrons in the outermost shell. (Except for helium.)
Atomic numberEquals the total number of electrons.
A detailed periodic table of elements showing the organisation of all known elements. The table is categorised by colour-coded groups, including alkali metals, alkaline earth metals, transition metals, metalloids, non-metals, halogens, noble gases, lanthanoids and actinoids. Each element is represented by its atomic number, symbol, name and relative atomic mass. The table is also marked with vertical columns indicating groups 1 through 8 and horizontal rows indicating periods, from 1 to 7, and horizontal groups indicating groups, from 1 to 8.. The lanthanoids and actinoids are displayed separately at the bottom. Arrows and annotations explain the information provided for each element, such as atomic mass and chemical symbol, using Hydrogen as an example.

Example: Aluminium (Al)

Find the element aluminium (Al) in the periodic table.

  • It is in Period 3 (the third row down), so it must have three electron shells.
  • It is in Group 3 (the column labelled 3), so it has three electrons in its outermost shell.
  • Its atomic number is 13, so it must have thirteen electrons in total.

Therefore, aluminium’s electronic configuration is 2.8.3, adding up to 13 electrons (2+8+3=13).

Reverse Engineering Electronic Configurations

We can also use electronic configurations to identify elements. For example, if we are given the electronic configuration: 2.8.2., we can do the following:

Periodic Table InformationInformation for Electronic Configuration
Period number = Number of shells3 – We know this because the electronic configuration has 3 digits.
Group number = Number of electrons in the outermost shell2 – We know this because the electronic configuration is written from the innermost layer to the outermost layer and the last number is 2
Atomic number = Total number of electrons in all shells2+8+2=12

Because the atomic number is 12, we know that the electronic configuration is 2.8.2., which corresponds to Magnesium (Mg).

The electrons of a magnesium atom can be drawn like this:

An atomic model of a magnesium (Mg) atom. In the centre, there's a grey circle labelled "Mg," representing the nucleus. Surrounding the nucleus are three concentric circles, which represent the electron shells of the atom. Two electrons are depicted on the innermost shell, eight electrons on the second shell, and two electrons on the outermost shell, all represented by black dots with a minus sign, indicating their negative charge.

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