# 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:

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:

## Using the Periodic Table

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

### 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:

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:

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