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Atoms, Elements and Compounds » Electronic Structure

What you'll learn this session

Study time: 30 minutes

AQA spec: 4.1.1.7

How electrons are arranged in energy levels (shells) around the nucleus
Why electrons fill the lowest available energy level first
How to write an electronic structure as numbers, such as 2,8,1
How to draw electronic structure diagrams for the first 20 elements

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Electrons live in energy levels

That orange glow comes from sodium, electron structure 2,8,1 - its electrons jump between energy levels and give out light

Remember, an atom has a tiny nucleus in the middle, with electrons around it. The electrons are not scattered at random. They sit in energy levels, which are also called shells.

You can picture the shells as rings around the nucleus. The first shell is closest to the nucleus. The second shell is a bit further out, then the third, and so on.

The shell closest to the nucleus is the lowest energy level. Each shell further out is a higher energy level.

Here is the key rule from the spec: the electrons in an atom occupy the lowest available energy levels. In other words, electrons go into the innermost shell that still has space. Only when a shell is full do electrons start to go into the next one.

In exams you can talk about either energy levels or shells. Both words get the marks.

Key terms:

Energy level (shell): a region around the nucleus where electrons are found. The innermost shell is the lowest energy level.
Electronic structure: the way the electrons in an atom are arranged in its energy levels, shown as numbers or as a diagram.

How many electrons fit in each shell?

Each shell can only hold a certain number of electrons. For the first 20 elements, which is all you need at GCSE, use these numbers:

① First shell

Holds up to 2 electrons. It always fills first.

② Second shell

Holds up to 8 electrons. It fills once the first shell is full.

③ Third shell

Takes up to 8 electrons before any go into the fourth shell.

So the pattern to learn is 2, 8, 8. Any electrons left over after that go into the fourth shell. In the first 20 elements, only potassium and calcium have electrons in the fourth shell.

Writing electronic structures as numbers

Sodium's yellow-orange flame comes from electrons in its 2,8,1 arrangement - the lone outer electron is in the third shell

The spec's example is sodium. Its electronic structure is 2,8,1. This means:

2 electrons in the lowest energy level (first shell)
8 electrons in the second energy level
1 electron in the third energy level

To work out any electronic structure, follow these steps:

Find the atomic number on the periodic table. In an atom, the number of electrons equals the number of protons, so this is the number of electrons.
Put up to 2 electrons in the first shell.
Put up to 8 in the second shell.
Put up to 8 in the third shell.
Put any that are left in the fourth shell.
Check: the numbers must add up to the atomic number.

Worked example 1: aluminium

Aluminium has atomic number 13, so it has 13 electrons.
First shell: 2 (11 left).
Second shell: 8 (3 left).
Third shell: 3 (0 left).
Electronic structure: 2,8,3. Check: 2 + 8 + 3 = 13.

Worked example 2: potassium

Potassium has atomic number 19, so it has 19 electrons.
First shell: 2 (17 left).
Second shell: 8 (9 left).
Third shell: 8 (1 left).
Fourth shell: 1 (0 left).
Electronic structure: 2,8,8,1. Check: 2 + 8 + 8 + 1 = 19.

The first 20 elements

You should be able to write the electronic structure of any of the first 20 elements. Here they all are. Try covering the right-hand side and working them out yourself.

⚛ Elements 1 to 10

H (1): 1
He (2): 2
Li (3): 2,1
Be (4): 2,2
B (5): 2,3
C (6): 2,4
N (7): 2,5
O (8): 2,6
F (9): 2,7
Ne (10): 2,8

⚛ Elements 11 to 20

Na (11): 2,8,1
Mg (12): 2,8,2
Al (13): 2,8,3
Si (14): 2,8,4
P (15): 2,8,5
S (16): 2,8,6
Cl (17): 2,8,7
Ar (18): 2,8,8
K (19): 2,8,8,1
Ca (20): 2,8,8,2

Notice that hydrogen has just one electron, so its structure is simply 1. You do not write a comma or a zero.

Drawing electronic structure diagrams

The spec says you must also show electronic structure as a diagram. This is a flat (2D) drawing of the atom, seen as if from above. Real atoms are 3D, but a 2D diagram is much easier to draw and read.

To draw one:

Draw a small dot or circle in the middle for the nucleus. You can write the chemical symbol in it.
Draw one circle around the nucleus for each shell that has electrons in it.
Show each electron as a small cross (x) or dot on its circle.
Put the right number of electrons on each circle, matching the numbers you worked out.
Space the electrons out neatly. Many people draw them in pairs, but any clear spacing is fine.

◯ Sodium, 2,8,1

Three circles around the nucleus. 2 crosses on the inner circle, 8 crosses on the middle circle and 1 cross on the outer circle.

◯ Oxygen, 2,6

Two circles around the nucleus. 2 crosses on the inner circle and 6 crosses on the outer circle.

A diagram and a set of numbers show exactly the same information. The numbers are quicker to write. The diagram makes it easy to see how many electrons are in the outer shell.

Common mistakes

Putting 8 in the first shell. The first shell only holds 2. Carbon is 2,4, not 6 (all in the first shell).
Putting the 19th electron in the third shell. For potassium and calcium, the third shell takes 8 and the rest go into the fourth shell. Potassium is 2,8,8,1, not 2,8,9.
Using the mass number. The number of electrons in an atom equals the atomic number, not the mass number. Argon-40 has 18 electrons, not 40.
Not checking the total. Always add up your numbers. They must equal the atomic number.
Drawing the wrong number of circles. Draw one circle per shell that has electrons. Do not draw empty shells.

Exam-style question

Magnesium has the atomic number 12.

(a) Give the electronic structure of a magnesium atom. [1 mark]

(b) Describe what a diagram of the electronic structure of a magnesium atom would look like. [2 marks]

Model answer

(a) 2,8,2
(b) A nucleus in the middle with three circles (shells) around it (1). 2 electrons on the inner shell, 8 on the second shell and 2 on the outer shell (1).
(c) Electrons occupy the lowest available energy level (1). The first shell is the lowest energy level and it can hold 2 electrons, so it fills first (1).

Exam tip

Learn the pattern 2, 8, 8 and always finish by adding up your numbers. If the total is not the atomic number, something has gone wrong.

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