Electronic Structure

The electronic structure of an atom describes the arrangement of electrons in its energy levels, which orbit the nucleus. In a neutral atom, the number of electrons is equal to the number of protons. So, the atomic number also tells us the number of electrons in an atom.

  • You can find the number of protons by looking at the atomic number of the element, which represents the number of protons in the nucleus.

Electron shells

Electrons occupy specific energy levels, also known as electron shells. Each energy level has a maximum number of electrons it can hold.

 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.

  • The first energy level can hold up to two electrons
  • The second energy level can hold up to eight electrons
  • The third energy level can hold up to eight electrons

Electrons occupy energy levels in a specific order. They occupy the lowest available energy level first, which is the level closest to the nucleus. When the lowest energy level is full, the electrons will then occupy the next energy level, and so on.

Stability of atoms

For an atom to be considered stable, it must have a full outer energy level. If the outer energy level is not full, then the atom will be unstable and will likely react with another atom to gain or lose electrons until it becomes stable. For example:

  • Neon has eight electrons in its outer energy level, so it has a full outer energy level – Stable
  • Chlorine has one electron in its outer energy level, so it needs to gain one electron – Unstable
  • Sodium has one electron in its outer energy level, so it needs to lose one electron – Unstable
A diagram showing the outer shell of neon which is stable, the outer shell of chlorine which is unstable and the outer shell of sodium which is also unstable.

Many single atoms, like sodium, are unstable and tend to react to form molecules or compounds. However, there are exceptions, such as the group 0 elements, which have a full outer shell. The group 0 elements can be found in the far right column of the periodic table. They are stable, so they don’t react with anything.

Predicting the Electronic Structure of an Element

You could be asked to work out the electronic structure of the first 20 elements. Let’s work out the electronic structure of chlorine.

1. First, find the element on the periodic table.

2. Determine the atomic number of the element, which tells you how many electrons the element has.

  • The atomic number of chlorine is 17, meaning it has 17 electrons

3. Fill up the electron shells with electrons.

To fill up the electron shells, you can start with the first energy level and move to the next one once it is full. Here’s the process for filling the electron shells of chlorine:

  • The first energy level can hold up to 2 electrons, so the first two electrons occupy this level – 15 electrons left
  • The second energy level can hold up to 8 electrons, so the next eight electrons occupy this level – 7 electrons left
  • The third energy level can hold up to 8 electrons, so the remaining seven electrons occupy this level

With this process, you can predict the electronic structure of any element. Chlorine has 17 electrons arranged in the following manner: 2 electrons in the first shell 8 electrons in the second shell 7 electrons in the third shell

So, we get:

A diagram of a chlorine atom with the first second and third energy labels highlighted. the first level contains 2 electrons, the second contains 8 and the third contains 7.

We can also write the electronic structure using numbers to represent the number of electrons in each energy level. For example, the electronic structure of chlorine can be written as 2,8,7. Each comma separates one energy level from the next.

The electronic structure of an atom can also tell us its position in the periodic table. The number of electrons in the outer energy level of an atom tells us the group number, while the number of energy levels tells us the period number. For example:

  • Chlorine has seven electrons in its outer energy level, so it is in group 7
  • Chlorine has electrons in three energy levels, so it is in period 3.

You’ve used 0 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up