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Lessons / Atoms and the periodic table / GCSE and A-level

Where are the electrons in an atom?

The GCSE picture: electrons in shells, and how they explain the periodic table.

  1. A copper wire1 mm
  2. One copper crystal10 µm
  3. A row of atoms1 nm
  4. One atom0.3 nm
Cu: 1s2 2s2 2p6 3s2 3p6 3d10 4s1
What the simulation shows, in words

Choose an element with the slider or the buttons. The drawing shows its nucleus in the middle and its electrons on rings around it, in pairs, with the count for each shell written beside the ring. The readout gives the shell numbers, for example 2, 8, 1 for sodium, and the full configuration.

Electrons live in shells

Electrons are arranged around the nucleus in energy levels, usually drawn as shells. The shell closest to the nucleus has the lowest energy and fills first.

For the first 20 elements, which is what GCSE uses:

  • the first shell holds up to 2 electrons;
  • the second holds up to 8;
  • the third takes 8, and then the fourth starts to fill.

Sodium has 11 electrons: 2 in the first shell, 8 in the second and 1 in the third. Its electron arrangement is 2,8,1. Calcium, with 20, is 2,8,8,2.

The table in disguise

Try a few elements in the simulation and the periodic table starts to explain itself.

  • The group number is the number of outer electrons (for groups 1 to 7). Lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) all have one, which is why they all react the same way, by losing it.
  • The period number is the number of shells. Everything in period 3, sodium to argon, has three.
  • The noble gases have full outer shells: helium 2, neon 2,8, argon 2,8,8. That is why they hardly react.

Chemistry is mostly about outer electrons. When atoms react they lose, gain or share them, so elements with the same number of outer electrons behave alike.

Why “shells” is not the whole story

Look at potassium. If the third shell could hold 18 electrons, potassium should be 2,8,9. It isn’t: the 19th electron goes into the fourth shell. To understand why, you need the more detailed picture on the Deeper page, where each shell splits into subshells with different energies. The shell picture is still the right answer at GCSE.

Safety

No experiment goes with this page. Elements such as sodium and potassium react violently with water and are handled only by teachers.