Electron Shells
Electrons occupy shells that hold 2, 8, 8 … electrons.
About this topic
Electrons occupy energy levels called shells around the nucleus. The first shell holds a maximum of 2 electrons, the second 8, and the third 8 (for the first 20 elements). Electrons fill the lowest shell first, and the outermost shell determines chemical behaviour.
Shells are numbered 1, 2, 3 and so on outward, and the further a shell is from the nucleus, the higher its energy. The maximum number of electrons in shell n is 2n squared, which is 2, 8, 18 and 32, though for simple school diagrams the third shell is treated as holding 8 before the fourth begins. Electron arrangements are written as numbers: sodium is 2,8,1 and chlorine is 2,8,7.
Take magnesium, atomic number 12. Two electrons go in the first shell, eight in the second, leaving two in the third, so its arrangement is 2,8,2 and it sits in period 3 and group 2. The period number equals the number of shells used, and for main-group elements the group tells the number of outer electrons.
Electron shell diagrams explain reactivity, since atoms react to complete their outer shells. A misconception is that shells are physical rings; they are energy levels, and electrons are really found in clouds. Another is that a shell must be full before any electron enters the next, which fails for heavier elements.
Key ideas
- Fill lower shells first
- Outer shell is the valence shell
- Valence electrons set chemistry
Quick check
Use cases: where Electron Shells helps
- Homework: review the key idea behind Electron Shells before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Electron Shells without notes.
- Visual learning: play with the interactive lab to see how Electron Shells changes when you adjust its inputs.
- Teaching a friend: explain Electron Shells in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Electron Shells?
Electrons occupy shells that hold 2, 8, 8 … electrons.
What will I learn in Electron Shells?
Fill lower shells first. Outer shell is the valence shell. Valence electrons set chemistry.
Is Electron Shells part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Electron Shells lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Electron Shells?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.