# Electron shell

A level around the nucleus that can hold a fixed number of electrons. The first shell holds 2, the second holds 8 and the third holds 8 (for the first 20 elements).

Chemistry (school) · Part 02: The periodic table · Card 1 of 9 · Last updated 2026-10-19

## Analogy

Shells are like rows of seats around a stage. The front row fills first.

## Example

**Filling shells**

Carbon (6): 2, 4
Oxygen (8): 2, 6
Sodium (11): 2, 8, 1
Chlorine (17): 2, 8, 7

## In practice

Sodium has 11 electrons, so its shells fill as 2, 8, 1.

## Practice

### Flashcards

- **How many electrons fit in the first and second shells?** 2 in the first shell and 8 in the second.
- **What is the electron arrangement of chlorine (atomic number 17)?** 2, 8, 7.
- **What does the period number tell you?** The number of electron shells the atom has.

## Questions people ask

### Why does the second shell hold only 8 electrons?

Each shell has a fixed number of spaces, set by how electrons are arranged in energy levels. The first has room for 2 and the second for 8. At school level you simply learn the rule: fill the first shell, then the second, then the third, working outwards.

### How do you write the electron arrangement of an atom?

Find the atomic number, which is the number of electrons in a neutral atom. Put 2 in the first shell, up to 8 in the second, and so on. For phosphorus, number 15, you write 2, 8, 5. The last number shows the group.

## Related concepts

- [Period](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-2/chemistry-school-part-2-concept-2)

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