# Period

A horizontal row of the periodic table. All elements in a period have the same number of electron shells.

Chemistry (school) · Part 02: The periodic table · Card 2 of 9 · Last updated 2026-10-19

## Analogy

A period is like a floor in a building. Every room on one floor is at the same height.

## In practice

Sodium, magnesium and chlorine are all in period 3, so each has three shells.

## Practice

### Flashcards

- **What does the period number tell you?** The number of electron shells the atom has.

## Questions people ask

### Why does the periodic table have rows of different lengths?

Each period begins when a new shell starts to fill. The first shell holds only 2 electrons, so period 1 has only two elements. The next shells hold more, so periods 2 and 3 have eight each. Later periods include the transition metals and are longer.

### What changes as you move across a period?

The number of protons and outer electrons rises by one each step, so atoms go from metals on the left to non-metals on the right. The number of shells stays the same. Reactivity changes, and the last element in each period is a noble gas.

## Related concepts

- [Electron shell](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-2/chemistry-school-part-2-concept-1)
- [Group](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-2/chemistry-school-part-2-concept-3)

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