# Group

A vertical column of the periodic table. Elements in a group have the same number of outer electrons, so they react in similar ways.

Chemistry (school) · Part 02: The periodic table · Card 3 of 9 · Last updated 2026-10-19

## Analogy

A group is like a family. Members share features because they share the same outer electrons.

## Example

**Group number rule**

Group 1: 1 outer electron
Group 2: 2 outer electrons
Group 7: 7 outer electrons
Group 0: full outer shell

## In practice

Lithium, sodium and potassium are all in group 1. Each has one outer electron.

## Practice

### Flashcards

- **What do elements in the same group have in common?** The same number of outer electrons, so similar reactions.

### Interview questions

**Explain why the elements in group 1 have similar chemical properties and why reactivity increases down the group.**

- Same number of outer electrons (one)
- Each loses one electron to react
- Lower in the group the outer electron is further from the nucleus
- Less attraction so electron is lost more easily

## Questions people ask

### How can you tell the group of an element from its electrons?

For main groups 1 to 7, the group number equals the number of outer electrons. Group 0, the noble gases, is the exception because those atoms have a full outer shell, meaning 8 electrons (2 for helium). So sodium with 2, 8, 1 is in group 1.

### Why did Mendeleev leave gaps in his periodic table?

Dmitri Mendeleev arranged elements by atomic mass and by chemical behaviour. Where an element did not fit a pattern, he left a gap and predicted an undiscovered element would fill it. Later discoveries, such as gallium and germanium, matched his predictions, which won support for his table.

## Related concepts

- [Alkali metals](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-2/chemistry-school-part-2-concept-6)
- [Period](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-2/chemistry-school-part-2-concept-2)

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