# Compound

A substance made of two or more different elements joined together by chemical bonds. It can only be split up by a chemical reaction.

Chemistry (school) · Part 01: Atoms and matter · Card 3 of 10 · Last updated 2026-10-19

## Analogy

A compound is like a cake. You cannot pick the flour and eggs back out once it is baked.

## Example

**Reading a formula**

H2O: 2 hydrogen, 1 oxygen
CO2: 1 carbon, 2 oxygen
NaCl: 1 sodium, 1 chlorine

## In practice

Water is a compound of hydrogen and oxygen. Its formula is H2O.

## Practice

### Flashcards

- **What is the difference between a compound and a mixture?** A compound has its elements chemically joined. A mixture is not chemically joined and can be separated physically.

### Interview questions

**Describe the difference between an element, a compound and a mixture, with one example of each.**

- Element has one type of atom
- Compound has different atoms chemically bonded
- Mixture is not chemically bonded
- Mixtures can be separated physically

## Questions people ask

### Why does a compound look different from the elements in it?

When elements bond, their atoms rearrange and share or swap electrons. That creates a new substance with new properties. Sodium is a soft, explosive metal and chlorine is a poisonous gas, yet together they form table salt, which is safe to eat.

### How do you write the formula for a compound?

A formula uses symbols and small numbers to show how many atoms of each element are in one unit. In H2O the 2 sits after H and means two hydrogen atoms. No number means one atom. So CO2 has one carbon and two oxygen atoms.

## Related concepts

- [Mixture](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-4)
- [Element](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-1/chemistry-school-part-1-concept-2)

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