Atoms, elements and compounds
The three words students use as if they mean the same thing — and don't. Sorting out atom, element, and compound is the foundation the whole of chemistry is built on.
Everything is made of atoms — tiny building blocks far too small to see. But "atom," "element," and "compound" are not interchangeable words, and the entire subject of chemistry depends on telling them apart. Here's the distinction, made concrete.
The three ideas
- Atom — the smallest particle of a substance. A single building block.
- Element — a substance made of only one type of atom. Oxygen, iron, gold, carbon. There are about of them, listed in the periodic table.
- Compound — two or more different elements chemically joined together. Water () is hydrogen joined to oxygen; it behaves like neither.
A useful mental image: if elements are the letters of an alphabet, compounds are the words you build by joining them. The same few elements make millions of different compounds.
Compound vs mixture — the key difference
This is where marks are won and lost. In a compound, the elements are chemically bonded and you'd need a chemical reaction to separate them. In a mixture, substances are just jumbled together and can be separated physically (filtering, evaporating).
| Joined how? | Separated how? | Example | |
|---|---|---|---|
| Compound | Chemically bonded | Chemical reaction | Water, salt (NaCl) |
| Mixture | Just mixed | Physical means | Air, sand in water |
Salt water is a mixture — boil it and the water leaves, the salt stays. But the salt itself is a compound of sodium and chlorine, which only a reaction could split.
Why a compound is genuinely new
When elements form a compound, the result has its own properties, nothing like the ingredients. Sodium is a metal that explodes in water; chlorine is a poisonous green gas. Join them and you get sodium chloride — ordinary, edible table salt. The bond changes everything.
"Made of one kind of atom" = element. "Different atoms chemically joined" = compound. "Just mixed, easily separated" = mixture. Run any substance through those three tests and you'll classify it correctly every time.
Last revised 9 January 2025.