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Compounds

Radiation and the atom · level 3, lesson 4 of 6

A question to start

Hydrogen is a gas that burns. Oxygen is a gas that helps things burn. Join them, and you get water, which puts fires out. How can that be?

The idea

When atoms of different elements join together, they make a compound. A compound is a new substance, often nothing like the elements that made it.

A group of atoms joined together is a molecule. Its formula says which atoms it has, and how many:

Compound Formula Made of
Water H₂O 2 hydrogen atoms, 1 oxygen atom
Carbon dioxide CO₂ 1 carbon atom, 2 oxygen atoms
Table salt NaCl sodium and chlorine, one of each
Rust Fe₂O₃ iron and oxygen, 2 to 3

Unlike a mixture, a compound cannot be pulled apart with a sieve or a magnet. It takes a chemical reaction.

OHHCOO water, H₂Ocarbon dioxide, CO₂

Worked examples

1. Counting atoms. CO₂ has 1 carbon and 2 oxygen atoms: 3 atoms in each molecule.

2. Salt. Sodium is a soft metal that fizzes in water; chlorine is a poisonous gas. Joined, they make table salt.

3. Rust. Iron joins with oxygen from the air, helped by water, to make rust: a new red-brown compound.

Try it

  1. What is a compound?
  2. How many atoms are in one molecule of water?
  3. What elements are in carbon dioxide?
  4. Can a magnet pull the iron out of rust?
Answers
  1. A substance made of atoms of different elements joined together.
  2. Three: two hydrogen and one oxygen.
  3. Carbon and oxygen.
  4. Not in the way it pulls iron filings: the iron is joined to oxygen in a new compound.

Common mistake

"A compound is a kind of mixture." In a mixture, the parts are just jumbled. In a compound, the atoms are joined, and the new substance behaves differently.

Where it leads