Atoms, elements and compounds — Chemistry, 14–17 years
Matter is built from atoms. Understanding the difference between elements, molecules and compounds gives chemistry a map for describing substances.
The tiny building blocks
All ordinary matter is made from atoms, far too small to see directly. An element contains one kind of atom, such as oxygen or iron; a compound contains different elements joined together, such as water, H₂O.
Why chemists sort matter
Chemists needed a way to predict whether an unknown substance was like a known one and whether a reaction had made something new. Naming elements and writing formulas makes invisible ingredients countable, so experiments can be compared instead of described only by appearance.
Reading H₂O
Start with H₂O. The capital H and O show two elements: hydrogen and oxygen. The small 2 applies only to H, so each molecule has two hydrogen atoms and one oxygen atom. That is why H₂O is a compound, not an element.
The formula is not a label
A common mistake is to think the small number applies to every symbol in a formula. It is reasonable because the symbols are printed together, but H₂O means two H atoms and one O atom, not two of each. Read each subscript next to its own symbol.
Labels in real life
Ingredient lists use chemical names and formulas to identify substances in medicines, fertilisers and cleaning products. A formula helps a manufacturer use the intended substance, but it does not by itself tell you whether that substance is safe at every dose.
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