Elements and compounds — Chemistry, 11–13 years
An element contains one kind of atom, while a compound joins different atoms in a fixed pattern. Knowing the difference helps you read formulas and predict what a substance is made of.
Different building blocks
Atoms are the tiny building blocks of matter. An element, such as iron or oxygen, has only one kind of atom. A compound, such as water, has atoms of different elements joined together, so it is a new substance with its own properties.
Why separate them?
People once gave substances names without knowing what was inside them. Scientists needed a careful way to describe materials, compare experiments and make the same substance again. The element-and-compound idea does this: it links visible properties to invisible combinations of atoms.
Reading H₂O
Look at H₂O. The small 2 means there are two hydrogen atoms, while the O without a number means one oxygen atom. So one water molecule contains three atoms altogether, in a fixed 2:1 ratio. Changing that ratio would make a different substance, not simply ‘more water’.
Water is not hydrogen plus oxygen
It seems reasonable to think water should behave like the gases hydrogen and oxygen that make it. But joining atoms changes how they share electrons and therefore changes the substance’s properties. Water puts out many fires, while hydrogen burns and oxygen helps fires burn.
Materials with a plan
Engineers choose substances by their properties and what their particles are made to do. Carbon can form soft graphite in a pencil or hard diamond, while steel combines iron with other elements to make useful tools and bridges. The arrangement matters, not just the ingredient list.
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