Elements and compounds — Science, 11–13 years
Materials can be described by the kinds of atoms they contain and how those atoms are joined.
What are elements and compounds?
An element is made from only one kind of atom, such as iron or oxygen. A compound contains atoms of different elements joined in a fixed pattern, such as water, made from hydrogen and oxygen. A compound has its own properties, not simply a mixture of its parts.
Why classify substances this way?
Chemists needed a clear way to describe thousands of substances and predict how they might behave. Calling something an element, compound or mixture tells us whether its particles are one kind, chemically joined kinds, or simply together. This makes recipes, tests and safety instructions much less confusing.
Reading water’s formula
Water has the formula H₂O. First, read H as hydrogen and O as oxygen. The small ₂ means that each water particle has two hydrogen atoms; no number after O means one oxygen atom. So water is a compound with a fixed 2:1 atom ratio, not a random mixture.
A compound is not a mixture
It is easy to think that a compound is just a mixture because it contains different elements. That seems reasonable: salt contains sodium and chlorine, after all. But in a compound the atoms are chemically joined, so the new substance has different properties and cannot usually be separated by simple sieving or picking.
Where does this help?
Food labels, medicines and cleaning products name substances so people can judge what they contain and how they should be handled. Engineers also choose compounds for particular jobs: glass for transparent screens, salts for batteries, and alloys for strong tools. The particle description helps explain why each material behaves as it does.
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