Elements, compounds and mixtures · Year 7
Everything is made of atoms. A substance made of only one type of atom is an element; there are about 100 of them, each with its own symbol. When atoms of different elements join together chemically they make a compound, with a fixed formula and new properties. Substances that are together but not joined make a mixture, which can be separated without a chemical reaction.
Key facts
- An atom is the smallest part of an element that can exist. A molecule is two or more atoms joined together.
- An element contains only one type of atom. Each element has a symbol of one or two letters: the first is a capital, the second is small (C, Ca, Cl).
- Some symbols come from old Latin names: Na (sodium), K (potassium), Fe (iron), Cu (copper), Ag (silver), Au (gold), Pb (lead).
- A compound is two or more elements chemically joined in fixed proportions. Its properties are different from those of its elements.
- A formula shows the atoms in a compound: H2O has 2 hydrogen atoms and 1 oxygen atom. A symbol with no number counts as 1.
- A compound of two elements ends in -ide (sodium chloride). A name ending in -ate means oxygen is there too (copper sulfate).
- A mixture contains substances that are not chemically joined. Each keeps its own properties, and the mixture can be separated by physical methods.