Elements, compounds and mixtures

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.

Worked examples

Example 1

Each circle is an atom; different colours are different elements. Which box shows an element, which a compound and which a mixture?

  1. Left: every particle is the same single atom. One type of atom: an element.
  2. Middle: every particle is the same, and each has two different atoms joined. That is a compound.
  3. Right: there are two different substances (single blue atoms and red molecules), not joined to each other: a mixture.

Answer: left: element; middle: compound; right: mixture

Example 2

What is the formula of water, and how many atoms are in one molecule?

  1. A water molecule has one oxygen atom joined to two hydrogen atoms.
  2. Write each symbol with the number of its atoms after it (no 1s): H2O.
  3. Add them up. 2 + 1 = 3

Answer: H2O; 3 atoms

Example 3

Iron filings and sulfur powder are mixed, then heated. How do the mixture and the product differ?

  1. In the mixture the iron and sulfur keep their own properties: a magnet pulls the iron out.
  2. Heating makes them react. The word equation is: iron + sulfur → iron sulfide
  3. Iron sulfide is a compound with new properties: it is black and not magnetic, and it can only be split up by another chemical reaction.

Answer: the mixture can be separated with a magnet; the product, iron sulfide, is a new compound that is not magnetic

Example 4

Name the compound that contains calcium, carbon and oxygen, and the compound that contains only magnesium and chlorine.

  1. The metal comes first and keeps its name.
  2. Carbon with oxygen in the same compound makes -ate: carbonate. So calcium carbonate.
  3. Just two elements makes -ide: chlorine becomes chloride. So magnesium chloride.

Answer: calcium carbonate; magnesium chloride