# Elements, compounds and mixtures

### Element

• A pure substance that is listed in the and only has one type of atom in it.
• There are over 100 elements.
• Most are metals, a few are metalloids (also known as semi-metals), and the rest are non-metals.

### Compound

• A pure substance made from more than one type of element chemically bonded together.
• Elements bond in fixed ratios and so can be represented by a chemical formula. For example, sodium chloride has the same number of sodium ions and chloride ions, so its formula is NaCl, whereas water is always made from twice the number of hydrogen atoms as oxygen atoms, so it is H2O.

### Mixture

• An impure substance made from different elements or compounds mixed together that are not chemically joined.
• Mixtures can usually be separated by physical techniques such as and .
• Air is a mixture that contains the elements nitrogen, oxygen and argon, and also the compound carbon dioxide.