Compounds (AQA GCSE Chemistry: Combined Science)

What is a compound?

• Elements take part in chemical reactions in which new substances are made in processes that most often involve an energy change
• In these reactions atoms combine together in fixed ratios that will give them full outer shells of electrons, often producing compounds
• A compound is a pure substance made up of two or more elements chemically combined and which cannot be separated by physical means
• The properties of compounds are usually quite different from the elements that form them
• There is an unlimited number of compounds, and the chemical formula is what tells you the ratio of atoms in a compound
• For example, H₂O is a compound containing 2 hydrogen atoms and 1 oxygen atom
• The chemical formula can be deduced from the relative number of atoms present
• For example, a molecule containing 3 atoms of hydrogen and 1 atom of nitrogen is NH₃
• Diagrams or models can also be used to represent the chemical formula

A molecule of ammonia

The ammonia molecule consists of a central nitrogen atom bonded to 3 hydrogen atoms

How to name a compound

• Naming compounds depends on what type of elements make up a compound

Metals and non-metals

• Ionic compounds contain metal and non-metal elements joined together as particles called ions
• The metal element’s symbol is always written first
• The non-metal element always takes on the name ending ‘– ide’ unless oxygen is also present,
  • For example, PbS is called lead sulfide and MgCl₂ is called magnesium chloride

• When oxygen is present the name ending is usually ‘-ate’
  • For example, CuSO₄ is copper sulphate, KClO₃ is potassium chlorate and Na₂CO₃ is sodium carbonate

• Some formula names are similar so be careful with spelling
  • For example, NaNO₃ is sodium nitrate and NaNO₂ is sodium nitrite

• The ending ‘-ite’ will always have less oxygen than ‘-ate’
• The number of oxygen atoms varies, so you cannot tell how many oxygen atoms are present from the name ending

Non-metal only compounds

• Covalent compounds contain only non-metals
• They are named using prefixes (mono, di, tri, tetra, penta) to show how many of each element is present in the formula
  • For example, CO₂ is carbon dioxide, NO is nitrogen monoxide and SiCl₄ is silicon tetrachloride

• A few covalent compounds are known by their common names, and you are expected to learn them, for example:
  • H₂SO₄ is sulfuric acid
  • NH₃ is ammonia
  • CH₄ is methane
  • HCl is hydrochloric acid (or hydrogen chloride if it is a gas)
  • C₂H₅OH is ethanol
  • HNO₃ is nitric acid