AQA GCSE Chemistry

Revision Notes

1.1.2 Compounds

Forming Compounds

  • 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

Ammonia-Displayed-Formula, downloadable IGCSE & GCSE Chemistry revision notes

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

Exam Tip

A common error is to say elements are pure while compounds are impure. Pure substances contain only one element or compound and are not mixed with anything else e.g., water that contains H2O molecules only is pure whereas if salt is added it then becomes impure.

Naming Compounds

  • 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)
    • C6H12O6 is glucose
    • C2H5OH is ethanol
    • HNO3 is nitric acid

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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