Forming compounds
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
Examiner 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.
