Atoms, Elements and Compounds — AQA GCSE Chemistry
This topic introduces the basic particles of chemistry and how they combine.
Atoms, elements and compounds
- All substances are made of atoms — the smallest part of an element that can exist.
- An element is a substance made of only one type of atom. There are about 100 elements, shown in the periodic table.
- A compound is two or more elements chemically combined in fixed proportions. Compounds can only be separated into elements by chemical reactions.
Chemical symbols and formulae
Each element has a chemical symbol (e.g. O for oxygen, Na for sodium). A formula shows the elements and the number of each atom in a compound, e.g. H₂O (2 hydrogen, 1 oxygen), CO₂, NaCl.
Chemical equations
Reactions are shown by equations:
- Word equation: magnesium + oxygen → magnesium oxide.
- Symbol equation: 2Mg + O₂ → 2MgO.
Equations must be balanced — the same number of each type of atom on both sides — because atoms are never created or destroyed (conservation of mass).
Mixtures
A mixture is two or more substances not chemically combined. The substances keep their own properties and can be separated by physical methods.
Exam tips
- Define element (one type of atom) and compound (elements chemically combined).
- A mixture is not chemically joined and is separated physically.
- Always balance symbol equations.
- Be able to count atoms from a formula (e.g. Ca(OH)₂ has 1 Ca, 2 O, 2 H).