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Getting started: atoms, elements, compounds and formulae
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Chemistry has its own language of symbols and formulae. At KS3 you learned the basics; at GCSE you will read and write formulae in almost every lesson.
Atoms and elements
An atom is the smallest part of an element that can exist.
An element contains only one type of atom. There are over 100 elements, listed in the periodic table.
Each element has a symbol of one or two letters: the first is a capital, the second is lower case (Co is cobalt; CO is carbon monoxide).
Some symbols come from old names: Na sodium, K potassium, Fe iron, Cu copper, Ag silver, Au gold, Pb lead.
An element contains only one type of atom. There are over 100 elements, listed in the periodic table.
Each element has a symbol of one or two letters: the first is a capital, the second is lower case (Co is cobalt; CO is carbon monoxide).
Some symbols come from old names: Na sodium, K potassium, Fe iron, Cu copper, Ag silver, Au gold, Pb lead.
Compounds and formulae
A compound contains two or more elements chemically joined. Its properties are different from those of its elements.
A formula shows which atoms are joined and how many. A small number after a symbol counts that atom only.
H2O: 2 hydrogen atoms, 1 oxygen atom.
CO2: 1 carbon atom, 2 oxygen atoms.
A number after a bracket multiplies everything inside: Ca(OH)2 has 1 Ca, 2 O and 2 H.
In a mixture the substances are not chemically joined, so they can be separated by physical methods.
A formula shows which atoms are joined and how many. A small number after a symbol counts that atom only.
H2O: 2 hydrogen atoms, 1 oxygen atom.
CO2: 1 carbon atom, 2 oxygen atoms.
A number after a bracket multiplies everything inside: Ca(OH)2 has 1 Ca, 2 O and 2 H.
In a mixture the substances are not chemically joined, so they can be separated by physical methods.
Naming compounds
Metal + non-metal: the non-metal ending changes to -ide: sodium + chlorine → sodium chloride.
An -ate ending means oxygen is also present: calcium carbonate (CaCO3) contains calcium, carbon and oxygen; copper sulfate (CuSO4) contains copper, sulfur and oxygen.
An -ate ending means oxygen is also present: calcium carbonate (CaCO3) contains calcium, carbon and oxygen; copper sulfate (CuSO4) contains copper, sulfur and oxygen.
How many atoms are shown in the formula Ca(OH)2?
- Ca: 1 atom
- (OH)2: 2 oxygen + 2 hydrogen = 4 atoms
- Total = 1 + 4
Answer: 5 atoms
Element: one type of atom. Compound: elements chemically joined. Mixture: not joined. In a formula, a small number counts the atom before it; a number after a bracket multiplies everything inside.
The interactive lesson includes the diagrams for this topic.
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