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GCSE Combined Science · key term
A formula showing the actual number of atoms of each element in one molecule.
Used by AQA and Edexcel
Specification 5.2.1.4, 5.7.1.1 · Paper 3, Paper 4
Revise: Covalent Bonding and StructuresCounting gives two carbon atoms and six hydrogen atoms, so its molecular formula is C₂H₆. The three-dimensional position of an atom does not change the count: a hydrogen behind another atom still belongs to the molecule.
A formula showing the number of atoms of each element in one molecule.
Specification Calculations involving masses · Paper 3, Paper 4
Revise: Determining empirical and molecular formulaeA molecular formula gives the actual number of atoms of each element in one molecule. Hydrogen peroxide has the molecular formula H₂O₂: each molecule contains two hydrogen atoms and two oxygen atoms. Its empirical formula is HO, because 2:2 simplifies to 1:1.
To deduce a molecular formula, count the actual atoms in one molecule. Do not simplify their ratio: C₂H₆ is not CH₃.
When finding a molecular formula, multiply every empirical-formula subscript by the same whole-number factor, including unwritten subscripts of 1.
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