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GCSE Combined Science · key term
A group of atoms held together by covalent bonds.
Used by AQA and Edexcel
Specification 5.1.1.1, 5.2.1.4, 5.2.2.4 · Paper 3
A group of atoms chemically bonded together.
Revise: Atoms, Elements, Compounds and Chemical FormulaeHCl: one hydrogen atom and one chlorine atom per molecule.
A discrete group of atoms joined together by covalent bonds.
Specification Covalent bonding · Paper 3, Paper 4
Revise: Covalent bondingCarbon, oxygen and chlorine have full outer shells with eight electrons in the molecules considered here.
Hydrogen needs two electrons in its full outer shell; the other atoms in these eight molecules need eight. Count shared electrons for both bonded atoms.
To deduce a molecular formula, count the actual atoms in one molecule. Do not simplify their ratio: C₂H₆ is not CH₃.
A similar-looking formula does not guarantee a similar structure: CO₂ consists of small molecules, whereas SiO₂ is giant covalent.
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