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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

2 Atoms, molecules and stoichiometry

2.1 Relative masses of atoms and molecules 2.2 The mole and the Avogadro constant 2.3 Formulas 2.4 Reacting masses and volumes (of solutions and gases)

Relative Masses of Atoms and Molecules

Specification Reference Physical Chemistry: Atoms, molecules and stoichiometry 2.1

Quick Notes

  • Unified atomic mass unit (u): one twelfth the mass of a carbon-12 atom.
  • Relative atomic mass (Ar): average mass of an atom compared to 1/12 of a carbon-12 atom.
  • Relative isotopic mass: mass of a specific isotope compared to 1/12 of a carbon-12 atom.
  • Relative molecular mass (Mr): sum of the relative atomic masses of all atoms in a molecule.
  • Relative formula mass: used for ionic compounds — total of Ar values in a formula unit.

Full Notes

Unified Atomic Mass Unit (u)

The unified atomic mass unit (abbreviated as u) is a standard unit of atomic mass.

Definition:
1 u = one twelfth of the mass of a carbon-12 atom.

This gives us a convenient scale to compare the masses of atoms and molecules.

Relative Atomic Mass (Ar)

The relative atomic mass (Ar) of an element is the average mass of an atom of that element compared to 1/12 of the mass of a carbon-12 atom.

It takes into account:

Ar has no units.

Example Chlorine has two isotopes, 35Cl and 37Cl. The average relative mass of a chlorine atom is based on the amounts of each isotope in a natural sample of chlorine.

CIE A-Level Chemistry diagram showing chlorine isotopes 35Cl and 37Cl with relative abundances contributing to average relative atomic mass.

Relative Isotopic Mass

The relative isotopic mass is the mass of a single isotope of an element compared to 1/12 of the mass of a carbon-12 atom.

It refers to one specific isotope, not an average.

Example Relative isotopic mass of C-12 = 12.0; Relative isotopic mass of C-13 = 13.0

Relative Molecular Mass (Mr)

The relative molecular mass (Mr) is the sum of the relative atomic masses of all atoms in a molecule.

It is used for simple covalent molecules.

Example Water (H2O): Mr = (2 × 1.0) + (1 × 16.0) = 18.0

CIE A-Level Chemistry worked example showing calculation of relative molecular mass of water as 18.0.

Like Ar, Mr is unitless.

Relative Formula Mass

The relative formula mass is similar to Mr but used for ionic compounds, which do not form individual molecules.

It is the sum of Ar values in the compound’s formula.

Example NaCl: Na = 23.0, Cl = 35.5 → Formula mass = 58.5

Summary

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