Relative masses of atoms and molecules
Cambridge A-Level Chemistry (9701) · Unit 2: Atoms, molecules and stoichiometry · 7 flashcards
Relative masses of atoms and molecules is topic 2.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 2 — Atoms, molecules and stoichiometry , alongside The mole and the Avogadro constant and Formulas. In one line: The unified atomic mass unit (u) is defined as one-twelfth of the mass of a carbon-12 atom. It is used as a standard unit for measuring the mass of atoms and molecules.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 5 definitions and 2 key concepts — covering the precise wording mark schemes reward. Use the 5 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The unified atomic mass unit (u)
The unified atomic mass unit (u) is defined as one-twelfth of the mass of a carbon-12 atom. It is used as a standard unit for measuring the mass of atoms and molecules.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- define the unified atomic mass unit as one twelfth of the mass of a carbon-12 atom
- define relative atomic mass, Ar, relative isotopic mass, relative molecular mass, Mr, and relative formula mass in terms of the unified atomic mass unit 2.2 The mole and the Avogadro constant Learning outcomes
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Relative masses of atoms and molecules
- › Always check the subscripts in a chemical formula to ensure the correct number of metal ions are used in quantitative calculations.
- › Use the balanced equation to find the exact mole ratio and subtract the mass reacted from the initial mass.
Define the unified atomic mass unit (u).
The unified atomic mass unit (u) is defined as one-twelfth of the mass of a carbon-12 atom. It is used as a standard unit for measuring the mass of atoms and molecules.
Define relative atomic mass (Ar).
Relative atomic mass (Ar) is the weighted average mass of the naturally occurring isotopes of an element, relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Define relative isotopic mass.
Relative isotopic mass is the mass of an isotope relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Define relative molecular mass (Mr).
Relative molecular mass (Mr) is the sum of the relative atomic masses of all the atoms in a molecule, relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Define relative formula mass.
Relative formula mass is the sum of the relative atomic masses of all the ions in a formula unit of an ionic compound, relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Explain the relationship between relative atomic mass (Ar) and the unified atomic mass unit (u).
Relative atomic mass (Ar) is a ratio. It represents how many times heavier, on average, an atom of an element is compared to 1/12th the mass of a carbon-12 atom (1 u). Therefore, it is dimensionless.
Describe how relative atomic mass (Ar) is calculated from isotopic abundance data.
Ar = Σ (isotopic mass × % abundance) / 100. Multiply the mass of each isotope by its percentage abundance, sum the results, and divide by 100 to account for the percentage. This calculates the weighted average.
More Chemistry flashcards
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All Chemistry FlashcardsMore topics in Unit 2 — Atoms, molecules and stoichiometry
Relative masses of atoms and molecules sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Relative masses of atoms and molecules deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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