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
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define one unified atomic mass unit (u) as 1/12 the mass of a single carbon-12 atom.
- Define relative formula mass, relative molecular mass (Mᵣ), relative atomic mass (Aᵣ) and relative isotopic mass, each measured against unified atomic mass units.
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 define relative atomic mass as the weighted average mass of atoms of an element compared to 1/12th the mass of a carbon-12 atom.
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 weighted average mass of a molecule compared with one-twelfth of the mass of an atom of carbon-12. It has no units. In practice it is found by adding the relative atomic masses of all the atoms in the molecule.
Define relative formula mass.
Relative formula mass is the weighted average mass of one formula unit of a compound (for example an ionic compound) compared with one-twelfth of the mass of an atom of carbon-12. It has no units and is found by adding the relative atomic masses of the atoms in the formula.
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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