Explain why the mass number of an atom is always a whole number, yet the relative atomic mass of most elements is not a whole number.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Mass number is the total number of protons and neutrons in the nucleus of one atom.
Because protons and neutrons are discrete particles, their numbers are whole numbers, so the mass number is always an integer.
Most elements exist as a mixture of isotopes – atoms with the same number of protons but different numbers of neutrons, i.e. different mass numbers.
The relative atomic mass is the weighted mean of the masses of all naturally occurring isotopes, weighted by their relative abundances; this average is therefore usually not a whole number.
Because protons and neutrons are discrete particles, their numbers are whole numbers, so the mass number is always an integer.
Most elements exist as a mixture of isotopes – atoms with the same number of protons but different numbers of neutrons, i.e. different mass numbers.
The relative atomic mass is the weighted mean of the masses of all naturally occurring isotopes, weighted by their relative abundances; this average is therefore usually not a whole number.
Examiner tips
- State that mass number = protons + neutrons; both are whole numbers.
- Explain that isotopes give a range of mass numbers.
- Show that relative atomic mass is a weighted average of isotope masses.
- Use the word ‘average’ and ‘weighted’ to match the mark scheme.
Common mistakes
- Confusing mass number with atomic mass.
- Forgetting to mention isotopes or weighted average.
- Using ‘average’ without specifying ‘weighted by abundance’.
Mark scheme (4 marks)
- Mass number is the total number of protons and neutrons (nucleons) in the nucleus of one atom.
- Since the number of protons and neutrons must be whole numbers, mass number is always a whole number / integer.
- Most elements exist as a mixture of isotopes, which have the same number of protons but different numbers of neutrons / different mass numbers.
- Relative atomic mass is the weighted mean / average of the masses of all naturally occurring isotopes of the element, taking into account their relative abundances, so it is rarely a whole number.
Key terms in this question
mass number · relative atomic mass
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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