#3
Atoms (structure, isotopes, relative mass)
Paper 1 · Higher combined
Key steps (method)
- Atomic number = protons (and electrons in a neutral atom).
- Mass number = protons + neutrons.
- Isotopes: same protons, different neutrons.
- Relative atomic mass is a weighted mean of isotopes.
Common mistakes
- Swapping atomic number and mass number.
- Forgetting electrons = protons (neutral atoms).
- Treating relative atomic mass as a mass number (it’s an average).
Exam-style questions (quick)
- An atom has atomic number 13 and mass number 27. How many protons, neutrons and electrons?
- Explain why chlorine has a relative atomic mass of 35.5.
- Define isotope and give one example.