Isotopes
Isotopes for Cambridge IGCSE Chemistry 0620: the exact definition, particle numbers, electronic structure and Supplement calculations of relative atomic mass from abundance.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-11.
An isotope question can test several different outcomes, and they are not all at the same tier. The definition and particle counting are Core. The electron-based explanation and weighted relative atomic mass calculation are Supplement.
Definition of isotopes (Core)
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
An equivalent definition is: atoms with the same proton number but different nucleon numbers.
Both halves matter:
- same number of protons
- different number of neutrons
“Same element with different masses” is too vague because it does not state which subatomic particles are the same and different.
Particle numbers in isotopes (Core)
Chlorine-35 and chlorine-37 are both chlorine because both have proton number 17.
| Particle or value | Chlorine-35 | Chlorine-37 |
|---|---|---|
| Protons | 17 | 17 |
| Neutrons | 18 | 20 |
| Electrons in a neutral atom | 17 | 17 |
| Nucleon number | 35 | 37 |
Use:
number of neutrons = nucleon number − proton number
For chlorine-37:
37 − 17 = 20 neutrons
Why the chemistry is the same (Supplement)
Neutral isotopes of an element have the same number of electrons and the same electronic configuration. Chemical reactions involve electrons, especially the outer-shell electrons. Therefore, isotopes of the same element have the same chemical properties.
Their masses differ because their neutron numbers differ. Mass-dependent physical behaviour can therefore differ slightly, but the required chemical explanation is based on identical electronic structure.
Relative atomic mass from isotopic abundance (Supplement)
Relative atomic mass, Ar, is a weighted mean because a natural sample can contain more than one isotope.
For percentage abundances:
Ar = Σ(isotopic mass × percentage abundance) ÷ 100
For chlorine containing 75% chlorine-35 and 25% chlorine-37:
Ar = [(35 × 75) + (37 × 25)] ÷ 100
Ar = (2625 + 925) ÷ 100 = 35.5
The answer must lie between the isotope masses and closer to the mass of the more abundant isotope. That is a useful reasonableness check.
If the abundances are given as a ratio rather than percentages, divide by the total number of ratio parts.
Example for isotopes 10 and 11 in a 1:4 ratio:
Ar = [(10 × 1) + (11 × 4)] ÷ 5 = 10.8
Worked examination question
Boron has proton number 5. A sample contains 20% boron-10 and 80% boron-11.
(a) State the numbers of protons and neutrons in boron-11. [2]
- protons = 5 [1]
- neutrons = 11 − 5 = 6 [1]
(b) Explain why boron-10 and boron-11 have the same chemical properties. [2]
- they have the same number / arrangement of electrons [1]
- chemical reactions involve electrons [1]
(c) Calculate the relative atomic mass of the sample. [2]
- [(10 × 20) + (11 × 80)] ÷ 100 [1]
- 10.8 [1]
Part (a) is Core. Parts (b) and (c) use Supplement depth.
Common mistakes
- Giving half a definition. State both same protons and different neutrons.
- Saying isotopes have different electron numbers. Neutral isotopes of the same element have the same electron number.
- Confusing nucleon number with neutron number. Nucleon number is protons plus neutrons.
- Calling isotopes different elements. Element identity is fixed by proton number.
- Using an unweighted average. Add the mass × abundance contributions first.
- Producing an Ar outside the isotope range. The weighted mean must lie between the isotope masses.
- Treating the abundance calculation as Core. It is Supplement for this syllabus cycle.
Exam-ready wording
| Weak wording | More precise wording |
|---|---|
| “Different versions of the same atom” | “Atoms of the same element with the same number of protons but different numbers of neutrons” |
| “They react the same because they are the same element” | “They have the same electronic configuration, and chemical reactions involve electrons” |
| “Chlorine is 35.5 because it is an average” | “35.5 is the weighted mean of the isotopic masses using their relative abundances” |
Test yourself
Q1. Define isotopes. [2]
Show answer
Atoms of the same element with the same number of protons [1] but different numbers of neutrons [1].
Q2. Copper contains 69% copper-63 and 31% copper-65. Calculate Ar to three significant figures. [2]
Show answer
[(63 × 69) + (65 × 31)] ÷ 100 = 63.6.
Q3. Explain why the two copper isotopes have the same chemical properties. [2]
Show answer
They have the same number and arrangement of electrons [1], and chemical reactions involve electrons [1].
Continue with atomic structure and relative atomic and molecular mass.
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