IGCSE · Practice questions · Isotopes

Weight it by abundance.

Six original Cambridge-style questions on isotopes: the definition, particle counts, the reason for identical chemistry and three relative atomic mass calculations.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Same protons, different neutrons.

Isotopes: same proton number, different mass number.
Same chemistry: same number of electrons and the same configuration.
A_r: sum of (mass number × % abundance) ÷ 100.
Sense check: A_r lies nearer the most abundant isotope.

Recall

[2 marks]

Define the term isotopes.

Different atoms of the same element that have the same number of protons, ✓
but different numbers of neutrons. ✓

Analysis

[3 marks]

State the number of protons, neutrons and electrons in an atom of 126C and in an atom of 146C, and explain why they are isotopes.

Carbon-12: 6 protons, 6 neutrons, 6 electrons. ✓
Carbon-14: 6 protons, 8 neutrons, 6 electrons. ✓
Same number of protons (same element) but different numbers of neutrons, so they are isotopes. ✓

Explain

[2 marks]

(Extended) Explain why ³⁵Cl and ³⁷Cl react in the same way with sodium.

Both have 17 electrons with the same configuration, 2,8,7, so the same number of outer-shell electrons. ✓
Chemical reactions depend on the electrons, so both gain one electron to form Cl⁻ in the same way; the extra neutrons do not affect this. ✓

Calculation

[2 marks]

(Extended) Chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Calculate the relative atomic mass of chlorine.

(35 × 75) + (37 × 25) = 2625 + 925 = 3550. ✓
A_r = 3550 ÷ 100 = 35.5. ✓

Calculation

[2 marks]

(Extended) Boron is 20% ¹⁰B and 80% ¹¹B. Calculate the relative atomic mass of boron.

(10 × 20) + (11 × 80) = 200 + 880 = 1080. ✓
A_r = 1080 ÷ 100 = 10.8. ✓

Calculation

[2 marks]

(Extended) Bromine is 50.5% ⁷⁹Br and 49.5% ⁸¹Br. Calculate the relative atomic mass of bromine to one decimal place.

(79 × 50.5) + (81 × 49.5) = 3989.5 + 4009.5 = 7999. ✓
A_r = 7999 ÷ 100 = 79.99 = 80.0. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.