AS · Practice questions · Isotopes

Same chemistry, different weight.

Six original Cambridge-style questions on isotopes: definitions, similarities and differences, nuclide notation, and relative atomic mass from abundance data.

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

Keep these straight

Protons fix the element, neutrons fix the isotope.

Definition: same number of protons, different number of neutrons.
Chemistry: identical, because the electron configuration is identical.
A_r: sum of (mass × fraction) over all isotopes.
Reverse: let the unknown fraction be x and solve one equation.

Recall

[2 marks]

Define the term isotopes, and state why the relative atomic mass of most elements is not a whole number.

Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (different nucleon numbers). ✓
Relative atomic mass is a weighted average of the masses of the naturally occurring isotopes, so it is usually not a whole number. ✓

Explain

[3 marks]

Copper exists as ⁶³Cu and ⁶⁵Cu. Describe two ways in which atoms of these isotopes are the same and one way in which they differ.

Both have 29 protons (the same proton number). ✓
Both have 29 electrons in the same configuration, so they have the same chemical properties. ✓
They differ in neutron number: 34 neutrons in ⁶³Cu and 36 in ⁶⁵Cu, so ⁶⁵Cu has the greater mass. ✓

Explain

[2 marks]

Explain why isotopes of an element have the same chemical properties but slightly different physical properties.

Chemical properties depend on the electron configuration, which is the same for all isotopes of an element because they have the same number of protons and electrons. ✓
Physical properties such as density and rate of diffusion depend on mass, and isotopes have different masses because they contain different numbers of neutrons. ✓

Analysis

[2 marks]

Magnesium consists of ²⁴Mg (79.0%), ²⁵Mg (10.0%) and ²⁶Mg (11.0%). Calculate its relative atomic mass to two decimal places.

A_r = (24 × 79.0 + 25 × 10.0 + 26 × 11.0) / 100 = (1896 + 250 + 286) / 100. ✓
A_r = 2432 / 100 = 24.32. ✓

Analysis

[3 marks]

Bromine has two isotopes, ⁷⁹Br and ⁸¹Br, and a relative atomic mass of 79.9. Calculate the percentage abundance of each isotope.

Let the fraction of ⁸¹Br be x, so the fraction of ⁷⁹Br is (1 − x). ✓
79(1 − x) + 81x = 79.9, so 79 + 2x = 79.9 and x = 0.45. ✓
Abundances: 45% ⁸¹Br and 55% ⁷⁹Br. ✓

Analysis

[2 marks]

An ion contains 20 protons, 20 neutrons and 18 electrons. Write its full nuclide symbol including the charge.

20 protons is calcium; A = 20 + 20 = 40; charge = 20 − 18 = 2+. ✓
Symbol: 4020Ca²⁺. ✓

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.