Isotopes
Cambridge A-Level Chemistry (9701) · Unit 1: Atomic structure · 6 flashcards
Isotopes is topic 1.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 1 — Atomic structure , alongside Particles in the atom and atomic radius, Electrons, energy levels and atomic orbitals and Ionisation energy. In one line: Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. This results in the same atomic number but a different mass number.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 6 flashcards — 4 definitions and 2 key concepts — covering the precise wording mark schemes reward. Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The term 'isotope' in terms of the number of protons and neutrons
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. This results in the same atomic number but a different mass number.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define the term isotope using how many protons and neutrons the atoms contain.
- Understand isotope notation, in which the symbol A carries the nucleon (mass) number x as a superscript and the proton (atomic) number y as a subscript, both on its left, e.g. ³⁵₁₇Cl.
- State that isotopes of one element have identical chemical properties, and explain why.
- State that isotopes of one element differ in their physical properties, and explain why; only mass and density are required.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Define the term 'isotope' in terms of the number of protons and neutrons.
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. This results in the same atomic number but a different mass number.
Explain the isotope notation where mass number (A) is written as a superscript and atomic number (Z) as a subscript before the element symbol X. What do A and Z represent?
In this notation the mass number A is written as a superscript and the atomic number Z as a subscript, both before the element symbol X (for example ¹²₆C). A is the mass number (number of protons + neutrons, also called the nucleon number), Z is the atomic number (number of protons) and X is the element symbol.
Why do isotopes of the same element have the same chemical properties?
Isotopes of the same element have the same number of electrons and the same electronic configuration. Chemical properties are determined by the number and arrangement of electrons; therefore isotopes react similarly.
Why do isotopes of the same element exhibit different physical properties?
Isotopes have different physical properties (such as mass and density) because they contain different numbers of neutrons. These differences in neutron number affect the mass of the atom and subsequently macroscopic properties dependent on mass.
Describe the difference between the mass number and the atomic number of an isotope.
The mass number is the total number of protons and neutrons in the nucleus of an atom. The atomic number is the number of protons in the nucleus of an atom, defining the element's identity. The number of neutrons is found by subtracting the atomic number from the mass number.
Give an example of two isotopes of carbon, showing their mass and atomic numbers.
Carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are isotopes of carbon. Both have 6 protons (atomic number = 6), but carbon-12 has 6 neutrons (mass number = 12) and carbon-14 has 8 neutrons (mass number = 14).
More Chemistry flashcards
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All Chemistry FlashcardsMore topics in Unit 1 — Atomic structure
Isotopes sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Isotopes deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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