Definition: Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons.
Why this matters:
- Same element = same chemical properties (because they have the same electrons)
- Different mass = different physical properties (density, rate of diffusion)
- Some isotopes are radioactive (unstable nuclei that decay)
Example — Carbon isotopes:
- Carbon-12 (⁶¹²C) — 6 protons, 6 neutrons — most common, stable
- Carbon-13 (⁶¹³C) — 6 protons, 7 neutrons — stable, less common
- Carbon-14 (⁶¹⁴C) — 6 protons, 8 neutrons — radioactive, used in carbon dating
Quick Check: Two atoms have atomic number 6 but one has mass number 12 and the other has mass number 14. What is the relationship between these atoms?
They are isotopes of carbon. Both have 6 protons (same element) but different numbers of neutrons (6 and 8 respectively), giving different mass numbers.
This key facts covers Isotopes — Same Element, Different Mass within Atomic Structure for GCSE Chemistry. Revise Atomic Structure in Atomic Structure for GCSE Chemistry with 28 exam-style questions and 22 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 7 of 14 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Atomic Structure
What does the atomic number of an element tell you?
Explain what is meant by the relative atomic mass of an element and how it is calculated from isotopic data. [3 marks]