3.2.1.1
Isotopes have the same proton number but different nucleon numbers
Atomic Structure & Nuclear Stability — AQA A-Level Physics
Key Definition
Isotopes — Nuclei that have the same number of protons but different numbers of neutrons.
- Isotopes of an element have the same proton number $Z$ but different nucleon number $A$.
- Example: hydrogen has three isotopes — hydrogen (${}_{1}^{1}$H), deuterium (${}_{1}^{2}$H), and tritium (${}_{1}^{3}$H). All have 1 proton.
- Some isotopes are unstable and decay by emitting radiation. Decay timescales range from nanoseconds to hundreds of thousands of years.
Isotopic data
Key Definition
Isotopic data — The relative amounts of different isotopes of an element present within a substance.
- The percentage abundance of isotopes in a sample is measured using a mass spectrometer.
- Relative atomic mass can be calculated from isotopic abundances: multiply each isotope's mass number by its fractional abundance and sum.
- Carbon dating uses the ratio of stable carbon-12 to unstable carbon-14 to determine the age of dead organic matter.