Mass number, atomic number and isotopes
Atoms are described by two numbers: $Z$ (atomic number) and $A$ (mass number). Together they identify which nuclide (specific nucleus) we mean.
📖Definition— Definitions
- Atomic number $Z$: the number of protons in the nucleus. This determines which element the atom is.
- Mass number $A$: the total number of protons + neutrons (collectively called nucleons).
So number of neutrons $N = A - Z$.
Standard notation: $^A_Z X$ where X is the chemical symbol.
Example: $^{14}_6 C$ — carbon, 6 protons, 8 neutrons (14 − 6).
Isotopes
Isotopes are atoms of the same element (same $Z$) but with different numbers of neutrons (different $A$).
- Same element → same chemistry (same electron arrangement).
- Different mass → different physical properties (e.g. some are radioactive).
Examples:
- Hydrogen isotopes: $^1_1 H$ (protium, no neutrons), $^2_1 H$ (deuterium, 1 neutron), $^3_1 H$ (tritium, 2 neutrons).
- Carbon isotopes: $^{12}_6 C$ (stable, ~99% of natural C), $^{14}_6 C$ (radioactive, used for dating).
- Uranium isotopes: $^{235}{92} U$ (fissile), $^{238}{92} U$ (more abundant, not easily fissile).
Why isotopes matter
- Nuclear power and weapons use specific isotopes (U-235, Pu-239).
- Radiocarbon dating relies on C-14.
- Medical tracers use specific radio-isotopes (e.g. technetium-99m).
- Chemistry is the same — biological systems treat isotopes nearly identically (a small "kinetic isotope effect" exists at the level of reaction rates).
Reading nuclide symbols
Given $^{16}_8 O$:
- Z = 8 → 8 protons → element is oxygen.
- A = 16 → 16 nucleons total.
- N = A − Z = 8 → 8 neutrons.
- Neutral atom → 8 electrons.
⚠Common mistakes
- Reading A as the number of neutrons. A = protons + neutrons.
- Forgetting that isotopes have the same chemistry but different stability.
- Confusing isotope (same element) with ion (same atom, different electron count).
- Saying isotopes are "different elements" — they aren't. They're variants of the same element.
AI-generated · claude-opus-4-7 · v3-deep-physics