Most hydrogen atoms contain no neutrons. The rare isotopes deuterium and tritium contain one and two neutrons, respectively.
During the early 20th century, the proton and electron were well characterized, yet atomic mass numbers consistently exceeded atomic numbers, implying another particle of similar mass—the neutron—was present in the nucleus. In 1932, physicist James Chadwick confirmed its existence.
The atomic number indicates the number of protons. Stable, neutral atoms have equal numbers of protons and electrons, giving them a net zero charge. Atomic mass units (amu) are defined as one‑twelth the mass of a carbon‑12 atom, which contains 6 protons and 6 neutrons; therefore a proton or neutron ≈ 1 amu.
Hydrogen, the lightest element, exists in three stable isotopes:
¹H – 0 neutrons (protium) ²H – 1 neutron (deuterium) ³H – 2 neutrons (tritium)
All isotopes carry a single proton and a single electron, maintaining electrical neutrality.
Protium is the most abundant isotope. While free hydrogen rarely exists on Earth, it forms compounds such as water (H₂O) and hydrocarbons. Hydrogen combustion emits only heat and water, making it a clean energy vector.
Deuterium occurs naturally at a ratio of ~1 in 6,420 hydrogen atoms. When paired with oxygen, it forms heavy water (D₂O), which has a higher freezing point (3.8 °C) than ordinary water. Heavy water serves as a neutron moderator in nuclear reactors and is valuable in scientific research, though high concentrations (>25 %) can be harmful to biological tissues.
Tritium is radioactive, decaying with a half‑life of 12.28 years. It is produced in nuclear reactors. Despite its radioactivity, tritium is used in long‑lasting luminous signs, in tracing experiments, and in certain nuclear weapons designs.
