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The neutron was the last of the three main atomic particles to be discovered, because it’s the hardest to detect: with no electric charge, it isn’t pushed or pulled by electric fields and passes through matter without leaving an obvious trail. Yet without neutrons, only hydrogen could exist in any quantity. Neutrons hold heavier nuclei together, create isotopes, and made nuclear power possible. This article explains what the neutron is and why it matters.
The neutron at a glance
| Property | Value |
|---|---|
| Symbol | n (¹₀n) |
| Charge | 0 |
| Mass | 1.675 × 10⁻²⁷ kg (about 1.009 u), slightly more than a proton |
| Location | in the nucleus (every atom except hydrogen-1) |
| Made of | one up quark and two down quarks |
| Discovered | 1932, James Chadwick |
| Free lifetime | about 15 minutes (mean lifetime about 880 s) |
See protons, neutrons and electrons for a side-by-side comparison.
Why scientists suspected a neutral particle
By the early 1920s, the proton and electron were known, and Rutherford had shown that atoms have a nucleus. But the numbers didn’t add up.
- A helium nucleus has a charge of +2 (two protons) but a mass about four times that of a proton.
- Where does the extra mass come from?
Rutherford suggested in 1920 that the nucleus might contain a neutral particle with about the mass of a proton. For over a decade, no one could find it.
Chadwick’s discovery (1932)
In 1930, German physicists found that bombarding beryllium with alpha particles produced a very penetrating radiation. At first it was thought to be high-energy gamma rays. Then Irène Joliot-Curie and Frédéric Joliot showed that this radiation could knock protons out of paraffin wax with considerable energy.
James Chadwick, working in Rutherford’s laboratory in Cambridge, realised that gamma rays couldn’t knock heavy protons about so forcefully, just as a ping-pong ball couldn’t send a bowling ball flying. He carried out careful experiments, measuring how the radiation affected different target nuclei, and showed it must consist of neutral particles with a mass very close to a proton’s:
⁹₄Be + ⁴₂He → ¹²₆C + ¹₀n
He had found the neutron, and received the Nobel Prize in Physics in 1935. The discovery completed the basic picture of the atom and opened the door to nuclear physics.
Neutrons and isotopes
Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These are isotopes.
| Isotope | Protons | Neutrons |
|---|---|---|
| hydrogen-1 | 1 | 0 |
| hydrogen-2 (deuterium) | 1 | 1 |
| hydrogen-3 (tritium) | 1 | 2 |
| uranium-235 | 92 | 143 |
| uranium-238 | 92 | 146 |
Isotopes have almost identical chemistry (same electrons) but different masses and different nuclear stability. Number of neutrons = mass number − atomic number. See mass number explained and isotopes, ions, isomers and allotropes.
Why nuclei need neutrons
Protons repel each other electrically. The strong nuclear force attracts all nucleons (protons and neutrons) to each other over very short distances. Neutrons add strong-force “glue” without adding electrical repulsion.
- Light stable nuclei have roughly equal numbers of protons and neutrons: carbon-12 (6p, 6n), oxygen-16 (8p, 8n).
- Heavy stable nuclei need progressively more neutrons than protons: iron-56 (26p, 30n), lead-208 (82p, 126n).
As protons increase, electrical repulsion grows quickly, because every proton repels every other proton. Extra neutrons spread the protons out and add attraction.
If a nucleus has too many or too few neutrons for its number of protons, it’s unstable and radioactive:
- Too many neutrons → beta-minus decay: a neutron turns into a proton, emitting an electron. Example: carbon-14 → nitrogen-14.
- Too few neutrons → positron emission or electron capture: a proton turns into a neutron.
See alpha, beta and gamma radiation and the island of stability.
Free neutrons don’t last
Inside a stable nucleus, a neutron can last indefinitely. But a free neutron outside a nucleus is unstable. It decays, with a mean lifetime of about 15 minutes, into a proton, an electron and an antineutrino:
n → p + e⁻ + ν̄
This happens because a neutron is slightly heavier than a proton plus an electron, so the decay releases energy.
Neutrons and nuclear fission
Because neutrons have no charge, they aren’t repelled by nuclei. Even slow neutrons can wander right up to a nucleus and be absorbed. This makes them ideal for triggering nuclear reactions.
In 1938, Otto Hahn and Fritz Strassmann found that bombarding uranium with neutrons produced barium, a much lighter element. Lise Meitner and Otto Frisch explained that the uranium nucleus had split in two: nuclear fission.
A typical fission of uranium-235:
¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy
Each fission releases two or three new neutrons, which can cause more fissions: a chain reaction.
- In a nuclear reactor, the chain reaction is carefully controlled. Moderators (water or graphite) slow the neutrons so they’re more easily absorbed by uranium-235, and control rods (containing boron or cadmium, which absorb neutrons) regulate how many neutrons are available.
- In a nuclear weapon, the chain reaction is uncontrolled.
See nuclear fission vs fusion.
Neutrons at work
- Carbon-14 dating: cosmic-ray neutrons in the upper atmosphere hit nitrogen-14 nuclei, forming carbon-14, which plants absorb. See carbon-14 dating.
- Making medical isotopes: research reactors bombard targets with neutrons to make radioactive isotopes for diagnosis and treatment.
- Neutron scattering: beams of neutrons reveal where atoms (especially hydrogen) sit in materials, from proteins to battery electrodes. Neutrons are sensitive to light atoms that X-rays barely see.
- Neutron activation analysis: bombarding a sample with neutrons makes some elements radioactive; the gamma rays they emit identify and measure trace elements.
- Neutron stars: when massive stars collapse, electrons and protons are forced together to form neutrons, creating objects so dense that a teaspoon would weigh hundreds of millions of tonnes.
Common misconceptions
- “Neutrons are a proton and an electron stuck together.” This was an early idea, but the neutron is a particle in its own right, made of quarks. It decays into a proton and an electron (plus an antineutrino); it doesn’t contain them.
- “Adding neutrons changes the element.” No: only the number of protons does. Extra neutrons give a heavier isotope of the same element.
- “Neutrons have no effect on chemistry.” Almost true, but the extra mass slightly changes reaction rates and bond vibrations. Heavy water (D₂O) freezes at 3.8 °C rather than 0 °C.
Key takeaways
- The neutron is an uncharged particle in the nucleus with a mass just over that of a proton.
- James Chadwick discovered it in 1932 by showing that radiation from beryllium consisted of neutral, proton-mass particles.
- Different neutron numbers give isotopes; heavy nuclei need more neutrons than protons to be stable.
- Free neutrons decay in about 15 minutes into a proton, an electron and an antineutrino.
- Uncharged neutrons easily enter nuclei, which makes them the key to fission, reactors, isotope production and neutron-based analysis.
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