Neutron: The Glue of the Atom
The Discovery of the Neutron
For a long time, scientists knew atoms had a dense, positive core called a nucleus, surrounded by negative electrons. But there was a problem. If the nucleus only contained positive protons, the repulsive force between them would tear it apart. There had to be something else inside, something that helped hold it all together. In 1932, the English physicist James Chadwick solved the mystery. He performed an experiment where he bombarded a thin sheet of beryllium with alpha particles. This produced a new, highly penetrating radiation that was not deflected by electric or magnetic fields. Chadwick concluded this radiation was made of neutral particles with a mass similar to the proton. He had discovered the neutron.
Basic Properties: What is a Neutron Made Of?
We now know that neutrons are not truly fundamental particles; they are made up of smaller particles called quarks. A neutron is composed of one "up" quark and two "down" quarks. The charge of an up quark is $+2/3$, and the charge of a down quark is $-1/3$. So, the total charge of a neutron is $(+2/3) + (-1/3) + (-1/3) = 0$. This explains its neutrality.
Here are the key properties of a neutron compared to a proton and an electron:
| Particle | Symbol | Location | Charge | Mass (kg) |
|---|---|---|---|---|
| Proton | $p$ or $p^+$ | Nucleus | $+1$ | 1.6726 × 10^{-27} |
| Neutron | $n$ | Nucleus | $0$ | 1.6749 × 10^{-27} |
| Electron | $e$ or $e^-$ | Outside Nucleus | $-1$ | 9.1094 × 10^{-31} |
Notice that the mass of a neutron is slightly greater than that of a proton. The mass of an electron is so small that it is almost negligible when calculating the mass of an atom. The atom's mass comes almost entirely from its nucleus—the protons and neutrons.
The Neutron's Role: Atomic Stability and Isotopes
The primary job of the neutron is to provide the "nuclear glue" that holds the nucleus together. Protons, all having a positive charge, repel each other strongly due to the electromagnetic force. The neutrons, being neutral, do not add to this repulsion. Instead, they contribute to the strong nuclear force, an incredibly powerful force that acts over a very short range inside the nucleus. This force binds protons and neutrons together, overcoming the electrical repulsion between protons.
This leads to the concept of isotopes. Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons. For example, carbon always has 6 protons. But it can have different numbers of neutrons:
- Carbon-12: 6 protons + 6 neutrons. This is the most common, stable form of carbon.
- Carbon-13: 6 protons + 7 neutrons. This is also stable but less common.
- Carbon-14: 6 protons + 8 neutrons. This isotope is unstable, or radioactive, and decays over time. It is used in "carbon dating" to determine the age of ancient organic materials.
The number of neutrons determines the isotope of an element and is crucial for its stability. For lighter elements, a 1:1 ratio of protons to neutrons is usually stable. For heavier elements, more neutrons are needed to provide enough strong force to hold the larger nucleus together.
Free Neutrons and Radioactivity
While neutrons are stable inside the nucleus, a free neutron—one existing alone outside a nucleus—is not stable. A free neutron will decay into other particles with a half-life[1] of about 14 minutes and 40 seconds. It decays into a proton, an electron, and an antineutrino. This process is a type of radioactive decay called beta-minus decay.
The decay can be represented by this equation: $$n \to p^+ + e^- + \bar{\nu}_e$$
This is why neutrons are not found lying around in nature; they are always bound inside atomic nuclei or are produced for a short time in nuclear reactions.
Neutrons in Action: Nuclear Fission and Fusion
Neutrons are the key players in the nuclear reactions that power stars and nuclear reactors. Because they have no charge, neutrons are not repelled by the positive charge of a nucleus. This allows them to get very close to and enter atomic nuclei, triggering reactions.
Nuclear Fission: This is the process of splitting a heavy nucleus into lighter ones. It is the principle behind nuclear power plants and atomic bombs. For example, when a uranium-235 nucleus absorbs a slow-moving neutron, it becomes unstable and splits into two smaller nuclei (like krypton and barium), along with releasing more neutrons and a tremendous amount of energy. The newly released neutrons can then go on to split other uranium nuclei, creating a chain reaction.
Nuclear Fusion: This is the process of combining two light nuclei to form a heavier nucleus. It is the power source of the sun and other stars. In the core of the sun, protons (hydrogen nuclei) fuse together under immense pressure and temperature. Neutrons are involved in the later stages of these fusion processes. For instance, the fusion of deuterium[2] and tritium[3], two isotopes of hydrogen, produces a helium nucleus and a neutron, releasing vast energy: $$^2_1H + ^3_1H \to ^4_2He + ^1_0n + \text{energy}$$
Practical Applications of Neutrons
Beyond power generation, neutrons have many practical uses thanks to their unique properties.
Neutron Scattering: Beams of neutrons are used like super-powered X-rays to study the structure of materials at the atomic level. Because neutrons interact with the nuclei of atoms, they can see light elements like hydrogen very well and can distinguish between elements that are next to each other on the periodic table. This is invaluable for research in physics, chemistry, biology, and materials science.
Medical Applications: Neutrons are used in a cancer treatment called Boron Neutron Capture Therapy (BNCT). A patient is injected with a drug containing boron-10, which collects in tumor cells. When the tumor is exposed to a beam of low-energy neutrons, the boron-10 nuclei absorb a neutron and immediately split, releasing high-energy particles that destroy the cancer cell from the inside while sparing surrounding healthy tissue.
Industrial Uses: Neutrons are used to inspect airplane parts for hidden cracks, analyze the composition of oil wells, and even help authenticate priceless works of art by determining their elemental makeup without causing any damage.
Common Mistakes and Important Questions
A: They don't, directly. The chemical properties of an element are determined almost entirely by the number and arrangement of its electrons. Since the number of electrons is equal to the number of protons, chemistry is governed by the proton number (the atomic number). Neutrons only affect the mass and the nuclear stability of the atom, not its chemical behavior. This is why isotopes of the same element have identical chemical properties.
A: Yes, that is correct. The mass number (A) is the total number of protons (Z) and neutrons (N) in an atom's nucleus: $A = Z + N$. The electrons contribute so little mass that they are ignored in this calculation. For example, a carbon-12 atom has 6 protons and 6 neutrons, so its mass number is 12.
A: Yes, but only one common element does: the most abundant isotope of hydrogen, called protium. Its nucleus is a single proton and no neutrons. All other elements require neutrons in their nucleus to be stable. Even helium, the next lightest element, needs at least one neutron to form a stable nucleus (Helium-3 has 2 protons and 1 neutron).
Footnote
[1] Half-life: The time required for half of the atoms in a radioactive sample to decay.
[2] Deuterium ($^2_1H$): An isotope of hydrogen whose nucleus contains one proton and one neutron.
[3] Tritium ($^3_1H$): A radioactive isotope of hydrogen whose nucleus contains one proton and two neutrons.