A neutron is a subatomic particle that resides in the nucleus of an atom, alongside protons. It contributes significantly to an atom’s overall mass. Neutrons act as a nuclear “glue,” helping to stabilize the positively charged protons within the dense atomic core. Their presence is fundamental to the concept of isotopes, which are atoms of the same element that differ only in the number of neutrons they contain.
Electrical Property: Zero Charge
The most recognized property of the neutron is its lack of a net electrical charge. This electrical neutrality means it is unaffected by the electromagnetic forces that govern charged particles like protons and electrons. This zero-charge state allows the neutron to penetrate the positively charged atomic nucleus without being repelled by the electrostatic force. This unique ability to bypass the Coulomb barrier makes neutrons effective at initiating nuclear reactions, such as fission.
The neutron is not an elementary particle; it is composed of smaller constituents called quarks. Specifically, a neutron contains one up quark and two down quarks, a combination known as a baryon. The up quark carries a fractional positive charge of +2/3, and each down quark carries a fractional negative charge of -1/3. When these charges are summed, the total net charge is zero, explaining the neutron’s neutral state.
Physical Properties: Mass and Size
The mass of a neutron is a defining physical property, though it is often approximated as one unified atomic mass unit (amu) for simplicity. More accurately, the mass is approximately 1.00866 amu. This value is fractionally greater than the mass of a proton, which is about 1.00728 amu. This slight mass difference is a factor in the neutron’s decay behavior.
Compared to an electron, the neutron is nearly 1,839 times heavier. Since neutrons and protons are much more massive than electrons, the nucleus accounts for almost the entire mass of an atom. The neutron is not a point particle; it has a measurable volume, with a mean-square radius of about 0.8 femtometers. This finite size and slight internal charge distribution are consistent with its composite structure of three quarks held together by the strong nuclear force.
Behavior: Stability and Decay
The stability of a neutron depends entirely on its environment. A bound neutron, residing within a stable atomic nucleus, is generally stable and does not decay. The nuclear binding energy required to dislodge it prevents its spontaneous decay.
In contrast, a free neutron—one existing outside the confines of a nucleus—is inherently unstable. The weak nuclear force mediates this instability, causing the free neutron to undergo beta decay. A free neutron has a mean lifetime of approximately 878 seconds (about 15 minutes) before it decays.
In the process of beta decay, a free neutron transforms into three distinct particles. The neutron converts into a positively charged proton, which involves a down quark changing into an up quark via the weak force. To maintain conservation of charge, the decay also emits a high-energy electron (a beta particle) and an electron antineutrino. This transformation is represented by the equation n -> p + e- + anti-v_e.