Neutron

Summary: A neutron is an electrically neutral subatomic particle found in the nucleus of an atom, with relative charge 0 and relative mass 1, whose number accounts for the difference between mass number and atomic number and which determines isotopic identity. Tags: igcse chemistry Created: 2026-07-18

The Neutron is a subatomic particle located in the nucleus of an atom, carrying no electric charge (relative charge 0) and having a relative mass of 1 on the atomic mass scale. Alongside the Proton, the Neutron is one of the two types of nucleon that make up the nucleus. Although electrically neutral, neutrons play a crucial role in nuclear stability by contributing to the strong nuclear force that binds nucleons together, without adding to the electrostatic repulsion between protons. The number of neutrons in an atom is not fixed for a given Element — atoms of the same Element with different numbers of neutrons are called isotopes, which is why the mass number (A = protons + neutrons) can vary for the same Element. The number of neutrons can be calculated by subtracting the atomic number (Z) from the mass number (A): N = A - Z.

Properties of the Neutron

Charge and Mass

The Neutron has the following key properties:

The Neutron is slightly heavier than the Proton (by approximately 0.14%), though for IGCSE purposes both are assigned a relative mass of 1. Like the Proton, the Neutron is roughly 1838 times heavier than an Electron.

Calculating Neutron Number

The number of neutrons in an atom is determined by:

Number of neutrons = Mass number (A) - Atomic number (Z)

For example:

• Carbon-12 (^12C): Z = 6, A = 12, therefore N = 12 - 6 = 6 neutrons
• Carbon-13 (^13C): Z = 6, A = 13, therefore N = 13 - 6 = 7 neutrons
• Chlorine-35 (^35Cl): Z = 17, A = 35, therefore N = 35 - 17 = 18 neutrons
• Chlorine-37 (^37Cl): Z = 17, A = 37, therefore N = 37 - 17 = 20 neutrons
• Uranium-235 (^235U): Z = 92, A = 235, therefore N = 235 - 92 = 143 neutrons

This calculation is essential for identifying and distinguishing between isotopes of an Element.

Role in Isotopes

Isotopes are atoms of the same Element (same number of protons, same atomic number) that differ in the number of neutrons (different mass number). It is the varying number of neutrons that defines isotopic identity. Key points about isotopes and neutrons:

• Isotopes have the same chemical properties because they have the same number of electrons and the same outer-shell electron arrangement, and chemical reactions involve electrons.
• Isotopes have different physical properties because they have different masses. For example, a heavier isotope diffuses more slowly and has a slightly higher density.
• Many elements exist naturally as mixtures of isotopes. For instance, chlorine is approximately 75% chlorine-35 (18 neutrons) and 25% chlorine-37 (20 neutrons), giving relative atomic mass a value of 35.5.

Role in Nuclear Stability

Neutrons are essential for the stability of the nucleus. Without neutrons, the electrostatic repulsion between positively charged protons would cause the nucleus to fly apart. neutrons contribute to the strong nuclear force — which is attractive and acts between all nucleons — but do not contribute to electrostatic repulsion. This is why:

• Light elements (Z up to ~20) tend to have roughly equal numbers of protons and neutrons (N/Z ratio approximately 1).
• Heavier elements require more neutrons relative to protons to remain stable (N/Z ratio > 1).
• If the neutron-to-proton ratio is too high or too low, the nucleus becomes unstable (radioactive), and the atom may undergo nuclear decay to achieve a more stable configuration.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.2, Cambridge Assessment International Education

Common Misconceptions