Neutron

A neutron is a subatomic particle in the nucleus with a relative mass of 1 and no charge (relative charge 0). Malay: neutron · Chinese: 中子.

The neutron sits in the nucleus alongside the proton. Because it has a relative mass of 1, it adds to the atom’s mass just as a proton does, but because it has no charge it does not affect whether the atom or nucleus is positive or negative. You can work out the number of neutrons in an atom by subtracting the proton number from the nucleon number.

Example. Chlorine-35 has 17 protons and 18 neutrons, while chlorine-37 has 17 protons and 20 neutrons. Both are chlorine because both have 17 protons, but they differ in the number of neutrons, so they are isotopes of chlorine. Their different neutron numbers give them different nucleon numbers (35 and 37) and slightly different masses.

Confusion to avoid. Do not confuse the neutron with the proton or the electron. The neutron and the proton have the same relative mass of 1, but the proton is charged (+1) and the neutron is not. The electron, by contrast, has almost no mass. A common error is to think changing the neutron number changes the element, it does not; only the proton number decides the element.

In the exam, a common task is to be given the proton number and nucleon number of an atom and asked for the number of neutrons. The rule is simply neutrons = nucleon number − proton number, so an atom with nucleon number 23 and proton number 11 (sodium) has 12 neutrons. Practise this subtraction until it is automatic, because it appears in nearly every atomic-structure question.

The neutron is the particle that makes isotopes possible, which is why it matters so much in the Matter and the Atomic Structure chapter. When you understand that neutrons add mass but not identity, the definitions of nucleon number and isotope both fall into place.