neutron

Meaning: A neutron is a neutral nucleon with mass about 1.675 × 10⁻²⁷ kg (roughly 1 u), slightly more than a proton. It adds to mass number but not to charge, so it defines the isotope. Neutrons add strong-force attraction without adding electric repulsion, so heavier stable nuclei need more neutrons than protons. Hydrogen-1 is a common nucleus with no neutrons. A neutron-rich nucleus can undergo β⁻ decay: n → p + e⁻ + antineutrino. Z rises by 1 and A stays the same.

Equation: A = Z + N, where A is the mass number (a count, no unit), Z is the atomic number, or number of protons (a count), and N is the number of neutrons (a count).

On the MCAT: A passage gives isotope notation such as ²³⁸₉₂U and asks for the neutron count (238 − 92 = 146). Another common question asks how Z and N change in a decay. Alpha decay lowers both Z and N by 2. β⁻ decay lowers N by 1 and raises Z by 1. Neutrons also appear in fission, in mass defect and binding energy questions, and in the fact that electric and magnetic fields do not deflect them.

Common confusion: Students think adding or removing neutrons changes the element or the charge. The element is set by Z alone, so a different N gives a different isotope of the same element. In β⁻ decay, the emitted electron is created in the nucleus when a neutron converts to a proton. It does not come from the electron shells.