Beta-negative (β-) decay

Meaning: A nucleus with too many neutrons for its number of protons can become more stable by converting a neutron into a proton. The nucleus ejects a fast electron (the β⁻ particle) and an electron antineutrino. The electron is created in the nucleus during the conversion. It is not taken from the atom's orbitals. The daughter nucleus has the same mass number A but an atomic number one higher.

Equation: n → p + e⁻ + ν̄, so parent (A, Z) → daughter (A, Z+1) + e⁻ + ν̄. A is the mass number and Z is the atomic number, and both are dimensionless. e⁻ is the electron (charge −1.60 × 10⁻¹⁹ C). ν̄ is the electron antineutrino (neutral, nearly massless). Example: carbon-14 → nitrogen-14 + e⁻ + ν̄.

On the MCAT: You are usually asked to complete a nuclear equation by conserving mass number and charge (total Z) on both sides, then name the daughter nuclide. Passages may use carbon-14 dating or a medical tracer and ask which decay type fits a neutron-rich isotope. You may also need to compare penetrating power. Alpha particles are stopped most easily, beta particles penetrate moderately, and gamma rays penetrate most.

Common confusion: Students subtract 4 from A as they would for alpha decay, but in β⁻ decay A does not change. Only Z increases by 1. Do not mix it up with β⁺ decay, which emits a positron when a proton becomes a neutron, so Z decreases by 1 and A is unchanged.