Beta Decay

A nucleus with no electrons in it can still shoot one out, because that electron is not pulled from anywhere. It's built from scratch. In beta-minus decay, a neutron inside the nucleus converts into a proton, an electron, and an antineutrino. The weak nuclear force drives this conversion, turning some of the neutron's rest mass energy into the mass and kinetic energy of the new particles. The proton stays behind in the nucleus; the electron and antineutrino fly out. That ejected electron is the beta particle.

Since a neutron became a proton, the nucleus now has one more proton and one fewer neutron. The number of protons sets the element, so the atomic number Z goes up by one and the element changes. But the total count of protons plus neutrons, the mass number A, stays the same, because one neutron just swapped identity for one proton. Beta-plus decay runs the opposite way: a proton converts into a neutron, a positron, and a neutrino, so Z drops by one while A again stays fixed. On a chart of nuclides, beta-minus decay shifts a nucleus one step toward higher Z, and beta-plus decay shifts it one step toward lower Z, both staying on the same vertical mass-number line.

Beta decay sits alongside alpha decay and gamma emission as one of the three main ways unstable nuclei shed energy. Alpha decay drops both Z and A by fixed amounts; beta decay adjusts Z by one while leaving A untouched, nudging a nucleus toward a more stable ratio of protons to neutrons.