In beta decay, the atomic number increases by 1 because a neutron in the nucleus of an atom is converted into a proton. Beta decay occurs in certain unstable isotopes where the nucleus contains an excess of neutrons compared to protons, making the nucleus unstable.
During beta decay, one of the neutrons in the nucleus is transformed into a proton, an electron, and an antineutrino. The neutron emits a high-energy electron, which is called a beta particle (β-), and also releases an antineutrino (ν̅e). The beta particle is ejected from the nucleus, while the antineutrino, being electrically neutral, escapes the atom.
The conversion of a neutron into a proton increases the number of protons in the nucleus, which, in turn, increases the atomic number by 1. This means that the identity of the element changes, as elements are defined by their atomic numbers. The emission of the beta particle allows the atom to transition to a more stable state by decreasing the neutron-to-proton ratio.
To summarize, in beta decay, the atomic number increases by 1 because a neutron is converted into a proton, resulting in the transformation of one element into another.