What is a beta particle? What happens to the mass number and atomic number of a nuclide that emits a beta particle?

Radioactive decay is a fundamental process by which an unstable atomic nucleus loses energy by emitting radiation. This process can result in the nucleus changing into the nucleus of a different element through the alteration of the number of protons—an essential characteristic which defines an element. There are several types of radioactive decay, including alpha decay, beta decay, and gamma decay.

In beta decay, which is the focus of our exercise, a beta particle is emitted. Beta particles are essentially high-energy, high-speed electrons (for beta-minus decay) or positrons (for beta-plus decay). The emission of beta particles results from the transformation of neutrons into protons or vice versa within the atom's nucleus. These transformations are crucial in changing the atom into a different element and are indicative of the instability within the atomic nucleus that prompts the decay. By emitting a beta particle, the atom attempts to reach a more stable nuclear configuration.

The atomic number, often denoted by the symbol 'Z', is a fundamental property of an element that signifies the number of protons in the nucleus of an atom. It defines what element the atom is; for instance, an atom with 6 protons will always be carbon. Since each element on the periodic table has a unique number of protons, the atomic number effectively serves as an identifier for the element.
When discussing beta decay, it's important to understand that the atomic number changes according to the type of beta particle emitted. In beta-minus decay, a neutron turns into a proton, releasing an electron (the beta particle). Consequently, the atomic number increases by 1. Conversely, in beta-plus decay, a proton is transformed into a neutron, with the emission of a positron, thus decreasing the atomic number by 1. These changes in the atomic number during beta decay are significant because they result in the formation of a different element.

The mass number, denoted as 'A', is the total number of protons and neutrons in an atom's nucleus. Unlike the atomic number, which changes during certain types of radioactive decay, the mass number does not always change. In the case of beta decay, which is our focus, the mass number remains constant as the emitted beta particle is either an electron or a positron, each having a negligible mass compared to protons and neutrons.

When a beta-minus particle is released during decay, it comes from the conversion of a neutron into a proton and an electron. While the atomic number increases due to the additional proton, the mass number stays the same because the total number of protons and neutrons in the nucleus remains unchanged—the neutron has simply split into a proton and an electron. This concept is crucial for students to understand because it distinguishes between the type of radioactive decay occurring and informs them about the new element that forms as a result, without a change in overall atomic mass.