When a radioactive nucleus emits an alpha particle, the mass number of the atom remains the same.

Explanation:

- Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation. There are several types of radioactive decay, including alpha decay, beta decay, and gamma decay.
- Alpha decay occurs when a radioactive nucleus emits an alpha particle, which consists of two protons and two neutrons. This emission reduces the atomic number and mass number of the parent nucleus.
- In alpha decay, the atomic number decreases by two because the emission of an alpha particle means the loss of two protons. The atomic number represents the number of protons in an atom, so when two protons are lost, the atomic number decreases.
- On the other hand, the mass number remains the same because the mass number represents the sum of protons and neutrons in an atom. Since the alpha particle contains two protons and two neutrons, when it is emitted, the mass number of the parent nucleus remains unchanged.
- For example, if we consider the alpha decay of uranium-238 (U-238), the parent nucleus emits an alpha particle (He-4) and transforms into thorium-234 (Th-234). In this decay process, the atomic number of U-238 decreases from 92 to 90, but the mass number remains the same at 238.
- This concept is consistent with the conservation of mass and charge. The total number of protons and neutrons in the nucleus, which is represented by the mass number, is conserved during alpha decay. However, the number of protons, which determines the atomic number and the chemical properties of an element, changes.
- It is important to note that the emission of an alpha particle is only one type of radioactive decay. Other types of decay, such as beta decay, can result in changes in both the atomic number and mass number of the parent nucleus.