An atom becomes a new element when it undergoes a nuclear reaction called nuclear transmutation, resulting in a change in its atomic number. The atomic number of an atom corresponds to the number of protons in its nucleus, and it determines the identity of the element.
There are a few ways in which an atom can become a new element:
Nuclear fusion: This occurs when two or more atomic nuclei combine to form a heavier nucleus. During nuclear fusion, the combined nucleus will have a higher atomic number than the original nuclei. Fusion reactions are responsible for the energy production in stars, including our Sun. For example, in the Sun, hydrogen nuclei (protons) undergo fusion to form helium nuclei, resulting in the release of energy.
Nuclear fission: This process involves the splitting of a heavy atomic nucleus into two or more smaller nuclei. During fission, the original nucleus will break apart into two or more fragments, and each fragment will have a lower atomic number than the original nucleus. Fission reactions are utilized in nuclear power plants and nuclear weapons. For instance, uranium-235 nuclei can undergo fission when bombarded with neutrons, resulting in the formation of smaller nuclei and the release of energy.
Radioactive decay: Some atoms are unstable, and their nuclei undergo spontaneous decay, emitting particles or radiation in the process. During radioactive decay, the original atom will transform into a different element with a lower atomic number. Various types of radioactive decay exist, including alpha decay (emission of an alpha particle, which consists of two protons and two neutrons), beta decay (emission of a beta particle, either an electron or a positron), and gamma decay (emission of gamma rays, which are high-energy photons).
It's important to note that the transformation of an atom into a new element through nuclear reactions typically occurs under specific conditions, such as high temperatures and pressures (for fusion) or in radioactive or highly energetic environments (for fission and radioactive decay). In everyday conditions, atoms of elements remain stable and maintain their identity.