Final answer:
When an atomic nucleus gains or loses protons, it undergoes transmutation, changing into a different element. The number of protons in the nucleus determines the atomic number, which is essential for the element's identity. This is a key concept in radioactivity and is distinct from the formation of ions, which involves changes in electron numbers.
Step-by-step explanation:
The atomic nucleus of an atom (of a given element) changes into a nucleus of a different element if it gains or loses protons. This process is known as transmutation, which is different from changes in the number of electrons that result in the formation of ions.
Radioactivity is a common way this can happen, where radioactive elements emit particles including protons and neutrons, and transform into different elements. For instance, all nuclei with 84 or more protons are radioactive, and those with fewer can have both stable and unstable isotopes.
The number of protons in the atomic nucleus is critical because it determines the element's atomic number, which defines its identity in the Periodic Table.