Elements are defined by their unique number of protons, known as the atomic number. The atomic number determines the element's identity and its position in the periodic table. Elements with different atomic numbers have different numbers of protons.
However, elements with similar atomic numbers can have different numbers of neutrons. These variants are called isotopes. Isotopes of an element have the same atomic number but different mass numbers. The mass number is the sum of protons and neutrons in the nucleus of an atom.
Isotopes occur because the number of neutrons in an atom can vary while the number of protons remains constant for a given element. This variation in the number of neutrons leads to different isotopes with different mass numbers.
Isotopes often have similar chemical properties because their electron configurations are the same, determined by the number of protons. However, isotopes can have different physical properties, such as varying stability, radioactive decay rates, or nuclear binding energies, due to differences in the number of neutrons.
For example, carbon-12 (6 protons and 6 neutrons) and carbon-14 (6 protons and 8 neutrons) are isotopes of carbon. They have the same atomic number (6) because they both have 6 protons. However, they have different mass numbers due to the differing number of neutrons. Other elements can exhibit similar variations in isotopes.
In summary, elements have different numbers of protons (atomic numbers) because that is what defines their identity. However, elements with similar atomic numbers can have different numbers of neutrons, resulting in different isotopes with distinct mass numbers.