The mass number of an atom refers to the total number of protons and neutrons in its nucleus. While all atoms of a given element have the same number of protons, known as the atomic number, the number of neutrons can vary among different atoms of the same element. This variation in the number of neutrons leads to different mass numbers.
The mass number is important because it provides information about the total mass of an atom. Neutrons have a mass that is slightly larger than that of protons, so the presence of additional neutrons increases the overall mass of the atom. Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons and, therefore, different mass numbers.
For example, let's consider the element carbon. Carbon typically has six protons in its nucleus (giving it an atomic number of 6), but it can have different numbers of neutrons. Carbon-12 has six neutrons, so its mass number is 12 (6 protons + 6 neutrons). Carbon-13 has seven neutrons and a mass number of 13, while Carbon-14 has eight neutrons and a mass number of 14.
By specifying the mass number alongside the atomic number, we can distinguish between different isotopes of an element. These isotopes may have similar chemical properties due to the same number of protons, but their differing mass numbers can affect their stability, nuclear properties, and behavior in certain reactions.