Why do isotopes have different mass numbers?

Isotopes have different mass numbers because they contain different numbers of neutrons in their atomic nuclei.

Isotopes are variants of a particular chemical element, which means they have the same number of protons in their atomic nuclei, but different numbers of neutrons. This difference in neutron number is what gives isotopes their different mass numbers.

The mass number of an atom is calculated by adding the number of protons and neutrons together. Since protons and neutrons both carry approximately the same mass (with neutrons being slightly heavier), the mass number is a good approximation of the actual atomic mass. For example, carbon-12, the most common isotope of carbon, has 6 protons and 6 neutrons, giving it a mass number of 12. Carbon-13, another isotope of carbon, has 6 protons and 7 neutrons, giving it a mass number of 13.

The existence of isotopes is a fundamental aspect of the atomic theory of matter. It explains why elements can exhibit different physical properties, such as different rates of reaction or different types of decay, even though they have the same number of protons. For instance, carbon-14, an isotope of carbon with 8 neutrons, is radioactive and decays over time, while carbon-12 is stable.

In summary, isotopes have different mass numbers because they contain different numbers of neutrons in their atomic nuclei. This difference in neutron number results in different physical properties and behaviours, making the study of isotopes a key part of understanding the atomic nature of matter.