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Isotopes are variants of a particular chemical element that have different numbers of neutrons while maintaining the same number of protons. Because of the constant proton number, isotopes of an element have identical atomic numbers but different mass numbers. This distinction in neutron count is what leads to the variation in atomic mass. Isotopes exhibit nearly identical chemical behavior but can differ in physical properties, like atomic mass. Each isotope of an element has its own relative abundance, indicating how much of that isotope is found compared to other isotopes of the same element. For example, bromine has two stable isotopes: bromine-79 and bromine-81. These isotopes have atomic masses of 78.9183 u and 80.9163 u, respectively.

Atomic mass, often expressed in atomic mass units (u), is the total mass of the protons and neutrons in an atom's nucleus. Since protons and neutrons have nearly equal masses, the atomic mass is approximately equal to the mass number of the isotope. For isotopes, each variant has a unique atomic mass because the number of neutrons varies while the number of protons remains constant. Understanding atomic mass is vital when calculating the atomic weight of an element with multiple isotopes. In the case of bromine, the atomic masses are specific to its isotopes—bromine-79 has an atomic mass of 78.9183 u, and bromine-81 is 80.9163 u. These values are crucial in determining the element's overall atomic weight.

Abundance refers to how common or rare an isotope is in comparison to other isotopes of the same element. The abundance of an isotope directly affects the average atomic weight calculated for a natural element. To calculate an element's atomic weight, you must consider the atomic masses and the relative abundances of its isotopes. The process involves multiplying each isotope's atomic mass by its abundance in decimal form and adding them together. For bromine, the abundances are 50.69% for bromine-79 and 49.31% for bromine-81. By converting these percentages into decimals and using them in calculations, we weight the isotopic masses to find bromine's atomic weight. Thus, the atomic weight balances the contributions of each isotope reflecting their natural occurrence.