How do isotopes affect atomic mass calculations?

Isotopes affect atomic mass calculations by contributing to the weighted average of an element's atomic mass.

Isotopes are variants of a particular chemical element which, while having the same number of protons, differ in the number of neutrons. This difference in neutron number results in different atomic masses for each isotope of an element. When calculating the atomic mass of an element, it's not as simple as taking the mass of one atom. Instead, the atomic mass is a weighted average that takes into account the masses of each of an element's isotopes and their relative abundances.

For example, consider carbon. It has two stable isotopes, carbon-12 and carbon-13. Carbon-12 makes up about 98.9% of all carbon, while carbon-13 makes up the remaining 1.1%. To calculate the atomic mass of carbon, you would multiply the mass of each isotope by its relative abundance, then add these values together. This gives a weighted average atomic mass that is slightly higher than 12, reflecting the small contribution of the heavier carbon-13 isotope.

This principle applies to all elements, not just carbon. Some elements have many stable isotopes, each of which must be considered when calculating the atomic mass. This is why the atomic masses listed on the periodic table are usually not whole numbers. They represent the average mass of an element's atoms, taking into account the masses and relative abundances of all its isotopes.

In summary, isotopes play a crucial role in atomic mass calculations. They contribute to the weighted average of an element's atomic mass, which is why atomic masses are usually not whole numbers but decimals. Understanding this concept is key to mastering atomic mass calculations in IGCSE Chemistry.