How is the relative atomic mass of an isotope calculated?

The relative atomic mass of an isotope is calculated by multiplying the mass number of each isotope by its abundance, then adding these values together.

The relative atomic mass (Ar) of an element is not simply the mass number of one atom of that element. It is actually a weighted average of the masses of the different isotopes of that element, taking into account how abundant each isotope is. This is because most elements in nature exist as a mixture of different isotopes.

To calculate the relative atomic mass of an isotope, you need to know two things: the mass number of each isotope and the abundance of each isotope. The mass number is the total number of protons and neutrons in the nucleus of an atom. The abundance is the proportion of atoms of that isotope compared to the total number of atoms in a sample of the element.

The calculation involves multiplying the mass number of each isotope by its abundance, and then adding these values together. This gives a value that takes into account both the mass and the abundance of each isotope.

For example, let's consider chlorine, which has two main isotopes: chlorine-35 and chlorine-37. If we assume that chlorine-35 makes up 75% of the atoms and chlorine-37 makes up 25%, we can calculate the relative atomic mass as follows: (35 x 0.75) + (37 x 0.25) = 35.5.

So, the relative atomic mass of chlorine is 35.5. This means that, on average, a chlorine atom is 35.5 times heavier than one twelfth of the mass of an atom of carbon-12. This method of calculation ensures that the relative atomic mass reflects the isotopic composition of the element in nature.