Atoms are incredibly small, and their actual masses are also very small. Therefore, we compare the masses of atoms using a relative scale, known as relative atomic mass (Ar). This allows us to compare the masses of different atoms in a more meaningful way.
To standardise this comparison, we use the carbon-12 atom as a reference point. The relative atomic mass of an atom is expressed as a ratio, with one atomic mass unit being equal to 1/12th of the mass of a carbon-12 atom. As the relative atomic mass is a ratio, it has no units.
For example, the relative atomic mass of hydrogen is 1. This means that 12 hydrogen atoms have the same mass as 1 carbon atom. Similarly, magnesium has a relative atomic mass of 24, which means that magnesium atoms are twice as heavy as carbon atoms.
The equation to calculate the relative atomic mass of an isotope is:
Let’s consider the two isotopes of chlorine:
The abundance of chlorine-35 is 75% and the abundance of chlorine-37 is 25%. So, to calculate this:
The element magnesium has three natural isotopes:
Let’s assume that 78.99% of all magnesium atoms are magnesium-24, 10% are magnesium-25, and 11.01% are magnesium-26.
Calculate the relative atomic mass of magnesium to three significant figures