To calculate the relative atomic mass (Ar) of an element, multiply the mass of each isotope by its abundance, add the results, and divide by the total abundance. The abundance can be given as a percentage or as a ratio. If it is a percentage, the total is 100.
Worked example for chlorine. Chlorine is 75% chlorine-35 and 25% chlorine-37. Ar = [(35 × 75) + (37 × 25)] ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5.
Second example for boron. Boron has isotopes of mass 10 and 11. If 20% of the atoms are boron-10 and 80% are boron-11, then Ar = [(10 × 20) + (11 × 80)] ÷ 100 = (200 + 880) ÷ 100 = 10.8. A good check is that the answer must lie between the masses of the isotopes and be closer to the more abundant one. If the two isotopes are present in equal amounts, the answer is exactly halfway between their masses.
Neon, with 90% of mass 20 and 10% of mass 22, has Ar = (1800 + 220) ÷ 100 = 20.2. Relative atomic mass is a ratio, so it has no units. Atoms are tiny, about 0.1 nanometres (1 × 10−10 m) across.