# Relative atomic mass

- Relative atomic mass (Ar) is the weighted average mass of the isotopes of an element compared to 1/12th of a carbon-12 atom
- Hydrogen weighs 1/12th as much as a carbon-12 atom so has an Ar value of 1
- For example, chlorine exists has two different isotopes naturally, 35-Cl and 37-Cl
- 35-Cl makes up 75% of all chlorine, and 37-Cl makes up 25% of all chlorine
- Therefore, to calculate the Ar of it, you have to multiple the percentages by the isotopes, and divide them by 100:
- ((25 x 37) + (75 x 35)) / 100 = 35.5

- Therefore, to calculate the Ar of it, you have to multiple the percentages by the isotopes, and divide them by 100:

- 35-Cl makes up 75% of all chlorine, and 37-Cl makes up 25% of all chlorine