1.2.1

# Relative Masses

Test yourself

## Relative Atomic Mass

Relative atomic mass (Ar) is the average mass of an element compared to one-twelfth the mass of a single carbon-12 atom.

### What does this mean?

• 1amu is defined as one-twelfth of the mass of a single carbon-12 atom.
• So Ar is the average mass of an element in units of amu.

### What is the average mass?

• Almost every element has multiple stable isotopes.
• This means that some atoms of an element will weigh more than others.
• The average mass is the average of the masses of the elements, weighted according to their abundance.
• E.g. 35Cl is 75% abundant and 37Cl is 25% abundant.
• Ar = (0.75 × 35) + (0.25 × 37) = 35.5

## Relative Molecular Mass

Relative molecular mass (Mr) is the average mass of a molecule compared to one-twelfth the mass of a single carbon-12 atom.

### What does this mean?

• 1amu is defined as one-twelfth of the mass of a single carbon-12 atom.
• So Mr is the average mass of a molecule in the units of amu.

### What is the average mass?

• Recall that the Ar takes into account the differences in mass and abundance of isotopes.
• This means that we can simply add the Ar values for all the atoms in a molecule to calculate the Mr!