# 1.2.1 Relative Atomic Mass & Relative Molecular Mass

### Relative Masses

#### Atomic Mass Unit

• The mass of a single atom is so small that it is impossible to weigh it directly
• Atomic masses are therefore defined in terms of a standard atom which is called the unified atomic mass unit
• This unified atomic mass is defined as one-twelfth of the mass of a carbon-12 isotope
• The symbol for the unified atomic mass is u (often Da, Dalton, is used as well)
• 1 u = 1.66 x 10-27 kg

#### Relative atomic mass, Ar

• The relative atomic mass (Ar) of an element is the ratio of the average mass of the atoms of an element to the unified atomic mass unit
• The relative atomic mass is determined by using the average mass of the isotopes of a particular element
• The Ar has no units as it is a ratio and the units cancel each other out #### Relative isotopic mass

• The relative isotopic mass is the mass of a particular atom of an isotope compared to the value of the unified atomic mass unit
• Atoms of the same element with a different number of neutrons are called isotopes
• Isotopes are represented by writing the mass number as 20Ne, or neon-20 or Ne-20
• To calculate the average atomic mass of an element the percentage abundance is taken into account
• Multiply the atomic mass by the percentage abundance for each isotope and add them all together
• Divide by 100 to get average relative atomic mass
• This is known as the weighted average of the masses of the isotopes #### Relative molecular mass, Mr

• The relative molecular mass (Mr) is the ratio of weighted average mass of a molecule of a molecular compound to the unified atomic mass unit
• The Mr has no units • The Mr can be found by adding up the relative atomic masses of all atoms present in one molecule
• When calculating the Mr the simplest formula for the compound is used, also known as the formula unit
• Eg. silicon dioxide has a giant covalent structure, however the simplest formula (the formula unit) is SiO2

#### Relative formula mass, Mr

• The relative formula mass (Mr) is used for compounds containing ions
• It has the same units and is calculated in the same way as the relative molecular mass
• In the table above, the Mr for potassium carbonate, calcium hydroxide and ammonium sulfates are relative formula masses
Close Close