# 1.2.1 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 Masses

#### 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

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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