# Relative Mass

## Relative Atomic Mass

• average mass of an atom of an element on a scale where an atom of carbon-12 is exactly 12
• Ar of each element is shown in periodic table
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## Relative Isotopic Mass

• mas of an atom of an isotope of an element on a scale where an atom of carbon-12 is exactly 12
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## Calculating Relative Atomic Mass

• relative isotopic mass - usually whole number
• relative atomic mass - average so not whole number
• given percentage
• multiply % by isotopic mass & divide by the total %
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## Relative Molecular Mass

• average mass of a molecule on a scale where an atom of carbon-12 is exactly 12
• to find Mr, just add up relative atomic mass values of all the atoms in the molecule
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## Relative Formula Mass

• average mass of a formula unit on a scale where an atom of carbon-12 is exactly 12
• used for compounds that are ionic or giant covalent
• to find relative formula mass, just add up relative atomic masses of all ions in formula unit
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