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
• • 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 isotopesIsotopes are represented by writing the mass number as 20Ne, or neon-20 or Ne-20

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

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