relative atomic mass

• The relative atomic mass (Ar) is the average mass of atoms of an element relative to the mass of an atom of carbon-12 (which is given a mass exactly of 12). The average mass must take into account the proportions of naturally occurring isotopes of the element
• Scientists decided that the atomic mass of an atom of an element would be compared to carbon-12. Therefore the atomic mass is known as relative as it is being compared to the mass of carbon-12. If the Ar of an atom is lower than 12 it has a mass smaller than carbon-12 atom.
• You can find the relative atomic mass of an element on a periodic table by looking at the number directly above the element symbol. For example the relative atomic mass of Copper (Cu) is 29.
• Relative atomic mass


  The average mass of an atom, taking into account the masses of each isotope and their proportions in the environment
• To calculate the relative atomic mass

  1. Identify the isotopes and their abundance

  2. Identify the mass number of each isotope

  3. Write out the relative atomic mass formula

  4. Substitute identified data and work out the answer
• Practice Question


  • A sample of chlorine gas is a mixture of 2 isotopes, chlorine-35 and chlorine-37, occurring in 75% and 25% proportions respectively
• The relative atomic mass of chlorine in the sample is 35.5
• 35.5 is closer to 35 compared to 37 as chlorine-35 is more abundant than chlorine-37
• You need to be able to rearrange the relative atomic mass equation to work out a missing abundance or mass
• How to Calculate Relative Atomic Mass

  The relative atomic mass of an element is the average mass of an atom, and it takes into account the masses of each isotope and their proportions in the environment.

  To calculate the relative atomic mass you require the following information:

  • The abundance of each isotope which tells you the quantity the isotope is present in the environment. This is different for each isotope of an element.
  • The mass number of each isotope of a particular element. The number of protons in all the isotopes of an atom remains the same, it is only the number of neutrons.
• How to calculate relative atomic mass example

  1. Identify the isotopes

  2. Identify the mass number

  3. Write out the relative atomic mass formula

  4. Substitute identified data
• Relative atomic mass


  The average mass of the atoms of an element, taking into account the masses and abundances of the element's isotopes
• Chlorine isotopes


  • Chlorine-35
  • Chlorine-37
• The chlorine sample is a mixture of 75% chlorine-35 and 25% chlorine-37
• The relative atomic mass of chlorine in this mixture was 35.5
• Chlorine-35 is more abundant than chlorine-37


  The relative atomic mass is closer to 35 compared to 37
• You need to be able to rearrange the relative atomic mass equation to work out a missing abundance or mass