Isotopes and RAM

• what is an Isotope?

  Atoms of the same element with different numbers of neutrons.
• What is a relative atomic mass?

  The average mass of atoms of an element taking into account the mass and amount of each isotope it contains on a scale where the mass of a 12C atom is 12 .
• How can you use isotopes to calculate the relative atomic mass?

  Ar = Sum of 9 isotope abundance x isotope mass number) / Sum of abundances of all the isotopes
• Rubidium is a soft, silvery-white metal that has two common

  isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the

  abundance of 87Rb is 27.8%, what is the average atomic mass of

  rubidium?

  Ar = ( 85 x 72.2 ) + ( 87 x 27.8 )/ 72.2 + 27.8

  6,137 + 2,418.6 / 100

  8,555.6 / 100

  = 85.56
• Uranium is used in nuclear reactors and is a rare element on earth.

  Uranium has three common isotopes. If the abundance of 234U is

  0.01%, the abundance of 235U is 0.71%, and the abundance of 238U is 99.28%, what is the average atomic mass of uranium?


  Ar = ( 234 x 0.01 ) + ( 235 x 0.71 ) / 0.01 + 0.71

  2.34 + 166.85 / 0.72

  169.19 / 0.72

  = 237.98