Isotopes and Atomic Mass (weight)

• What is the connection between an element’s isotopes and its atomic mass?
• The observed mass of an element is a weighted average of the weights of all the naturally occurring atoms.
• Average mass = atomic weight (amu)
• What information is needed to determine the atomic mass of an element?
• The natural abundance of each of the element’s isotopes
• The percentage of an element that is one isotope is called the isotope’s natural abundance.
• Example problem
• Boron has two isotopes.
• Boron is 19.9% 10B and 80.1% 11B.
• Boron atomic weight = 0.199 (10.0 amu) + 0.801 (11.0 amu) = 10.8 amu