The element boron consists of two isotopes, 105B and 115B. Their masses, based on the carbon scale, are 10.01 and 11.01, respectively. The abundance of 105B is 20.0% and the abundance of 115B is 80.0%.
What is the atomic mass of boron?
Solution: The percentages of multiple isotopes must add up to 100%. Apply the following equation to the problem:
atomic mass = (atomic mass X1) · (% of X1)/100 + (atomic mass X2) · (% of X2)/100 + ...
where X is an isotope of the element and % of X is the abundance of the isotope X.
Substitutie the values for boron in this equation:
atomic mass of B = (atomic mass of 105B · % of 105B/100) + (atomic mass of 115B · % of 115B/100)
atomic mass of B = (10.01· 20.0/100) + (11.01· 80.0/100)
atomic mass of B = 2.00 + 8.81
atomic mass of B = 10.81
The atomic mass of boron is 10.81.
Note that this is the value listed in the Periodic Table for the atomic mass of boron. Although the atomic number of boron is 10, its atomic mass is nearer to 11 than to 10, reflecting the fact that the heavier isotope is more abundant than the lighter isotope.