Subatomic ParticlesSubatomic Particles

Calculating Average Atomic Mass

• expressed in atomic mass units (amu)– done because actual masses so small and

hard to work with– based on carbon-12 assigned mass of

exactly 12 amu

• weighted average of all naturally occurring isotopes.

change % abundances to decimals

for each isotope

Multiply abundance by atomic mass

of isotope

Add products to get

average atomic mass for element

ExampleChlorine has two isotopes. The isotope with a mass of 34.969 amu has a relative abundance of 75.77%. The isotope with a mass of 36.966 amu has a relative abundance of24.33%. Calculate the atomic mass of chlorine.

x 34.969 = 26.49 amux 36.966 = 8.994 amu

Cl37for 24.33% = 0.2433

for 75.77% = 0.7577Cl35

for Cl atomic mass = 35.484 = 35.48 amu