Atomic Mass Units
Definition
An atomic mass unit (AMU, dalton, Da, unified atomic mass unit, u) is defined as a mass equal to one twelfth the mass of an atom of carbon-12
Actually, historically speaking, we shouldn't say "atomic mass unit", as it used to refer to
Figure
Atomic mass units for each element are often on the periodic table. In this table, they're labeled "standard atomic weight"
Average Atomic Mass
Formula
Average atomic mass is calculated as:
where:
is the percentage abundance of that particular isotope is the mass of that particular isotope
Example
Copper has two major naturally occurring isotopes, one with 34 and one with 36 neutrons. Determine the percentage abundance of copper atoms with 34 neutrons.
solution
Atomic number of copper is
- Mass of
: - Mass of