| An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per
atom
of the element to 1/12 of the mass of 12C in its nuclear and electronic ground state. |
| A sample of any element consists of one or more
isotopes
of that element. Each
isotope
is a different weight. The relative amounts of each
isotope
for any element represents the
isotope
distribution for that element. The atomic weight is the average of the
isotope
weights weighted for the
isotope
distribution and expressed on the 12C scale as mentioned above. |
| The standard atomic weights apply to the elements as they exist naturally on Earth, and the uncertainties take into account the isotopic variation found in most laboratory samples. |