Atomic weight

In chemistry and physics (less so for particle, atomic physics), for an element, this is the standard weight of one mole of  that element. A mole consists of an avogadro number or 6.022 141 79  (30) x1023 of atoms, ions or molecules in 12 grams of 12C.

However, relative atomic weight is not always fixed and can vary, due to differences in isotopes of elements.

There is some controversy over the usage of the word even among experts. But for practical laboratory work and calculations it's handy for calculating amounts of elements.

Some atomic weights are given as a range. This is partly because if the element is extracted from one geological location it can have an atomic weight (isotope) more in one extreme to another. For example bromine