Question

Difference between atomic percent and weight percent

Answer 1

Atomic percent is based on the number of atoms in a sample. So if the sample has x number of oxygen atoms and x number of iron atoms it would report 50% oxygen and 50% iron (atomic percent). Weight percent is based on the mass of the elements detected. So if we used the above example and reported the results as weight percent we would get 22.3% oxygen and 77.7% iron. Weight percent takes into consideration the mass or atomic weight of the elements and not just the number of atoms. Most people use weight percent although for chemistry atomic percent may be more useful.

Answer 2

at% signifies atomic percentage, so Cu-27at%Al would have 27% of the atoms in the alloy as Aluminium and 73% of the atoms in the alloy as Copper.
wt% signifies weight percentage, so Cu-27wt%Al would have 27% of the weight of the alloy as Aluminium and 73% of the weight of the alloy as Coppet