Atomic weight and atomic mass are two important concepts in chemistry and physics. Many people use the terms interchangeably, but they don't actually mean the same thing. Take a look at the difference between atomic weight and atomic mass and understand why most people are confused or don't care about the distinction. So, atomic mass and atomic weight may equal each other whenever . In this case, you use the atomic mass in calculations rather than the atomic weight of the element from the periodic table.