What is the relationship between atomic mass and mass number?
Let's talk about four atoms, one of hydrogen (
The mass number for hydrogen is 1, for carbon-12 it is 12, and for U-238 it is 238. See--simply counting the number of protons and neutrons in the nucleus.
The atomic masses are slightly different:
(yes--we used carbon-12 as the standard for atomic mass)
Notice how the mass changes for the atoms relative to each other? That change in mass is the difference in the nuclear binding energy! So, if you were to combine 12 hydrogens into one carbon-12 atom you would end up with extra mass. That mass, using Einstein's conversion factor (
Just remember--mass number is a count, atomic mass is a measured quantity.