Hey there!
The mass number is the total count of protons and neutrons in the nucleus of the atom.
The atomic mass is the weighted average of mass number. Many elements have isotopes, which have different amounts of neutrons, so the mass number is different for each isotope.
If boron has an atomic mass of 10.811, that means the weighted average of mass numbers for boron is 10.811 amu. One mole of boron will also have a mass of 10.811 grams.
Hope this helps!
