Boron has an atomic mass of 10.81 amu according to the periodic table However, no single atom of boron has a mass or 10.81 amu. How can you explain this difference?
The mass number given on the periodic table is the average mass of all the isotopes of an element. Since boron has isotopes, the periodic table will give its average mass, which is done by scientists calculating the total mass of all known boron isotopes, and then dividing it by the number of boron isotopes.
Have a look on the periodic table here:
Most common isotopes of boron are boron-10 and boron-11.