How can you explain this difference The element boron has an atomic mass of 10.81amu acoording to the periodic tablehowever no single atom of boron has a mass of exactly 10.81amu?

Answer 1

There are several isotopes of Boron, which have different levels of abundance. I don't know the accurate numbers, but say like 25% of Zu-isotopes are 1 amu, and 75% of Zu-isotopes is 2, it would be 1.75 amu for the Atomic Mass. Relating Zu to B of course, what I mean is Boron's different isotopes appear in different abundances and have different masses. 14% of boron may be about 6.882 amu, but just think about it like the above analogy of Zu, where Zu either weighs 1 or 2, but never 1.75.

By the way, to find amu for an element:

(%1*Iso1)+(%2*Iso2)+[...]