Boron has primarily two isotopes, one with an atomic mass of 11.0 amu and another with an atomic mass of 10.0 amu. If the abundance of the boron atom with a mass of 11.0 amu is 18.9 percent and the abundance of the other isotope is 81.1 percent, what would be the atomic mass of the boron atom

The atomic mass of the boron atom would be 10.135

Explanation:

This is generally known as relative atomic mass.

Relative atomic mass or atomic weight is a physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass of 1/12 of the mass of a carbon-12 atom. Since both quantities in the ratio are masses, the resulting value is dimensionless; hence the value is said to be relative and does not have a unit.

Note that the relative atomic mass of atoms is not always a whole number because of it being isotopic in nature.

Relative atomic mass of Boron = (18.5/100 x 11) + (81/100 x 10)

                                                 = 2.035 + 8.1

                                                 = 10.135