Question

Boron has two isotopes, Boron 10 which has a mass of 10.0129 amu and Boron-11 with a mass of 11.0093 amu. B-10 occurs 84.74% of the time and 11 occurs 15.26% of the time. What is the average atomic mass of boron?

Answer 1

The average atomic mass of Boron: 10.431 amu

Further explanation  

Isotopes are atoms whose no-atom has the same number of protons while still having a different number of neutrons.  

So Isotopes are elements that have the same Atomic Number (Proton)  

Atomic mass is the average atomic mass of all its isotopes  

In determining the mass of an atom, as a standard is the mass of 1 carbon-12 atom whose mass is 12 amu  

Mass atom X = mass isotope 1 . % + mass isotope 2.%  + ...

The average atomic mass of boron :

$\tt avg~mass=0.8474\times 10.0129+0.1526\times 11.0093\\\\avg~mass=10.431~amu$