The average atomic mass of an element is a weighted average of the masses of the isotopes of that element. The weights are proportional to the percent abundance of each isotope in nature. Boron has two naturally occurring isotopes: boron-10 and boron-11.
To calculate the average atomic mass of boron, you should use the following formula:
\( \text{Average Atomic Mass} = ( \text{mass of isotope 1} \times \text{fractional abundance of isotope 1} ) + ( \text{mass of isotope 2} \times \text{fractional abundance of isotope 2} ) \)
Let's say the abundances were something like 20% for boron-10 and 80% for boron-11 (these figures are for the purpose of illustration and are not the actual natural abundances). You would then calculate the average atomic mass as follows:
\( \text{Average Atomic Mass} = (10 \times 0.20) + (11 \times 0.80) \\
\text{Average Atomic Mass} = 2 + 8.8 \\
\text{Average Atomic Mass} = 10.8 \)
In reality, the actual natural abundances of these isotopes yield an average atomic mass of boron that is close to 10.81 amu.
So the correct answer to the question "What is the AVERAGE atomic mass of boron?" would be:
C. 10.81
