Calculate the average atomic mass for the following elements.

Boron has two naturally occurring isotopes.

80% of all Boron is B11 (6 neutrons)

20% of all Boron is B10 (5 Neutrons)

Average atomic mass =

Closest whole number =

All boron isotopes have 5 protons.

B11 has 6 neutrons, so its atomic mass is:

6 (1.008664 u) + 5 (1.007276 u) = 11.0884 u

B10 has 5 neutrons, so its atomic mass is:

5 (1.008664 u) + 5 (1.007276 u) = 10.0797 u

So the average atomic mass of boron is:

0.80 (11.0884 u) + 0.20 (10.0797 u) = 10.8866 u

The closest whole number is 11.