The average mass of a boron atom is 10.81. assuming you were able to isolate only one boron atom, the chance that you would randomly get one with a mass of 10.81 is

            0 %

Explanation:

                  As we know 10.81 is the average atomic mass of boron which can not be contained by a single boron atom. Therefore, when isolated, a single boron atom will weight either 10 amu for ¹⁰B and 11 amu for ¹¹B respectively.

                 The average atomic mass is calculated as;

Atomic Mass of ¹⁰B  =  10

Atomic Mass of ¹¹B  =  11

Natural Abundance of ¹⁰B  =  20 %

Natural Abundance of ¹¹B  =  80 %

So,

                          Average Atomic Mass  =  (10 × 0.2) + (11 × 0.8)

                          Average Atomic Mass  =  2 + 8.8

                          Average Atomic Mass =  10.8

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