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