Explanation:
The atomic mass of boron is 10.81 u.
And 10.81 u is a lot closer to 11u than it is to 10u, so there must be more of boron-11.
To convince you fully, we can also do a simple calculation to find the exact proportion of boron-11 using the following formula:
(
10
u
)
(
x
)
+
(
11
u
)
(
1
−
x
)
100
%
=
10.81
u
Where u is the unit for atomic mass and x is the proportion of boron-10 out of the total boron abundance which is 100%.
Solving for x we get:
11
u
−
u
x
=
10.81
u
0.19
u
=
u
x
x
=
0.19
1
−
x
=
0.81
