Boron exists in two isotopes, boron-10 and boron-11. Based on the atomic mass, which isotope should be more abundant?
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:
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:
And thus the abundance of boron-11 is roughly 81%.