Answer
Boron-11
Work Step by Step
From the Periodic table the mass of Boron (B) is $10.811 u$.
Based on the question:
Boron-10 $= 10.0129370 u$
Boron-11 $= 11.0093054 u $
Through observing the masses given, $11.0093054$ is closer to $10.811$ than $10.0129370$ , therefore, boron-11 is the more abundant isotope.
If you cannot tell which one is closer just by looking at the numbers you can subtract:
For Boron-10 : ( $10.811 - 10.0129370 = 0.798063$ )
For Boron- 11 : ( $ 11.0093054 - 10.811 = 0.1983054$ )
The difference in the average atomic mass of boron and the mass of the Boron-11 isotope is smaller than the the difference in the average atomic mass of boron and the mass of the Boron-10 isotope. This suggests that the mass of the boron-11 isotope is closer to the average atomic mass of all the boron isotopes and thus is the more abundant of the two.
