Answer
(a) $^{11}$B
(b) $^{7}$Li
(c) $^{14}$N
Work Step by Step
(a) To determine which of the two naturally occurring isotopes of boron is more abundant, we look at the average atomic mass of boron, which is 10.811 amu from our periodic table. Because 10.811 amu is closer to 11 than to 10, that means that $^{11}$B is more abundant than $^{10}$B.
(b) To determine which of the two naturally occurring isotopes of lithium is more abundant, we look at the average atomic mass of lithium, which is 6.941 amu from our periodic table. Because 6.941 amu is closer to 7 than to 6, that means that $^{7}$Li is more abundant than $^{6}$Li.
(c) To determine which of the two naturally occurring isotopes of nitrogen is more abundant, we look at the average atomic mass of nitrogen, which is 14.007 amu from our periodic table. Because 14.007 amu is closer to 14 than to 15, that means that $^{14}$N is more abundant than $^{15}$N.
