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Boron exists in two isotopes, boron 10 and boron 11 . Based on the atomic mass, 10.81 amu ,which isotope should be more abundant?

Answer
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Hint: Since it is given the isotopes of boron, thus we can find the percentage composition of each isotope by using the average atomic mass of the boron. The isotope having greater percentage of composition that isotope is more abundant. We will find the composition of each isotope by calculating average mass.

Complete Step By Step Answer:
We are given two isotopes of boron which are , boron 10 and boron 11 . Isotopes are those elements which have the same atomic number but different mass number. Thus we are also given the average atomic mass of boron as 10.81 amu . For finding the percentage composition of both isotopes in this average atomic mass we will assume the percentage composition of boron 10 as x then the percentage composition of boron 11 will be 100x . Thus the average atomic mass can be represented as:
 10 × x + (100x) × 11100 = 10.81
On cross multiplication and solving we get,
 10x + (100x) × 11 = 1081
 = 1100 - 1081
 = 19
Thus boron 10 is only 19 \%  and the rest is boron 11 . Thus we can say that boron 11 is more abundant than boron 10 .

Note:
The average atomicity of boron 10 and boron 11 theoretically is 10.5 . But practically the average atomic mass of boron is 10.81 amu . Thus it shows that boron 11 has greater abundance in the average atomic mass of boron. The atomic mass is measured in atomic mass per unit which is represented as amu . The atomic number of boron is five and it contains three valence electrons. Since the average atomic mass of boron is near to the atomic mass of boron 11 isotope we can also predict that boron 11 has greater abundance in nature.