Bromine is a red-orange liquid with an average atomic mass of 79.90 amu. Its name is derived from the Greek word bromos, which means stench. It has two naturally occurring isotopes: Br-79 (78.92 amu) and Br-81 (80.92 amu). What is the abundance of the heavier isotope?

The heavier isotope is Br-81.

to find the percentage abundance, you can use the following equation:

(exact weight of isotope #1) (abundance of isotope #1) + (exact weight of isotope #2) (abundance of isotope #2) = average atomic weight of the element

so we plug in the values, where x is our percentage abundance

(78.92) (x) + (80.92) (1 - x) = 79.90

we solve for x.

78.92X + 80.92 - 80.92X = 79.90

78.92X - 80.92X = 79.90 - 80.92

-2X = - 1.02

X = - 1.02 / - 2 = 0.51 or 51%

51% represents the abundances of the lighters isotope. to find the heaviest, we just subtract 100 to it. 100-51 = 49 %