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 %