Bromine has an atomic number of 35 but has two stable isotopes- bromine-79 and bromine-81. Given that 51% of bromine atoms are bromine-79 and 49% are bromine-81%, work out the relative atomic mass of bromine to the nearest whole number.

What is an isotope? (To ensure the students basic understanding on isotopes) Isotopes are different atomic forms of the same element, which have same number of protons but different number of neutrons. e.g ⁷⁹Br and ⁸¹Br. You can see here they have different mass numbers- so they have the same number of protons and different number of neutrons. To answer this question I would get my student to put the information in a table to help with the equation. Relative mass of isotope Relative abundance (%) Bromine 79 51 81 49
To calculate the relative atomic mass of the Bromine isotopes you use the following formula:
(mass of isotope x it's relative abundance) + (mass of isotope x it's relative abundance) (Sum of relative abundances)
To clarify:Multiply the mass of each isotope by its relative abundance. Add those togetherDivide by the sum of the relative abundance
Using the information in our table, we can apply the above formula:
(79x 51)+(81x49)= 79.98= 80 (nearest whole number) 51+49
Tip: we know the answer is correct as the relative atomic mass for the isotope will be between Br-79 and Br-81, and our answer is 80!