A nucleus of each chemical element consists of protons, neutrons and electrons. The mass number of an element refers to the sum of the number of protons and neutrons. However, the majority of elements exists as isotopes. Isotopes have the same number of protons but they vary in the numbers of neutrons. For instance, one isotope of oxygen has eight protons and eight neutrons, while another isotope comprises eight protons and 10 neutrons. Bromine belongs to the group of halogens and exists as two isotopes having 44 and 46 neutrons.
Navigate to the periodic table of chemical elements.
Locate the bromine element that has the symbol "Br" in the group "VIIA" of the periodic table.
Read the atomic number given above the element symbol. For bromine, the atomic number is "35." Note that the atomic number is equivalent to the number of protons as well as to the number of electrons.
Add up the number of protons acquired from Step 3 and the number of neutrons to calculate the mass number of bromine. For this bromine isotope, the mass number is 35 + 46 or 81.