## Chemistry: Atoms First (2nd Edition)

If we are given an atom's mass number, then we can find out how many neutrons that particular atom contains. We can find the atom's atomic number by looking at the periodic table. The atomic number is also the number of protons in an atom. The mass number is the sum of the number of protons and the number of neutrons in the atom. To find the number of neutrons in an atom if we are given the mass number, we just subtract the number of protons from the mass number. a. The mass number of this atom of bromine is 79. If we subtract the atomic number of bromine, 35, from this mass number, we get 44, which is the number of neutrons in $^{79}$Br. b. The mass number of this atom of bromine is 81. If we subtract the atomic number of bromine, 35, from this mass number, we get 46, which is the number of neutrons in $^{81}$Br. c. The mass number of this atom of plutonium is 239. If we subtract the atomic number of plutonium, 94, from this mass number, we get 145, which is the number of neutrons in $^{239}$Pu. d. The mass number of this atom of cesium is 133. If we subtract the atomic number of cesium, 55, from this mass number, we get 78, which is the number of neutrons in $^{133}$Br. e. The mass number of this atom of hydrogen is 3. If we subtract the atomic number of hydrogen, 1, from this mass number, we get 2, which is the number of neutrons in $^{3}$H. f. The mass number of this atom of iron is 56. If we subtract the atomic number of iron, 26, from this mass number, we get 30, which is the number of neutrons in $^{56}$Fe.