Chapter 2 - Atoms, Molecules, and Ions - Questions and Exercises - Exercises - Page 88: 2.128

94 protons 94 electrons 145 neutrons

We know that the mass number of this isotope of plutonium is 239. From the periodic table, we know that plutonium has an atomic number of 94. In a neutral atom, the number of protons equals the number of electrons; therefore, this neutral isotope of plutonium has 94 electrons as well. To get the number of neutrons, we subtract the atomic number (94) from the mass number given (239), and we get 145.