Answer
Plutonium-239 has 94 protons and 145 neutrons.
$$^{239}_{94}Pu$$
Work Step by Step
1. As its name says, plutonium-239 has a mass number of 239.
A = 239
2. Using a periodic table, we can see that for plutonium: Z = 94 = number of protons
Number of neutrons = A - Z = 239 - 94 = 145
3. Knowing the value of A and Z (239 and 94):
$$^{239}_{94}Pu$$
