Am is Americium, with an atomic number of 95. This means that 1 atom has 95 protons.
The "239" given is the number of nucleons, i.e. protons + neutrons in an atom
So, the nucleon number is 239. This is most commonly known as the atomic mass, as it is the protons and the neutrons that contribute more than 99% of an atom's mass
Now that we know the total number of nucleons, and the number of protons, we can easily work out the number of neutrons
This is done by subtracting the number of protons from the number of nucleons.
So, 239 - 95 = 144 neutrons
Don't hesitate to ask if you have nay further questions!
Hope i helped!! xx
