Question

How many neutrons are in $ {}^{239}Pu? $

Answer 1

Hint: Each atom has a specific number of protons, neutrons and electrons that make the atom unique among the ones in the periodic table. These protons, neutrons and electrons are the smallest proved particles that are supposed to be the constituents of the atoms. They are called subatomic particles.
They are arranged in the periodic table according to the number and charge of the protons and electrons in the atom. The protons and neutrons compose the nucleus and the charge on the nucleus has a huge impact on the electronic configuration of each atom.

Complete step by step solution:
We know that the elements are specified by a few sets of numbers that correspond to the characteristic of the element. The atomic number of an element is nothing but the number of protons in the nucleus of the atom, the atomic mass number is nothing but the sum of the number of protons and neutrons in the atom.
The elements with the exact same characteristics but differ in the number of neutrons and thus in the atomic mass number of the element is called an isotope. A few elements have isotopic forms where it has gained a few neutrons in the nucleus that is the usual steady state of the element.
Plutonium is a radioactive element in the periodic table with atomic number 94 but has different mass numbers in different isotopic forms. One such isotope of plutonium has mass number 239.
We know that the mass number is the sum of number of protons and number of neutrons, thus,
 $ N = A - Z $
 $ N = 239 - 94 $
 $ N = 145 $
Thus, the isotope of plutonium $ {}^{239}Pu $ has 145 neutrons in its nucleus.

Note:
Some other isotopes of plutonium have mass numbers: 238,240,241,242 and 244. All of which vary only in the number of neutrons in the nucleus and some of them are fissile.