Not all atoms of an element are the same, some elements have isotopes. These are forms of the element with varying numbers of neutrons. Isotopes of an element have different properties and can behave differently than each other. They have the same atomic number because they maintain the same number of protons. Remember, it is the number of protons that determine the element and therefore changing the number of neutrons does not change the element. The mass number changes, however, since the more neutrons an atom has the heavier it gets. There are three isotopes of uranium: uranium-238, uranium-235, and uranium-234. Uranium-238 is by far the most common form of uranium and comprises about 99% of all natural uranium found. Uranium-238 has 146 neutrons, uranium-235 has 143, and uranium-234 has 142. The math is very simple: Isotope number [Uranium-238] minus the number of protons (the atomic number)  equals the number of neutrons  present in that isotope.
This is the first on several posts discussing radioactivity. In the next segment I will discuss radioactive decay and the difference between alpha, beta, gamma, and neutron radiation.