Recall from Chapter 2 that each atom has a nucleus containing protons and neutrons and that the nucleus is orbited by electrons Electrons have a negative electrical charge, and protons have a positive charge. A neutron is actually a proton and an electron combined, so it has no charge (it is neutral).The atomic number (each element's identifying number) is the number of protons in the nucleus.Every element has a different number of protons and thus a different atomic number Atoms of the same element always have the same number of protons, so the atomic number stays constant.Practically all of an atom's mass (99.9%) is in the nucleus, indicating that electrons have virtually no mass at all. So, by adding the protons and neutrons in an atom's nucleus, we derive the atom's mass number. The number of neutrons can vary, and these variants, or isotopes, have different mass numbers. To summarize with an example, uranium's nucleus always has 92 protons, so its atomic number is always 92. But its neutron population varies, so uranium has three isotopes: uranium-234, uranium-235, and uranium-238. All three isotopes are mixed in nature. They look the same and behave the same in chemical reactions.