Atoms are composed of particles: electrons, protons, and neutrons. Based on their stability, atoms can be any one of four types:
1.) Stable atom: These are the typical atoms. Their constituent numbers of electrons, protons, and neutrons stay the same. For example, a stable sodium atom will always contain 11 protons, 11 electrons, and 12 neutrons.
2.) Isotopes: These are the atoms that have the same number of protons but a different number of neutrons. As a result, they have the same atomic number but different atomic masses. For example, uranium-235 and uranium-238 are isotopes of uranium. Both of these have 92 protons, but U-235 has 143 neutrons, while U-238 has 146 neutrons.
3) Radioactive atoms: These atoms are unstable and achieve stability by emitting excess particles. Uranium is a radioactive atom.
4) Ion: When an atom gains or loses an electron or electrons, it becomes an ion. For example, the chlorine atom accepts 1 electron to become the chloride ion.
Hope this helps.