Nuclei are the basic building blocks of atoms, which are the fundamental units of matter in the known universe. Their behavior is linked to the physical and chemical properties of elements, and they play a pivotal role in the structure and behavior of matter. Without nuclei, matter would not exist in its recognizable form. Nuclei are made up of protons and neutrons, which are bound together by an attractive force known as the strong nuclear force. This force is hundreds of times stronger than the electromagnetic force that binds electrons to protons in an atom and is responsible for a nucleus's stability. The number of protons in a nucleus determines its atomic number, which is what differentiates each element. Most elements have several isotopes, which have the same atomic number but a different number of neutrons. Nuclei are responsible for releasing the energy that powers stars, nuclear reactors, and weapons. It is through nuclear reactions, such as fission and fusion, that nuclei are able to release vast amounts of energy. Nuclei can also undergo radioactive decay, releasing particles and energy in the process. This process is what powers natural sources of radiation. Nuclei also help determine the size and shape of atoms, as well as their reactivity, due to their number of protons and neutrons. In summary, nuclei are the basic building blocks of atoms and the fundamental units of matter. They possess an attractive force that binds protons and neutrons together and the number of protons in a nucleus determines its atomic number. Furthermore, nuclei are responsible for releasing energy from nuclear reactions and radioactive decay, which powers stars and weapons. Finally, nuclei help determine the size and shape of atoms as well as their reactivity.