Isotope
Every atom has an atomic nucleus, which is made from protons and neutrons that are held together by the nuclear force. Because protons have positive electrical charge, they repel each other. Neutrons are neutral and they stabilize the nucleus. The neutrons hold the protons slightly apart. This reduces the electrostatic repulsion between the protons, so the nuclear force can hold the protons and neutrons together. One or more neutrons are necessary for two or more protons to bind into a nucleus. As the number of protons increases, so does the number of neutrons needed to have a stable nucleus.