Isotope
 Atomic nuclei are protons and neutrons held together by the strong nuclear force. Because protons have a positive charge, they push against other protons. Neutrons are electrically neutral, and they attract the protons more than the protons push against other protons. This makes the nucleus more stable. So, we need neutrons for more than two protons to be in a nucleus. As there are more protons, more neutrons are needed to have a stable nucleus.