This force must be very strong because it holds charged particles very close together. It is also short ranged. The most tightly bound of all the nucleii belongs to iron. Just as electrons are the glue that hold groups of atoms together, neutrons are the glue that hold the nucleus together. There is at least as many neutrons in a nucleus as there is protons.
Heavy elements have more neutrons than protons.
Discovered by Antoine Bequerel in 1896 with a photographic plate exposed by radiation from minerals in a drawer. Image of coin. Much of the early research was conducted by Marie Curie, the first 2 time Nobel prize winner. She died from radiation exposure. There are 3 types of radioactive decay.