The nucleus of an atom contain protons and neutrons. The nucleus is held together by the ‘strong nuclear force’ but it is in opposition to an electrical force from all the positively charged protons in the nucleus which are trying to push it apart. Elements are named based on the number of protons in the nucleus and higher the nuclear number means there is a larger electrical force. Uranium is the largest natural element with 92 protons because any more protons makes the electrical force greater than the strong nuclear force. Bombarding a uranium nucleus with neutrons can eventually split it. When it splits it creates more neutrons which are used to split more of the uranium atoms which creates the chain reaction.

(Book – The making of the Atomic Bomb – Richard Rhodes)