The force that a neutron exerts on a proton inside an atom is:

Nuclear force acts between nucleons—protons and neutrons—present inside an atomic nucleus. It is an extremely strong attractive force at nuclear distances and is responsible for holding nucleons together against the electrostatic repulsion between positively charged protons. A neutron is electrically neutral, so electrostatic force does not provide the binding interaction between a neutron and a proton. Nuclear force is a short-range force, effective over distances of the order of a few femtometres (10^-15 m), and is much stronger than gravitational force at nuclear dimensions.