Why are protons in the nucleus of an atom and not electrons?

Excellent question, with wide reaching consequences! Electrons belong to a class of particles called Leptons, which do not interact via the strong nuclear force (the strongest fundamental force). Due to their electrostatic repulsion, electrons cannot be confined to the nucleus. read more

An atom contains protons, neutrons, and electrons. The nucleus of an atom consists of bound protons and neutrons (nucleons). The negatively-charged electrons are attracted to the positively-charged protons and fall around the nucleus, much like a satellite attracted to the gravity of the Earth. read more

The nucleus of an atom cannot contain electrons because electrons do not respond to the force that holds it together. This force is called the"residual strong force", and has no effect on electrons. It does, however, affect neutrons, and any neutrons contained in an atom are confined to the nucleus by this force. read more

A many electron atom might seem to make the situation better since there are more protons and a higher electric charge, but in a many electron atom there is repulsion between the atomic electrons to deal with as well. read more

Uncharged atoms pair equal amounts of electrons and protons. The electrons should zoom right to the atomic nucleus. In classical physics, electrons should fall into the nucleus even if they weren't attracted by the charge of the proton. Early models of atoms had electrons orbiting the nucleus the way a planet orbits a sun. read more