A simple model of the hydrogen atom suggests that the electron orbits the proton. What is the force that keeps the electron in orbit?

A.     Electrostatic

B.     Gravitational

C.     Strong nuclear

D.     Centripetal

A

The question asks “What is the force?”, it does not ask “What is the nature/direction of the force?”. To distinguish the two it is probably best, when teaching, to use the adverb “centripetally” rather than centripetal. So we would say “The electrostatic force acts centripetally”. In this way the candidates understand that there are many different forces, all of which can act centripetally under certain conditions.