Explain to students that two protons repel each other and that two electrons repel each other. But a proton and an electron attract each other. Since opposite charges attract each other, the negatively charged electrons are attracted to the positively charged protons.

Two protons are subject to two forces; the nuclear force and the electrostatic force. When they are very close together the nuclear force is dominant and when they are far apart it is the electrostatic force which is dominant. Somewhere two protons experience zero net force because the two opposing forces are equal.

That is, a proton and an electron will attract each other. The closer they are together, the stronger this attraction will be. Two protons (or two electrons) will repel each other. And again, the closer together they are, the stronger the repulsion.

The strong nuclear force pulls together protons and neutrons in the nucleus. At very small distances only, such as those inside the nucleus, this strong force overcomes the electromagnetic force, and prevents the electrical repulsion of protons from blowing the nucleus apart.

Protons and neutrons aren’t electrically attracted to each other, but when they get close enough they can exchange particles called mesons and become bound together by the strong force. Protons electrically repel each other because of their positive charge.

In a sense, protons and electrons stick together as much as they can. They simply can’t stay together. An electron has a lot of kinetic energy. Its constant motion keeps it in orbit around the atomic nucleus, which contains the protons.

When an electron in an atom has absorbed energy it is said to be in an excited state. An excited atom is unstable and tends to rearrange itself to return to its lowest energy state. When this happens, the electrons lose some or all of the excess energy by emitting light.

When an electron is hit by a photon of light, it absorbs the quanta of energy the photon was carrying and moves to a higher energy state. Electrons therefore have to jump around within the atom as they either gain or lose energy.

The electron can absorb one quantum of energy and jump up to the excitation state. From that excitation state, the electron can then drop back down, releasing a photon with a fixed amount of energy based on the energy lost by the electron when it fell to the lower orbital.

For an isolated atom or molecule in a gas, excited states of electrons often don’t have any easy way to lose energy except by electromagnetic radiation- light of some frequency. A little of the energy can go into internal vibrations for a molecule. In a liquid or solid, however, there are more ways to lose energy.