The Energy of an Atom

An atom consists of a nucleus, which is made up of positively-charged protons and uncharged neutrons, surrounded by a cloud of negatively-charged electrons. To give you an idea of the scale of things, a proton and a neutron have about the same mass, while an electron has about 1/2000 of that mass. An electron, being so much lighter and moving quickly, cannot occupy as small a volume as the nucleus. You can identify the location of a nucleus (which is heavy) with a certain degree of accuracy, but the uncertainty in the location of any electron (which is light) belonging to the same atom is many times larger. The best you can say is that it is very likely to be within a certain distance of the nucleus. This is a consequence of the Heisenberg Uncertainty Principle. The diameter of a typical nucleus is about 10-15 meters, while the electrons whirl about it at a typical distance of about 10-10 meters. That's like the nucleus being the head of a pin in the middle of a stadium, and the electrons spending most of their time out in the cheap seats. The space between the electrons and the nucleus is completely empty.

Why do the electrons stay with the nucleus, then? It's because of their electrical charges. The electron's negative charge is attracted to the positively charged proton(s) in the nucleus. And, while an electron can be removed from an atom (in a process called ionization), that process takes a lot of energy.

An electron in an atom has a certain, definite energy. Because the electron is so much lighter than the nucleus, it must move around in a much larger volume than the nucleus does. This means that an electron is always moving, so it always has some kinetic energy. And because it is always at some distance from the nucleus, it always has some potential energy due to the electrical attraction between electron and nucleus. The sum of the kinetic and potential energies is the total energy of the electron. The instantaneous total energy of the electron is constant, as long as nothing is applying a force to it.

If an atom has more than one electron, we can call the total energy of the atom the sum of all of the individual electron energies. It turns out that the total energy of an electron in an atom is only allowed to have certain definite values. We call these definite values energy levels. Each of the electrons in an atom has a set of allowed energies. The total energy of the atom, then, can only have certain values, equal to the sums of all the possible electron energies.

When an atom's energy changes, it may only go from one allowed energy level to another allowed energy level. The atom's energy can change in any of a number of processes, including collisions with other atoms, and by absorbing or emitting photons.