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Posted 2/27/2006 7:00 PM     Updated 2/27/2006 7:36 PM
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Whence came the atom's energy
Q: Where does an atom get its energy? Apparently they have been in existence for billions of years, resisting forces around them, spinning electrons around, keeping the nucleus packed tight and surely using energy to do all this. Why don't they run out of energy? (Mike, London, England)

A: Atoms are almost as old as time; they got their energy from the Big Bang about 13.7 billion years ago. Sometime in the first three minutes of the Big Bang, all the basic stuff that atoms are made of got created. But things had to cool for about 300,000 years before atoms could form. Then, a proton could capture an electron, and form hydrogen. Some helium was formed then, too, and a scattering of other kinds of atoms but none heavier than lithium. Even now, after star furnaces have produced heavier elements for eons, hydrogen still makes up about 75% of the Universe's atoms. Helium comprises about 24%, and all the other atoms make up the remaining 1%.

As you say, an atom resists forces around it. Even though mostly empty space, an atom wards off other atoms by repulsing the would-be trespasser with an electric force emanating from its electrons. It's a war of surface electrons. If another atom gets close enough (but doesn't combine to make a molecule), the first atom's surface electrons repel the second atom's and keep the second atom at bay. The only reason our feet don't fall through the floor is because the foot atoms push against the floor atoms in this way.

Electrons don't actually spin; rather they have an intrinsic property somewhat misleadingly called "spin." Electrons do, however, have associated discrete energy levels called shells, probability clouds or probability waves. The higher the shell energy level, the farther away the shell is from the nucleus. See Figure 2.

I'm not simply being cavalier when I term the electron energy levels by so many names. We don't know what they are — only how to describe them. This we can do mathematically, though, with incredible accuracy.

The nucleus is, indeed, packed tightly with protons and neutrons. See Figure 3. A couple of forces are at work here. The protons all have the same charge — positive — and therefore repel each other with a huge force because they are so tightly packed together. Protons don't all fly apart, only because another even stronger force holds them together. It's called the strong interaction, and only works over extremely short distances. This force plays out when the distance stretches across the nucleus of a big atom, like uranium. More about this in a moment.

Yes. You are right. It takes energy to do all this (and even more that we haven't talked about). Why doesn't the energy fade away in time and atoms run out of energy? It just doesn't. Probably, because of energy conservation. This is the only thing that we truly know about energy; it is conserved. Let's consider a few energy changes involving atoms.

An atom can absorb energy easily enough if a light particle (photon) hits it with just the right amount of energy to kick an electron from one of its energy levels to a higher one. (That's how a material blocks light.) But the atom doesn't get to keep that energy. Pretty soon the excited electron loses the extra energy by giving off a photon (or many lower-energy photons) and, in one (or many) steps, comes back down to its original rest-energy level — the only stable place for it to exist. No net gain.

How about the energy that binds the nucleus together? Can we lose energy here or gain it? The answer is no for ordinary stable nuclei of light to medium atoms (atoms with less than about 80 protons — gold and below in the Periodic Table of Elements). The binding energy of these lighter atoms is always in its most-bound state; the atoms give up no nuclear energy spontaneously.

For heavier atoms — the answer is sort of, depending on how you look at it. A heavier atom can lose mass and energy spontaneously (called radioactive decay), but in so doing it changes from one element into another. So, it's difficult to say that the atom lost energy because the atom changed into a different atom. Energy was conserved.

Let me explain. Consider uranium U-238 — a stable but uneasy atom. Within a given atom, close-together protons do fine because the nuclear "strong interaction" overcomes their mutual repulsion. But a uranium atom has 92 protons! So some protons are far apart, which is bad news because the strong interaction is weaker at greater distance. The far-apart protons almost manage to repel each other, and escape the nucleus. It's nip and tuck.

Occasionally, this uneasy environment lets the nuclear "weak interaction" toss out a particle (an alpha or beta particle). In so doing, the atom gives off energy in the form of gamma radiation, kinetic energy of the alpha particle and kinetic energy of the recoiling new atom.

When uranium-238 (consisting of 92 protons and 146 neutrons) emits an alpha particle, the nucleus loses two protons and two neutrons (that's what an alpha particle consists of). Then, the nucleus is left with only 90 protons and 144 neutrons. But that's a thorium atom. It no longer is a uranium atom. True, the uranium atom lost mass and energy, but it is transformed into a new element.

Similarly, both fission and fusion change the mass and energy of an atom. But, in so doing, they transform the atom into different kinds of atoms. For example, uranium 235 splits into barium and krypton and releases much energy. Hydrogen isotopes, deuterium and tritium, fuse together to produce a helium atom and much energy.

But a particular kind of atom (like a hydrogen atom) never runs out of energy. Its energy is innate and conserved.


All things are made of atoms. They are the little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. Suppose something terrible were to happen to Earth, and we the living could only pass one sentence to the next generation. What would it be? Nobel Prize winning physicist Richard Feynman in his book, Six Easy Pieces, nominates the first sentence of this paragraph. The concept of atoms is that important.

Further Reading:

Quantum physics by Rod Nave, HyperPhysics

Nuclear physics by Rod Nave, HyperPhysics

An explanation of the Big Bang by April Holladay, WonderQuest

What is energy? by April Holladay, WonderQuest

Atom, Wikipedia

Conceptual physics, ninth edition, by Paul G. Hewitt. San Francisco, CA: Pearson Education, Inc., 2002.

Atoms and the Particle Adventure

Scanning Tunneling Microscope, National Institute of Standards and Technology

Six Easy Pieces by Richard Feynman

(Answered Feb. 28, 2006)


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