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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%.
A single cobalt atom (purple peak) over a copper surface
(orange, yellow, and pink). A
scanning tunneling microscope took this image. Courtesy of National
Institute of Standards and Technology, Electron and Optical Physics Division
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.
Figure 2. Energy levels of a hydrogen atom (yellow is
lowest energy, red highest). The nucleus is shown as the center blue blob, and
an electron (green) is shown at its highest energy state, farthest from the
nucleus. It will not last long in this state; soon it will emit a photon, and
return to its lowest energy state (yellow ring). Figure by the author.
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.
A model of the helium atom, showing the nucleus (center
glob), which contains two protons (blue) and two neutrons (red). Two electrons
(waves) orbit the nucleus. Courtesy of Wikipedia
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. Lets 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 — 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 nominated the above sentence. The concept of atoms is
important.
Richard Feynman on the book cover of Six Easy Pieces.
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)
Reader's Comments
- The question was: 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?
Good answer but the question has a basic misunderstanding. The energy
of the big bang set everything in motion, but the motion in atoms
after that requires no new energy input, because that is the result
of inertia and the attractions within the atoms don't require energy
either, because that is a result of the four forces of the universe,
gravity, electromagnetic, strong and weak. Atoms interact and energy
is transferred and transformed, matter is conserved and transformed,
but there is not an input of energy that keeps them from running out
of energy.
Of course some of the energy in the universe is being transferred
from the matter created in the beginning, in the fusion reactions of
stars, a small bit of mass is being transferred into energy. But this
is not a creation of energy, it is within the laws of conservation of
matter and energy, with the ultimate equation being E=mc2. That
energy is transformed in various manners but it is not why the atoms
have motion in the first place, and is not required to maintain the
motion.
You did touch on that in the last couple paragraphs.
A simplistic explanation (the really complex being beyond me) but
hopefully clears up a misunderstanding in the assumptions of the
question in the first place.
Eric
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