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Name: Beth
Status: other
Grade: N/A
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Date: 8/2/2005

How can you explain why an atom emits a definite line spectrum of only certain energies instead of a continuous spectrum of all energies?

An atom will emit line spectra when the electron is moves from one "bound" state to another "bound" state. Atoms in bound states are limited to specific energies described by the quantum mechanics of atoms. If an atom is excited into an "unbound" state the energy is no longer "quantized" and a continuous spectrum results. A detailed explanation of atomic spectra is way too long for a medium such as NEWTON. You can find treatments of varying complexity in chemistry texts. The recent book titled: "Hydrogen: The Essential Element" by John S. Rigden gives a reasonably non-mathematical treatment of the spectrum of the simplest atom -- its history and the people involved.

Vince Calder

Hi Beth,

I found a decent explanation in College Physics, sixth edition - Authors Franklin Miller Jr. and Dietrich Schroeer - Published by Harcourt Brace Jovanovich 1987 - page 783. (sorry it's an old book, but so am I)

" In a solid, a liquid, or a dense gas, the atoms are close to each other, and they interact to give a continuous spectrum with all frequencies (and wavelengths) represented. However, the spectrum emitted by atoms of an incandescent gas is discrete; that is, there are sharp lines of definite wavelength, with no light emitted at the intermediate wavelengths. Presumably the atoms in a gas are free enough to emit their own characteristic frequencies, much as a piano string vibrates with certain natural frequencies. Some energy must be supplied to start a string vibrating; likewise, an atom must absorb some energy in order to emit radiation."

I do not know if this explanation makes things better or worse. I would hit the library, go down into the reference section. (Why is it that every library I have ever been to has the reference section in the basement?) Anyway, look up emission spectra. If the explanation makes no sense, move on to another textbook. There is probably something out there that you can "tune into".

If I find a better explanation, I will send it on.

Happy Hunting!

Martha Croll


Atoms emit the line spectrum energies from their electrons. The electrons exist in atoms at specific energy levels. The electron orbit's distance from the nucleus and the electron's rotation about its own axis are the two greatest factors. Due to quantum mechanics rules of the universe, only specific orbits and spins are allowed. Also, only one electron per electron can exist in each state.

When energy, in any location, bumps an electron into a higher energy level, it does not stay there forever. It can drop back down very quickly to a lower energy level. When it drops down, the electron emits a unit of radiation, called a photon. This photon is a unit of what we call light. Every atom has a different arrangement of electrons and a different set of energy levels. As a result, every atom emits a different set of photon energies.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Dear Beth,

You have put your finger on one of the main questions that led to the understanding of quantum mechanics. Classically, using the laws of mechanics described by Newton, atoms should behave much like the solar system where the planets can exist in orbits that differ by arbitrarily small amounts from one another. For example, the earth can circle the sun in an orbit with a radius of 93 million miles or in an orbit with a radius of 92.999999999 miles. In atoms the electrons can circle the nucleus only in certain "states" which can be calculated using the rules of quantum mechanics. An electron can "jump" (make a transition) from one such state to a lower energy state while emitting a photon (quantum of light) with an energy equal to the difference in the energies of the two states. This is hard to believe and even harder to picture but, as you know, the experimental evidence is totally convincing.

To develop a deeper understanding, look for the Bohr model of the hydrogen atom in any elementary textbook. If you have familiarity with algebra, you will find the mathematics quite simple. The model pictures an electron circling a proton very much like the earth circles the sun, but then adds a quantum condition that only angular momenta which are integer multiples of Planck's constant divided by two pi are allowed. With this bit of magic, the line spectrum emitted by the hydrogen atom can be calculated almost exactly.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

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