Model The Bohr Model of the Atom (or Modified Solar System Model): That model of the atom which succeeded the Rutherford Model of the Atom when it was unable to account for the discrete spectrum of light emitted by atoms. The Bohr Model was a planetary model, but allowed only certain discrete possible orbits. Definition Discrete: Separate or individually distinct, consisting of distinct parts or discontinuous elements. Used here to mean the opposite of "continuous" or smoothly varying.

In the animation we show a much simplified Bohr Model of the atom with only three possible orbits (the actual Bohr Model has many, many such orbits.) In the Bohr Model, the larger the orbit, the greater the amount of energy the electron has as it moves in that orbit. In the atom shown, there are only three discrete possible orbits and three discrete possible electron energies to go with them. When the animation begins, the electron will be in the lowest energy orbit and would remain there forever if the electron were not given more energy by some mechanism.

You can "excite" the atom by clicking on the button labeled "Absorb a Photon." Exciting the atom means giving the electron some extra energy. There are two common ways that this can happen. You can bump the atom with an electron that strikes the atom, or you can bump the atom with a photon (light) that is absorbed by the electron in the atom. So, for this animation, the electron absorbs a photon from a continuous spectrum represented on the left, passing through the atom from left to right.

When the electron absorbs a photon, it moves to an orbit with higher energy. Which orbit it chooses depends on how much energy the electron absorbs which is determined by the color of photon it absorbs. Watch for a moment and the electron will randomly absorb a photon. If you watch the excited atom for a few moments, it will also spontaneously "relax," meaning that the electron will jump back down to a lower available orbit. This is also a random choice on the part of the atom. (You can also force the electron to jump down if you wish by clicking on the "Jump Electron Down" button.) When the electron jumps down, it must unload its extra energy which it does by creating and emitting a new photon.

This simplified Bohr atom cannot absorb orange, yellow, or violet photons because those photons don't have the right amount of energy to cause an electron jump. These colors pass right through the atom and can be viewed on the other side. However, red, green, and blue are absorbed. Even if these photons are reemitted (or some other mechanism relaxes the atom), they may go off in some other direction and are missing from absorption spectrum on the immediate right. Absence of color is black, so that if you were looking at the light coming directly through the atom towards you (on the right) you would see the orange, yellow and violet separated by black "lines" where the missing colors should have been. This spectrum is called an "absorption spectrum."

Click here to begin the animation.

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