What experimental evidence led to the development of this atomic model from the one before it?

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University of Chicago

Susan C.

Biology

5 months, 4 weeks ago

Thomson took the idea of the atom and tried to incorporate the evidence for the electron. In this model, the electrons are the small things and the rest of the stuff is some positive matter. This is commonly called the plumb pudding model because the electrons are like things in positive pudding.

Rutherford Scattering

Ernest Rutherford said one day "hey, I think I will shoot some stuff at atoms." I am sure his wife said "oh, Ernie" (she probably called him Ernie) "if it makes you happy to play with your little physics stuff, go ahead. I know how much you like it." So he did. He shot some alpha particles (which are really just the nucleus of a helium atom) at some really thin gold foil. Here is a diagram of his experiment.

If you shoot these positive alpha particles at this positive pudding atom, they should mostly bounce off, right? Well, that is not what happened. Rutherford found that most of them went right through the foil. Some of them did bounce back. How could that be if the plumb pudding model was correct? Rutherford's experiment prompted a change in the atomic model. If the positive alpha particles mostly passed through the foil, but some bounced back. AND if they already knew that the electron was small and negative, then the atom must have a small positive nucleus with the electrons around them.

Bohr Model

The model proposed by Niels Bohr is the one that you will see in a lot of introductory science texts. There are a lot of good ideas in this model, but it is not the one that agrees with all of the current evidence. The model tries to make a connection between light and atoms.

Suppose you take some light and you let different colors bend different amounts (think rainbow). This way, you could see what colors are present for different light sources. Here are three different light sources.

Maybe the light from the light bulb is what you would expect. These are the colors of the rainbow. However, suppose you took some hydrogen gas and excited it. There would only be certain colors (only certain wavelengths) of light produced. If you shine light through some hydrogen gas, there will be dark bands of light at those same colors.

So, Bohr said that these colors of light in the hydrogen gas correspond to different energy levels the electron in hydrogen can have. And this is the key to the Bohr model - electrons can ONLY be at certain energy levels in the atom. This is crazy (at least it was crazy for its time). Think about a planet orbiting the Sun. It can be at any energy level. In this case, there is a gravitational force attracting the planet which produces orbital motion. This will work anywhere in the solar system.

Early physicist thought of the electron in an atom a lot like a planet orbiting the Sun. The key difference is that the electron (in the Bohr model) orbits due to an electric interaction and not a gravitational interaction. Well, the other difference in the Bohr model is that the electron can not orbit (if it does orbit, which it doesn't) at any distance and any energy. Here is the essence of the Bohr model.