Inquiring Minds

Physics Questions People Ask Fermilab

Mr. Pordes-

I have a question for science. As you probably know, we have been studying all about particles and the particle model of matter and John Dalton and everything like that. My question is though, are all atoms the same color? And if so, or even if not, how do you know?! Since they are so small, how can you see what color they are. Or do you not know? Thank you so much for your time! If you could write back with an answer, I would appreciate it!

Thanks!

Sincerely,
Kelly Hall

Dear Kelly,
I am answering your question below..it's a very good question (this is Mr. Pordes)

OK..so what color is an atom and how could you see what it is..

This question is a very, very good one. Here is my answer and an explanation. I hope it's not too long.

1. atoms (as opposed to molecules) do not have colors - they are clear except under special conditions..
2. molecules have colors
3. you could not see the color of one atom or molecule - not because it is too small - but because the color of one atom would be too faint. How could you check the color of an atom or a molecule? Get a large number of them and shine light on them.

First of all how do we see things? We see things because they reflect light into our eyes.

Next what is the color of something? The color of something is the color of light that it reflects most when light of all colors shines on it. A green leaf is green because when the sun shines its light on the leaf, the color that the leaf reflects best to our eyes is the green. The sun's light contains all the colors of the rainbow (red orange yellow green blue indigo) but the color that is reflected most to your eye is the green.

The part of an atom or molecule that "reflects" the light is the electrons on the outside of the atom. Now..the color comes in two steps..

Step 1: The electrons first absorb some of the light that hits the atom or molecule.

Step 2: The electrons which absorbed the light then emit (give out) some light

If the electrons give out exactly the same light as they absorb, the substance is "colorless"..however if the color of the light emitted is different from the color absorbed, then the substance has a color. In the case of the leaf, the leaf absorbs all the colors but only reflects green.

For atoms and regular lighting - whatever the atom - the absorbed light and the emitted light are the same. That is because for a single atom the electrons have to absorb and emit the same light.

In molecules, where two or more atoms share some of their electrons, the molecules can absorb light of one color and emit another color. This works whether the atoms are the same (eg two Nitrogen atoms) or different elements. Paints and dyes (the chemicals used to color clothes) are complicated molecules where the arrangement of the electrons make the molecules reflect specific colors very well.

Here are some questions you might want to ask.

Why is the sky blue? Does that mean that oxygen is blue or nitrogen is blue?
Why is chlorine gas green?
Why are neon signs red?

Stephen Pordes

Dear Kelly,

I hope you have time to read a correction I have to what I wrote. (You can see how we enjoy thinking about a good question).

It turns out that there are some elements whose atoms DO have a color.. (I said there aren't any - and, guess what, I was wrong).

Now this doesn't mean the atoms have little dabs of color paint on them - it means that when you shine white light on them, their electrons DO absorb some particular color and then re-emit it. How this gives the atoms a color is like this. Picture a glass-jar with some of these atoms floating around in it. When you shine a flashlight at the jar, most of the light will just go right through the jar - you could see the light coming out and making a spot on a wall for example. But the inside of the jar will also seem to glow with a definite color.

This is the color that the atoms are absorbing. The light of that color doesn't all go straight through the jar; some of it gets absorbed by the atoms floating around which then emit it (send it back out) in ALL directions - not just along the line of the flash-light beam. So if you look at the jar from the side, say, you WILL see light of a particular color and the inside of the jar will look like it is glowing. Sodium (Na in the periodic table) does this and gives a bright yellow glow. There may be some other elements, too. I will check on them.. If I get a complete list, I will send it to you.

sorry to make you read so much - and for the wrong answer the first time -

ps.it's not very easy to demonstrate this with sodium..you would need to heat the sodium in an evacuated jar. (Evacuated means all the air has been pumped out). The heat is needed because sodium is a solid at room-temperature and to get enough sodium atoms floating around you would have to get the sodium really hot; you need to get the air out of the jar because sodium reacts very strongly with the oxygen (it would catch fire) in the air.

Stephen Pordes

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