(republished on 07/21/06)
It's great to hear you have such a curious 6 year old! When he asks about what they look like, there are at least two sides to the question. One is what shape they are and the other is their color.
It turns out that visible light is no good for seeing the shape of atoms, because its wavelength is way bigger than the atom. In order for the shape to be clearly seen, an object has to be larger than the wavelength of light. The wavelength of visible light is about 400 nanometers to 700 nanometers (a nanometer is one billionth of a meter or about 1/15,000 to 1/200,000 of a human hair) but the largest atom is around 270 picometers in diameter or more than 1000 times smaller than visible light. So, while individual atoms can reflect visible light, because atoms are so much smaller than the light itself, you will never be able to see any more than a fuzzy dot.
In order to clearly visualize atoms, a scanning tunneling microscope is used which is a microscope that uses electrons instead of light to see very small objects. The resolution of these microscopes is good enough so that individual atoms can be seen as bumps. Some of the most famous pictures of individual atoms is the letters "IBM". Beyond actual images, you could take a look at some earlier responses to what atoms look like and some of the links provided here. One picture of the letters "IBM" spelled from atoms is below.
As for the color, atoms can have visible colors, altough that's hard to see unless you have a lot of them. By an object's color, people sometimes mean the color of the light that bounces of it, and sometimes they mean the color of the light that it emits.
Some atoms have very clear visble colos of emission, such as the sodium atoms that emit yellow light in some street lights. So when you see the yellow glow from a streetlamp, that could be the color of sodium atoms! Some others don't usually emit visible light.
All atoms will scatter some of the light that hits them. Usually they scatter more blue light than red. The molecules (pairs of atoms) in the air do that, and that's why the sky is blue. Any one atom or molecule doesn't scatter much, though.
Just as atoms can emit light, they can also absorb light. The colors you see when light goes through a collection of atoms that can absorb are just the colors that that particular type of atom doesn't absorb.
Erik +Mike W.
(published on 05/18/13)