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Does an atom have a color?
Category: Physics
Published: January 17, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
The answer really depends on how you define "having a color". The term "color" refers to visible light with a certain frequency, or a mixture of visible light frequencies. Therefore, the word "color" describes the frequency content of any type of visible light. Anytime visible light is present, we can describe it as having a certain color. With this in mind, there are many different ways an object can reflect or emit visible light. Thus, there are many ways an object can "have a color". While a single, isolated, atom can reflect or emit visible light in several of these ways, it does not participate in all the ways. If you define "having a color" very narrowly such that it only includes certain mechanisms, then atoms do not have color. If you define "having a color" more broadly, then atoms do have a color. Let us look at the different ways an object can reflect or emit visible light and apply each one to an atom.
1. Bulk reflection, refraction, and absorption
The most common, everyday manner in which objects can send visible light to our eyes is through bulk reflection, refraction, and absorption. These three effects are all part of the same physical mechanism: the interaction of an external beam of light with many atoms at the same time. When white light, which contains all colors, hits the surface of a red apple, the light waves that are orange, yellow, green and blue get absorbed by the atoms in the apple's skin and converted to heat, while the red waves are mostly reflected back to our eyes. Some of the light is also transmitted through the apple skin and bent slightly as it goes through. We call this bent transmission of light "refraction". Some materials such as glass transmit a lot of the light while other materials such as apples transmit very little.
The key point here is that traditional reflection, refraction, and absorption constitute a bulk phenomenon where each ray of light interacts with dozens to millions of atoms at the same time. This makes sense when you consider that visible light has a wavelength that is about a thousand times bigger than atoms. Visible light waves have a wavelength from 400 nanometers to 700 nanometers, depending on the color. In contrast, atoms have a width of about 0.2 nanometers. This discrepancy is why you can't see individual atoms using an optical microscope. The atoms are far smaller than the light you are trying to use to see them. The color of an object that results from traditional bulk reflection, refraction, and absorption is therefore a result of how several atoms are bound together and arranged, and not a result of the actual color of individual atoms. For example, take carbon atoms and bind them into a diamond lattice, and you get a clear diamonds. In contrast, take carbon atoms and bind them into hexagonal planes and you get gray graphite. The nature of the bonds between many atoms is what determines the traditional color of a material and not the type of atoms themselves. If you have no bonds at all between any atoms, you get a monoatomic gas, which is invisible (at least according to traditional reflection, refraction, and absorption).
The color of most of the everyday objects around us, from apples to pencils to chairs, arises from traditional bulk reflection, refraction, and absorption. This mechanism of light delivery is so common and intuitive that we could define "having a color" narrowly to only include this mechanism. With this narrow definition in mind, therefore, a single atom is too small to have a color.
2. Thermal radiation
Heat up a bar of iron enough and it glows red. You could therefore say that the color of a hot iron bar is glowing red. The red color of the iron bar in this case, however, is due to thermal radiation, which is a mechanism that is very different from bulk reflection, refraction, and absorption. In the mechanism of thermal radiation, the atoms of an object knock into each other so violently that they emit light. More accurately, the collisions cause the electrons and atoms to be excited to higher energy states, and then the electrons and atoms emit light when they transition back down to lower energy states. Since the collisions due to thermal motion are random, they lead to a wide range of energy excitations. As a result, the thermal radiation emitted contains many colors that span a broad band of frequencies. The interesting thing about thermal radiation is that its color is more a result of the temperature of the object and less a result of the material of the object. Every solid material glows red if you can get it to the right temperature without it evaporating or chemically reacting. The key to thermal radiation is that it is an emergent property of the interaction of many atoms. As such, a single atom cannot emit thermal radiation. So even if we expand the definition of "having a color" to include thermal radiation, individual atoms still have no color.
3. Rayleigh scattering
More informatively called "long-wavelength scattering", Rayleigh scattering is when light does bounce off of single atoms and molecules. But because the light is so much bigger than the atoms, Rayleigh scattering is not really the "bouncing" of a light wave off of a small particle such as an atom, but is more a case of immersing the particle in the electric field of the light wave. The electric field induces an oscillating electric dipole in the particle which then radiates. Because the mechanism is so different, Rayleigh scattering of white light off of small particles always creates the same broad range of colors, with blue and violet being the strongest. The color of Rayleigh scattering is always the same (assuming the incident light is white) and is mostly independent of the material of the scattering object.
Therefore, a single atom does have a color in the sense that it participates in Rayleigh scattering. For example, earth's atmosphere is composed mostly of small oxygen molecules (O2) and nitrogen molecules (N2). These molecules are far enough apart that they act like single, isolated molecules. When the daytime white sunlight hits isolated air molecules, it scatters according to Rayleigh scattering, turning the sky whitish-bluish-violet. The fact that we can see the daytime sky attests to the fact that small, individual molecules can exhibit some form of color. While we are talking about small molecules when it comes to the sky, the same principle applies to single atoms. Properly understood, the color in Rayleigh scattering belongs more to the interaction itself than to the actual types of atoms involved. Just because the sky is blue does not necessarily mean that nitrogen atoms are blue. Raman scattering is much rarer than Rayleigh scattering, but is nearly identical in the context of this discussion. Raman scattering is different in that some of the energy of the incident light is lost internally to the particle so that the scattered light is shifted lower in frequency.
4. Gas Discharge
Gas discharge (e.g. a Neon light) is perhaps the mechanism that would best fit the notion of an individual atom "having a color". Gas discharge is what happens when you take pure atoms, isolate them from each other in a low-density gas state and then excite them using an electric current. When the atoms de-excite, they emit visible light. The key here is that a particular atom can only being excited, de-excited, and emit light in certain ways. This leads to the color of an atom during gas discharge being very strongly tied to the type of atom involved. The frequency spectrum of an atom during gas discharge is considered the color "fingerprint" of that particular type of atom. For instance, true neon signs are always red because neon atoms themselves are red under gas discharge. Argon atoms are lavender under gas discharge, while sodium atoms are yellow and mercury atoms are blue. Many of the colors generated by "Neon" lights are attained by mixing different gases together. The "flame test" used in chemistry to detect certain atoms is essentially a less-controlled, less-pure version of a gas discharge lamp.
Note that florescence (such as in a florescent light bulb), phosphorescence, and gas laser emission are all similar to gas discharge in that they involve exciting electrons in single atoms or simple molecules. As opposed to gas discharge, which forces an atom to emit all of its characteristic colors; florescence, phosphorescence, and laser emission all involve exploiting certain transitions so that only certain atomic colors are emitted. They can be considered special cases of gas discharge, as far as atomic color characterization is concerned.
There are many other ways an object or material can emit or reflect visible light; such as through semiconductor electron-hole recombination (in LED's), Cherenkov radiation, chemical reactions, synchrotron radiation, or sonoluminescence; but all of these involve the interaction of many atoms or no atoms at all, and so are not pertinent to the current discussion.
In summary: in the sense of traditional reflection, refraction, absorption, and thermal radiation, individual atoms are invisible. In the sense of Rayleigh scattering and gas discharge atoms do have a color.