Are atoms colored?

El Sol expulsa un torrente de protones y electrones de alta energía que incide en la atmósfera cerca de los polos, donde chocan con los átomos de oxígeno y nitrógeno del aire, emitiendo luz: son las auroras, australes o boreales, como la mostrada en la imagen obtenida en la región del Yukón, en el Ártico canadiense.
The Sun expels a torrent of high-energy protons and electrons that hits the atmosphere near the poles, where they collide with the oxygen and nitrogen atoms in the air, emitting light: they are the auroras, austral or boreal, like the one shown in the image obtained in the Yukon region, in the Canadian Arctic.

By color we mean the different components of the visible spectrum, light to which the human eye is sensitive. When we say that an object is of a certain color, in general we are referring to how it absorbs and reflects the light that falls on it. This means that color is not a characteristic of matter at the atomic level, but arises from the interaction of light with atoms and molecules.

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When we illuminate an object with visible light that contains all colors, that light can be absorbed by the object, it can be transmitted if the object is transparent or it can be reflected. And this happens according to the energy of the light, that is, according to the wavelength of the light. The reflected light is what makes our eyes see an object of a certain color. If, for example, most of the reflected light has a wavelength between 380 and 450 nanometers, we see that object from violet color, but if the reflected light is in another wavelength range, for example, enter 620 and 750 nanometers which is the other extreme, we say that this object is red. And in between are the rest of the colors: blue, green, yellow and orange. If the wavelength is outside that range our eyes can no longer see it, it is no longer part of what we call the visible spectrum.

The size of the atoms is 1. 000 times less than the wavelength of visible light, so that it is not possible to see individual atoms using an optical microscope and therefore it would not make sense to talk about the color of these

Focusing on the case of atoms, their size is 1. 000 times shorter than the wavelength of visible light, so it is not possible to see atoms using an optical microscope and therefore it would not make sense to speak of the color of atoms in the above sense. In reality, and due to the difference in scales, the processes of absorption, reflection and transmission of visible light do not occur with the atoms individually but with many of them simultaneously and color emerges as a macroscopic property that does not it depends exclusively on individual atoms, but also on how atoms and molecules are organized in solids or compounds. An example of this that is very illustrative is that we can find two solids such as graphite or diamond that are formed by the same type of atoms, carbon, and have very different properties and colors.

So if we consider color as I explained at the beginning, the answer to your question is that individual atoms have no color, but groupings of atoms. Although an individual atom could emit radiation of a certain frequency, and therefore of a specific color, as a consequence of the excitation and de-excitation of electrons, this emission is so weak that our eyes cannot see it. We need many atoms to have radiation that is visible to the naked eye.

If we consider other definitions of color, we can find that atoms can have it

However, if we consider other color definitions, we can find with which atoms can have it. An example would be an electrical discharge in gases, which is the mechanism used in discharge lamps such as fluorescent ones. This phenomenon occurs when we put together a group of atoms of the same type, isolate them and create a low-density gas. If we excite this gas with an electric current, when these atoms de-excite they emit visible light. And that light is characteristic of each type of atom, there are discharge lamps with neon atoms and that light is red, the sodium ones produce an orange light and the argon, violet. But also in this case we are not talking about an individual atom but rather a group of atoms.

Another example is the dispersion of the light on individual atoms or molecules. This phenomenon is responsible for the blue sky. The white light that comes from the Sun is scattered on the oxygen and nitrogen molecules in the atmosphere and in this scattering the color that diffuses the most is blue and that is why we see the sky of that color. Although, in this case, the color does not depend on the nature of the atoms, we have oxygen and nitrogen atoms that produce the same color, but it depends on the nature of the dispersion.

The answer is that individual atoms do not have color, but if we expand the concept of color then we can see that of atoms as occurs in discharge lamps.

Mariam Tórtola is a professor at the Faculty of Physics of the University of Valencia and a researcher at the Institute of Corpuscular Physics, a mixed center of the CSIC and the University of Valencia.

Question sent via email by Jozsef Sakovics

Coordination and writing: Victoria Toro

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