Atoms And Color

Today, I would like to discuss one of those questions of the ages that I have wondered about for a long time but cannot find much in the way of satisfactory answers. We see things all around us as being of different colors. Some things are red, while others are blue or green. But exactly why would an object be red, rather than blue or green or any other colour? We will alternate the two global spellings of the word color (colour).

In the article about color on http://www.wikipedia.org/ , for example, it has all of the usual information about how the eye perceives colour as well as how light is reflected and refracted and optical illusions and, so on. But, while that is valuable information, it still does not tell us why a red object is red while a blue object is blue.

Color is not actually real, at least not in terms of inanimate matter. Outside of living things, the concept of colour is meaningless. The colors that we see are the result of how our brains interpret information from the eyes. The colours of visible light are a narrow section of the entire electromagnetic spectrum.

Color represents electromgnetic waves, received by the eyes, of different wavelengths. The colours, from longest wavelength and lowest frequency to shortest wavelength and highest frequency are: red, orange, yellow, green, blue and, violet. The entire electromganetic spectrum, from longest wavelength is: radio waves, infrared (heat), visible light, ultraviolet (provides suntan), X-rays and, gamma rays.

Most of the light that we see is not of one color, but of pastels and shades. White is all colours mixed together, and black is the complete absence of light. Gray is a mix of black and white, and brown is a mix of some colors but not all of them.

A lot of work has been done with spectroscopy, analyzing the light from various glowing matter in distant stars and laboratories to determine composition. But that still does not tell us why, in ordinary light, a red object is red while a blue object is blue.

We know that an object appears as of a certain colour because the object absorbs most of the light which falls on it, but reflects the light of a certain wavelength. It is the light which is reflected, rather than absorbed, by the object that our eyes and brains interpret as the color of the object. It must be the nature of the atoms and molecules of which the object (or the paint covering it) is composed which will determine which colour the object will appear to us.

One interesting effect of atomic and molecular structure on light is transparency. A few materials, such as water and some crystals, appear to us as transparent to light. This is because atoms arranged in an orderly structure throughout the material permit light to pass right through the gaps in between.

This alignment of atoms is not perfect, however, and if a transparent material is thick enough then light will be absorbed before it can reach the other side. Have you ever noticed that, in underwater photos, nothing appears as red below depths of around 30 feet (about 9 meters)?

This is because red light, with the longest wavelength, has the most difficulty "squeezing" between the molecules of water and so is the first to be absorbed. The reason that deep water tends to appear blue is that only blue light, with the shortest wavelength, can get through enough water to be refracted back to the surface. Violet light actually has the shortest wavelength, but our eyes are not very sensitive to violet.

I decided to begin by going over the chemical elements, and seeing if there was any patterns in their colors.

Let's briefly review the periodic table of the elements, in which elements are arranged in rows by the number of protons and electrons in an atom of the element, and in columns by the number of electrons in the outermost electron shell, which tends to determine the chemical properties of the element.

Here is a link to an excellent periodic table. Hold the pointer over an element and it will give you the electron configuarion of that element: http://www.ptable.com/

In ordinary atoms, there is an equal number of positively-charged protons in the nucleus and negatively-charged electrons in orbital shells around the nucleus, so that the atom has an overall neutral charge. If an atom has one more, or one less, electron, so that there is not an overall charge balance, the resulting charged atom is known as an ion.

The lightest atom is hydrogen, with one electron in orbit around one proton. The next is helium, with two electrons in orbit around a nucleus of two protons. This is what defines the elements, the number of protons in the nucleus. Each element has it's own number of protons, an atom with 26 protons has to be iron and cannot be anything else.

Atoms get heavier as the atomic number, the number of protons, increases. But we cannot determine the weight of the element with this alone because their is another particle in the nucleus called the neutron, so-called because of it's neutral charge. Neutrons are necessary as a kind of glue to hold the nucleus together against the mutually repulsive force of the positively-charged protons. Some elements can have several different numbers of neutrons in one of their atoms, each of these possible different numbers of neutrons in a stable atom of an element is known as an isotope.

When I name an element, I will give it's atomic number in parenthesis such as carbon (6).

The electrons which orbit the nucleus in an atom fit into very orderly shells, and are not haphazard like the planets in orbit around a star. The numbers of electrons in shells, moving outward from the nucleus, are known as the atom's electron configuration.

Carbon, for example, has two atoms in it's innermost shell, and four in an outer shell. This totals six electrons, which matches the number of protons in it's nucleus. The column of elements below carbon all have four electrons in their outer orbitals, meaning that they tend to have properties similar to that of carbon, even though they are all heavier than carbon.

We know that electromagnetic waves tend to be reflected by barriers that are similar in size to their wavelengths. A simple example of this is how the signal from a long-wave radio station, such as AM (amplitude modulation) in North America, will fade as you drive under a bridge, while a shorter-wave station, such as FM (frequency modulation), will not. The longer wave is reflected away by the bridge, while the shorter wave can reflect off the road and sides to get under the bridge.

I found that there are three separate ways in which elements handle reflected light, and this explains why objects are the colours that they are.

THE SIMPLE SPECTRUM OF LIGHT NON-METALS

Let's begin with carbon (6). Carbon atoms are below the range of reflecting light alone. This is why substances composed of carbon appear black. We know that black is the absence of light, meaning that virtually all light falling on carbon is absorbed by it. We also know that carbon forms extremely complex molecular structures. In fact, it forms far more different molecules than all of the other elements combined.

What happens is obvious. The carbon atoms themselves are too small to reflect any wavelength of light that we can see, but light can "get lost" and be absorbed in the very complex molecular structures associated with carbon. This is why coal and graphite appear as a dense black.

A few years ago, I happened to place several copper pennies that I had in my pocket on a shelf. I recently came across them and found that the pennies were partially covered by a bright blue-green corrosion. We know that this corrosion is caused by the copper slowly combining with oxygen (8).

We usually cannot see oxygen (8) because it is a gas in the air, and it's atoms are too far apart to relect any light that we can see. But this means that, if we could see oxygen, it would be a blue-green. Blue is the light with the shortest wavelength, and this is the beginning of the simple spectrum of light non-metals.

Generally, the elements are not very colorful in appearance. But notice that there is a block of four elements together on the periodic table which are quite colourful. There is oxygen (8), and below it on the table is sulfur (sulphur) (16), this element is a brilliant yellow.

Immediately following oxygen in atomic number is fluorine (9), which is a yellow gas. Below fluorine on the table is chlorine (17), which is a greenish-yellow gas.

It may be that nitrogen (7) would be blue, if we could get enough of it together to reflect light. But it is very unreactive so that it usually remains as a gas making up most of the air.

Notice that, to be colorful, an element in this spectrum must have enough electrons in it's outer shell. Oxygen (8) and sulfur (sulphur) (16) have six, while fluorine (9) and chlorine (17) have seven.

It would seem that atoms to the right of these, neon (10) and argon (18) should also be brilliant in colour. But, as with nitrogen (7) that is prevented by chemistry. The atoms of elements in the rightmost column on the table are very unreactive.

The simple spectrum of light non-metals is completed by bromine (35). This atom is much larger than the others, and so reflects a longer wavelength. Bromine (35) has the same seven electrons in it's outermost shell that fluorine (9) and chlorine (17) have. It's color is a deep red, it is known as the only element other than mercury that is liquid at ordinary temperatures.

THE METAL SPECTRUM

The thing that makes colour so complicated is that metals handle light in a fundamentally different way than non-metals, such as those described above. Most elements are metals, let's briefly review the difference between metals and non-metals.

In a metal, outer electrons are shared with sorrounding atoms. This creates a bond that allows metals to be bent and shaped without breaking. This is why metals appear with the luster that non-metals do not have, light is reflecting off this "sea" of shared electrons. If a voltage pressure is applied to the metal, it causes these free, or non-local, electrons to move in one direction, thus creating electricity.

In non-metals, this is not the case as outer-shell electrons tend to remain with their own atoms. Although a non-metal atom can lose an electron to another, creating an ionic bond between the two atoms because one would then have a positive charge, and the other a negative. Non-metal atoms sometimes can share electrons, creating a covalent bond, but only within the same molecule.

For our purposes here, imagine the atoms of a metal as several bowling balls, or other heavy spheres, in a shallow tub. If we now add water to the tub, so that most, but not all, of the surfaces of the spheres is under water, we have a good model of metal atoms in that the spheres represent the atoms and the sorrounding water represents the mass of electrons shared between the atoms.

Almost all metals appear as silvery, some with a whitish or grayish component. This is because the shared electrons, the surface of the water in our model, provides a virtually flat surface off which all colors will reflect. All colours mixed together give us white, and the effect of the shared electrons on the light is to give it that metallic silvery appearance.

But there are two commonly-known metals that are an exception to this, copper (29) and gold (79). It is these two elements which I have identified as forming the metal spectrum. The thing that is of such great significance is that the order of the metal spectrum is actually a reversal of that of the simple spectrum of non-metals. This is because metals handle light differently.

I did not investigate colors of the so-called rare earth metal elements at the upper end of the periodic table in doing this study because I wanted to stay with elements that most people would be familiar with, or at least have heard of.

To understand how the heavier elements, in particular, handle light, we have to remember that an atom is mostly empty space. If an atom were the size of a house, the nucleus might be the size of a pea in the center of the house. The rest of the atom would be the electrons in their orbitals around the nucleus.

My hypothesis is that only light which approaches the outer electrons of an atom at close to a perpendicular angle to the orbital path of the electron will be reflected by the atom, the rest of the light will be absorbed. I find that, once we accept that, the colours of the elements seem to fall right into place.

Why on earth, if objects including atoms reflect electromagnetic radiation similar in wavelength to their sizes, does copper (29) have it's red-orange color, while the much larger gold (79) atom reflect yellow light, which is of a shorter wavelength?

The answer lies in the difference between metals and non-metals. The delocalized electrons, shared between atoms, in metals act as a flat reflecting surface which should reflect all colours. But in copper (29), the top parts of the atoms protruding above this flat surface act as "speedbumps" which hinder the reflection of light by changing the angle at which the light is approaching the surface. The red and orange light, having the longest wavelengths, is thus affected least by this disruption in the surface of the copper, and these are the colors that end up being reflected.

But in the much larger gold (79) atom, the red and orange which is reflected by copper does not have a long enough wavelength to be reflected in the same way. However a shorter wavelength, such as yellow, will perceive the curvature of the gold (79) atom as more of a flat surface, relative to it's wavelength, than will the longer wavelength red and orange light will. But if the light is too short in wavelength, such as blue, there is less surface of the curved atom off which it will reflect. This is why gold reflects yellow light as that shimmering colour that has enchanted people for thousands of years.

The result is that the metal spectrum is reversed so that larger atoms reflect light of shorter wavelength. If there was a much larger metal atom, which reflected light in the same way as copper and gold, it would appear as green or blue.

THE BLUE METALS

Finally, we come to the third way in which elements handle light. There are a number of metals that are of the usual silvery appearance, but which also have a blue tinge to their colour. These include chromium (24), zinc (30), gallium (31), tin (50), osmium (76) and, lead (82).

There is a simple, and fascinating, explanation for this. All of these elements, except chromium (24), have eighteen electrons in their semi-outer shell, and four or fewer electrons in the outer shell. Light reflects off the semi-outer shell, with the electrons in the outer shell acting as a shutter which very quickly opens and closes. Only light with the shortest wavelength, which is blue, can get through the "shutter" system to reflect back.

By the way, there is a definite connection between a shell of eighteen electrons and the reflection of light. Copper and gold, as well as bromine, all have eighteen electrons in their semi-outer shells. Copper and gold only have one outer electron, and this is not enough to act as a shutter to cut off all except blue light. Chromium is an exception in that it is a bluish metal with only thirteen electrons in the semi-outer shell, but it has only one electron in the outer shell.

Of course, most of the matter that we see consists of compounds rather than elements. But similar rules apply in explaining which wavelengths of light are absorbed, and which are reflected to give the compound it's color.

Consider rust, for example. Iron is a silvery metal in appearance. But when it combines with oxygen, this breaks up the metallic surface which reflects all colours of light. There are now gaps in the surface, into which light is absorbed. It is only light of the longest wavelength, red, which can span these gaps to be reflected back. Rust is dark red because the red is combined with the black of light lost and absorbed.