|MadSci Network: Chemistry|
For the quick answer, scroll to the last few paragraphs.
The colour of a substance is determined by the wavelength of the light that bounces off it. As you may already know, light is a form of electromagnetic radiation (as are X-rays, ultraviolet rays, radio waves, and so on). Waves are the means by which electromagnetic energy is propagated (i.e. transferred from place to place).
Electric fields, when moving, induce (cause to be formed) magnetic fields, and vice versa. Hence, an electromagnetic wave is formed when a moving electric field induces a magnetic field which induces an electric field and so on and so forth. The two of them travel perpendicular to each other. The wave itself represents a variation in the magnitude of the electric and magnetic fields. If you observe it closely, you'll realise that the waves are actually sinusoidal, that is, shaped like a sine curve.
Now in a wave, there are basically three components that matter: the wavelength, the frequency, and the velocity of the waves. If you draw out a sine curve, you'll realize that there are many 'hills and valleys' or crests and troughs. A wavelength is the distance between one crest ('hill') and the next. The frequency is the number of waves that come by per unit of time (the number of waves every second is expressed in a unit known as the Hertz, or Hz). The velocity is how fast the wave propagates (moves forward). Light is a form of electromagnetic radiation, and refers to the narrow range of frequencies that we can see with our eyes. The boundaries are fuzzy. As the wavelength gets longer, it fades off to red, then to the infrared, then to the so-called millimeter waves and off to the radio waves, which can range from centimeters to meters in length. On the other end, as the wavelength gets smaller, the colour of the visible light tends to violet, then heads off to the ultraviolet range, then to the X-rays and the gamma ray ranges. This variation of the wavelength of EM radiation is known as the spectrum of electromagnetic (EM) radiation.
Now you may be curious. If this spectrum is determined by the wavelength alone, and radio waves are part of the EM spectrum, then why are we always talking about which frequency to tune in to? Now you must remember that all forms of EM radiation travel at the speed of light, which is 2.99 x 108 m/s (metres per second). Hence, the velocity is constant for all types of EM radiation (i.e. for all wavelengths). The standard wave equation is
Wavelength x frequency = velocity
Frequency = velocity / wavelength
Hence, if the velocity remains the same all the while, when the wavelength increases, then from the equation above, the frequency gets smaller, and vice versa. Thus, we can describe a wave in terms of either its frequency or wavelength, since as the velocity is constant, you can convert between the two.
Now what is colour? When you have normal light shining on an object, the object will absorb some of the light, and scatter the rest, which eventually reaches our eyes. The light which is absorbed, depending on the chemical and physical properties of the object, is limited to a certain range of wavelengths. The remaining light of the remaining unabsorbed wavelengths is what we see, and from there we can tell what wavelengths have been absorbed. For example, in the case of gold, it is yellowish, so the gold absorbs the light that is not in the 'yellow' range of the light spectrum, and reflects it off instead, allowing us to see it.
This is true, as you said, for large chunks of gold (ooh lala), but not necessarily so for finely divided gold. Bear in mind that gold when finely divided is still consisting of large bunches of atoms, and hardly can be considered individual atoms. On an atomic scale, they are actually pretty huge. Gold, like other metals, can be found (in its pure form) as either amorphous or crystalline. Crystals have an ordered structure, that is the atoms of gold are lined up in very orderly strict rows and columns. Amorphous means that no such structure exists, and instead the gold atoms are randomly distributed in the solid. The gold we see is normally in crystalline form, though the whole solid chunk we see is usually consisting of many small crystals bonded to each other, rather than one huge crystal. Crystalline solids may even come in different types, viz. the atoms may be arranged in different manners, and this causes a difference in the properties of the substance. Take, for example, carbon. You should know that there are two main forms of crystalline carbon -- graphite and diamond. Graphite is black and slippery/soft, while diamond is hard and colourless. These differences in both the colour and hardness are due to the different arrangements of the carbon atoms in the crystal matrix.
Furthermore, surface properties may affect colour. For example, very fine ridges on a reflective surface can cause it to sparkle and appear iridiscent. The colour on the wing of a butterfly, for example, is due to very fine scales arranged in different ways to diffract light, but they are made of the same substance as our hair! As you can see, many factors can affect the colour of a substance or more specifically an object. I am not very sure of the details surrounding gold, but I think the finely divided gold may appear the way it is because of the surface properties. The individual small grains reflect light differently from a large smooth surface, and this causes the difference in colour.
Now as for your question (finally I get to the point =p ) whether atoms have colour, the answer is no.
Why? As mentioned above, colour depends on the wavelength of the light that is reflected off the surface of an object. Now the so-called 'visible band' of wavelengths, that is the range of wavelengths of electromagnetic radiation that we can see with our eyes, is very narrow, from about 400nm (nanometers) to 700nm. One nanometer is 10-9 meters, or 0.0000000001 meters. In contrast, the radius of an atom is expressed in terms of picometers (pm), or 10-12 meters. The radius of a caesium atom, for example, is 265pm, quite big in comparison to other atoms. This is roughly an order of magnitude too small for it to reflect light on its own. The light wave is simply too big to be reflected and show us 'colour'. Therefore, to think of the 'colour' of an individual atom is meaningless. However, once you have an aggregate (a bunch) of atoms big enough, then you can begin to discern something approaching 'colour'.
I hope this answer has been useful to you. Some useful references are given at the back of this answer. If you have any more questions, don't hesitate to post them to us mad scientists!
Tan "Tombolo" Thiam Hock
Properties of Light and Color
http://www.geoci ties.com/HotSprings/8018/Brain.html (How the brain sees colour)
http://www.rese arch.ibm.com/image_apps/colorsci.html (Colour science)
http://w ww.hia.nrc.ca/moffatt/spectrum/sintro2/sintro2.html (What colours represent)
http://www.sa sked.gov.sk.ca/docs/physics/u6c32phy.html (Optics -- colour)
Try the links in the MadSci Library for more information on Chemistry.