[The third in a series of posts on the elements in art, by D.C. writer and Sci-Arts guest blogger Sam Kean. Check back each Wednesday for a new artistic element.]
I remember doing some gorgeous experiments over the years in chemistry labs. Flame tests with tongues of yellow or lilac or pure white fire leaping up. Scraps of paper turning all sorts of pastels in acids or bases. A supersaturated solution that, when startled, spread a shock wave of crystals through the liquid. Some elements, though, are beautiful even without the aid of litmus paper or Bunsen burners. Today’s look at the aesthetics of elements delves into the physics of the periodic table and the properties that give elements such visual variety.
We’ll start with Einstein. His theory of relatively puts a maximum speed limit on anything in the universe: 671 million miles per hour, the speed of light. Einstein also laid out all the funny things that happen to speeding objects as they approach the velocity of light. For instance, the closer they get, the heavier they become.
We don’t notice this drag in everyday life because nothing we do comes close to 671 million mph. And for the most part, smaller elements don’t bump up against that limit, either. For them, quantum mechanics works perfectly well to explain their behavior. For instance, quantum mechanics explains how different electrons (negatively charged bits) in an atom spend their lives in different “orbits” around their “sun,” the atom’s nucleus. And how, when light falls on an atom, electrons can jump between different orbits—and absorb or emit light as they do so, behavior that influences the color of an element.
But in heavier elements near the bottom of the periodic table, some electrons do reach appreciable fractions of the speed of light. (What gives electrons speed is the tug they feel from the positively charged nucleus; the bigger the atom, the bigger the nucleus and the stronger the tug.) For those elements, scientists must bring relativity into play, to make corrections for exactly how their electrons behave. That’s especially true for elements 79 and 80, gold and mercury. They’re two of the most alluring elements on the table—and it’s no coincidence that they’re neighbors.
The most fascinating feature of gold is its color, a contrast to the drab gray of almost every other metal. The details get a little hairy, but the key point is that when some of gold’s electrons get faster and heavier—as relativity says they must—the extra bulk distorts the orbits those electrons take. Changing the orbits naturally affects how easily electrons can jump between orbits—and therefore the absorption and production of light when they do jump. It just so happens that the distorted jumps that Einstein’s theory predicts will produce light we see as a rich yellow.
Gold is also very soft—and that softness is exaggerated in its neighbor, mercury, the only metal that’s a liquid at room temperature. Other metals form solids because their atoms share electrons freely; these shared electrons form bonds between neighbors. In mercury, however, the very electrons that would get shared are the ones that relativity saddles with the most extra weight. This makes it harder to share, and so the atom-to-atom bonds remain weak. And when those bonds are weak, you end up with a liquid.
Elements beyond mercury and gold have a different arrangement of electrons, and this ruins the neat relativistic effects like liquidity. But for some of these elements, especially bismuth, number 83, other physical properties can create beautiful effects. Normally we don’t think of tarnish—a thin layer formed when oxygen or another substance reacts with atoms on the surface of a metal—as a benefit. But in bismuth, its natural tarnish gives the metal an iridescent sheen.
Again, the cause has to do with light reflecting off the surface. Some light hits the very top surface and bounces back. Some light penetrates a short distance into the bismuth before bouncing back. That is, it slips through the bismuth-oxygen layer, hits the pure bismuth just underneath, and then reflects.
This means there are two sets of light waves reflected back at your eye: one from the top surface, one from just below. Because the latter waves had further to travel, their peaks and troughs will be slightly out of phase with the others’ peaks and troughs. And any two waves slightly out of phase will “interfere” with each other. This interference (also seen in soap bubbles and oil stains in parking lots) makes some colors stronger and destroys others, depending on exactly how thick the layer of bismuth and oxygen is. Small differences in thickness radically change what colors are strengthened or destroyed, which explains the great variety in color across the surface.
Other metals, like titanium, form an iridescent tarnish, too, but with bismuth, there’s a beautiful bonus. If you melt bismuth and very slowly cool it back into a solid, it will form some of the most gorgeous crystals around. Called “hopper” crystals, they look like grand spiral staircases that intersect each other at wild angles. (This shape arises because crystals on the outer edges form faster than crystals on the inside edges. So instead of filling in the center of each square, the rapidly expanding crystal moves up and out, where there’s more room.) When done right, hopper crystals can grow as large as a human hand. And they resemble nothing else on earth. They’re like M.C. Escher drawings come to life—but, because of the tarnish, M.C. Escher in full Technicolor glory.
That’s a remarkeable post and an interesting insight to some of the metals.