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Periodic Table
Posted February 11, 2001

I've had an awful lot of requests lately, but I'm going to disregard all of them today. Instead of telling you where the sky ends, or whether the earth would be warm inside if we didn't have the Sun, I'm going to talk about one of those things that most people learned to hate back in high school chemistry class. Of course I'm not talking about crucibles and bunsen burners -- everyone loves those -- and I'm not talking about graduated cylinders, or test tubes, or even beakers, which people love so much that they take them home and keep them. Except graduated cylinders -- I think that's such a dumb name for a measuring cup that not even a frustrated chemistry student would want to steal one. No... I want to talk about the periodic table.

But of course, I can't say anything about the periodic table until we've heard some Mozart.


The periodic table, or "the periodic table of the elements," as we call it on formal occasions, started out as a catalogue of all the basic substances which couldn't be broken down into parts. For example, water is not an element because it can be decomposed into hydrogen and oxygen. Hydrogen and oxygen, on the other hand, can't be decomposed into parts. Well, technically speaking, they can, but if you lived in the 18th century and you had never been inside the Sun, you would have no way of knowing that.

It wasn't long before chemists started to notice patterns appearing in their lists of elements. Patterns like, every fourth element is inert, which is to say it won't start a chemical reaction with any other element. The patterns aren't quite as simple as "every fourth element," of course; they're a bit more subtle. So chemists simply came up with different ways to write out the list, so the patterns would be more visible. What they came up with was a grid, which places all of the non-reactive elements, like Neon, Krypton, and Argon, in a vertical row. The most obvious pattern on the periodic table, of course, is that the lightest element, Hydrogen, is at the top left, and they get heavier from left to right and from top to bottom.

One of the many things the periodic table has helped us figure out, is what constitutes an element. It turns out that there's one thing about an atom that determines its chemical properties, and that one thing is how many protons it has. Hence, the number of protons in a given element is called its atomic number. A hydrogen atom is nothing more than a single proton, so its atomic number is 1. A helium atom has two protons. It might have 2 neutrons in there as well, or it might have more, but they won't affect how it reacts chemically. It will still be helium, and it will still have atomic number 2.

Carbon atoms always have 6 protons. Usually they have 6 neutrons as well, in which case we call it carbon-12; but sometimes there are 8 neutrons, and we call it carbon-14. Carbon-12 and carbon-14 act exactly the same in chemical reactions.

So what's the difference between carbon-12 and carbon-14? Is there a difference? Yes, there is a difference. For one thing, carbon-14 is heavier. Neutrons and protons weigh almost exactly the same, so carbon-14 is almost 20% heavier than carbon-12. But that's not all. Different isotopes, which is what we call atoms like carbon-12 and carbon-14 that have the same atomic number but different atomic weights, different isotopes have different properties when it comes to nuclear reactions. Nuclear reactions aren't limited to bombs and stars, of course; there's also the matter of radioactive decay.

Radioactive decay is something that just happens. At what rate it happens, depends on which element is decaying. If it's carbon-14 it has a half-life of 5730 years, which means that if you have a pound of pure carbon-14, and you wait 5730 years, you'll have half a pound of carbon-14 and half a pound of carbon-12. There's nothing you can do to stop it, or to speed it up. Not by heat, not by pressure, not by chemical reaction, not by the hair on your chinny-chin-chin. It just happens. Half of it is gone in 5730 years.

5730 years is a very convenient interval from an archaeologist's perspective. All organisms spend all day exchanging their carbon atoms with carbon atoms in the atmosphere. This only happens while they're alive, of course. When they die, whatever carbon they have is stuck inside them. Some of it is carbon-14, and some of it is carbon-12. The carbon-14 starts to decay. At a very predictable rate. Thousands of years later, someone can measure the proportion of carbon-14 to carbon-12, and compare that proportion against the proportion of carbon-14 to carbon-12 in the atmosphere. How different the proportions are is a direct reflection of how many years ago the organism died. And in case you haven't guessed, this process is called carbon dating.

Now that was a fascinating tangent into the world of carbon, and this new style of dating which you've all been wondering about, but I bet you're wondering, what about the rest of the elements? Surely they also have isotopes and weights and funny stories behind them. Well, they do, but I don't have enough time to tell you all those stories. I do have just enough time to list all of the known elements, though.