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The Periodic Table

© Copyright 1998, Jim Loy

Forget, for a moment, that you know all about chemistry. We are going to try to discover the periodic table. It is 1869. And here are the known chemical elements:

There are some patterns in the above data. Let's look at the most obvious ones. Metals and non-metals appear in alternate groups. There are few light metals; there are many, many heavier metals (and few heavier non-metals). There is one group of consecutive gasses (at standard conditions), nitrogen, oxygen, and fluorine. There are only two liquids, bromine and mercury, which don't make any obvious pattern.

There are gaps, where we might expect to discover new elements. Mostly, the weights of the above elements progress in steps of about 2. There are some big gaps, between cerium and terbium, between bismuth and thorium, and a smaller gap between thulium and tantalum. There are much smaller gaps, like that between fluorine and sodium. We cannot be confident of discovering an element between these elements. As yet, there is no theory about the structure of the atom. So we do not know whether there should be gaps in the sequence, or not.

We determine valency by chemical experiments. Many of these may not have been known very well, in 1869. These valencies are somewhat confusing, except that at the beginning of each group of metals, we encounter the sequence 1-2-3 (with one interesting exception, described below). That looks very promising. Also, we have only one element with a valency of 0, the inert gas helium. With hindsight, it seems obvious that there should be others, as there are gaps for these. And their inertness would make them difficult to discover. So, we have an excuse for not having discovered them already.

Let's build a little table:

Well, we've got a periodic table. We can fill in most of the blanks, just because the other elements probably should be filled in by order of their weights. By the way, titanium ("Ti?" in the above table) appears before a gap in the sequence of weights. We can guess that there is a missing element with a valency of 3.

In the above table, nickel and cobalt have very nearly the same weight. It turns out that cobalt should appear before nickel, on the periodic table.

The periodic table (as well as the accompanying periodic law) was discovered by Dimitri Mendeleev (Mendeleyev), in 1869. Before that, several scientists made contributions toward this discovery. They deserve partial credit for helping to discover the periodic table. But, Mendeleev did most of the work, and actually was the one who predicted the existance and properties of several of the missing elements.

A few of the properties, of elements, which I didn't mention, are very useful in building a more complete periodic table. These properties are boiling point, melting point, and compressability. You can fill in almost the entire table, using these properties.

Addendum:

This article will be expanded later. I received email asking: "why cobalt comes before nickel it its atomic weight is higher." Here is my response:

Thank you. Good question. The truth is that atomic weight (which is often easily deduced) is not really important when it comes to chemical properties. By arranging the chemicals into a table, by weight, it becomes obvious that there are recurring patterns. But the elements which don't quite fit the pattern are very interesting, and suggest a stronger pattern. It becomes apparent that a little reordering makes the elements line up better, by chemical properties. And so Nickel, Palladium, and Platinum really are fairly similar, and cobalt wouldn't quite fit that pattern. So something else is more important that atomic weight (protons + neutrons), and that is atomic number (number of protons). And Mendeleyev was bright enough to figure out these patterns before he knew very many of the atomic numbers. In fact he left gaps in his table, where he predicted elements would be discovered, and he was right every time. That really made his table impressive, and convincing.

Also, sometimes atomic weight has a different problem. Notice that many of them are very close to being whole numbers. An oxygen atom is almost exactly 16 times heavier than a hydrogen atom. It would seem that oxygen is made up of 16 nearly equal pieces (particles), while hydrogen is just made up of one. Well, Chlorine is 35.453. Chlorine is a mixture or two kinds of Chlorine, one of which is made up of 35 particles, and the other is made up of 37 (it turns out) particles. Well, what makes Chlorine 35 seem identical to Chlorine 37 chemically? Well, they have the same number of protons, which makes them have the same number of electrons, which makes them almost identical chemically. So here, atomic weight confused the issue; atomic number was all that really mattered.

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