One thing about science that people who have had no exposure to it find hard to grasp is the role of being wrong. Being wrong is usually thought of as a bad thing, but it’s a necessary part of science. In short, acknowledging when science is wrong lets science get better.
As an example, I’ve pulled out my old-but-not-too-moldy copy of Richter’s Inorganic Chemistry (5th American edition, published in 1900 – pages 243-250). What can a hundred -and-five year old book tell us about science? Plenty, if we’re looking for how science has progressed in that time.
In 1900, the periodic table was somewhat new, and didn’t quite have its modern form. What it did have were groups and periods. Chemists had long known that some elements were strikingly similar to others in terms of how they reacted with other elements. Sometimes that similarity even went as far as physical appearance and other properties. Mendeleev had, in the 1800s, surmised that if you arranged the elements in order of atomic weight (a relatively new measurement at the time), the properties of the elements would repeat at regular intervals. This became known as the periodic law, and enabled early chemists to arrange elements in a new way. Elements with similar chemical properties were placed into groups. Periods were essentially runs through the different groups. In other words, the first period contained the lightest member of the first group, the second group, the third group, etc. until the next element belonging to the first group was reached. Then the next period started. There’s a little more to it, but that is the essential idea.
The periodic law, organized as a table, looked like this in 1900. The numbers by the elements are the relative atomic weights of each element as known in 1900. Elements that had not yet been discovered but were thought to exist based on the periodic law are the “???”s.
|
|
Group I |
Group II |
Group III |
Group IV |
Group V |
Group VI |
Group VII |
Group VIII |
|---|---|---|---|---|---|---|---|---|
|
Period I |
Li, 7 |
Be, 9 |
B, 11 |
C, 12 |
N, 14 |
O, 16 |
F, 19 |
|
|
Period II |
Na, 23 |
Mg, 24 |
Al, 27 |
Si, 28 |
P, 31 |
S, 32 |
Cl, 35.4 |
|
|
Period III |
K, 39 Cu, 63 |
Ca, 40 Zn, 65 |
Sc, 44 Ga, 70 |
Ti, 48 Ge, 72 |
V, 51 As, 75 |
Cr, 52 Se, 79 |
Mn, 55 Br, 80 |
Fe, 56 / Ni, 59 / Co, 59 |
|
Period IV |
Rb, 85 Ag, 108 |
Sr, 87 Cd, 112 |
Y, 89 In, 114 |
Zr, 90 Sn, 118 |
Nb, 94 Sb, 120 |
Mo, 96 Te, 127 |
???, 100 I, 126.5 |
Ru,102 / Rh,103 / Pd, 106 |
|
Period V |
Cs, 133 ??? ??? Au, 197 |
Ba, 127 ??? ??? Hg, 200 |
La, 138 ??? Yb, 173 Tl, 204 |
Ce, 140 ??? ??? Pb, 207 |
Pr, 140 / Nd, 144 ??? Ta, 183 Bi, 208 |
??? ??? W, 184 ??? |
Sm, 150 ??? ??? ??? |
??? Os, 191/ Ir, 193 / Pt, 195 |
|
|
|
|
|
Th, 232 |
|
U, 239 |
|
|
The periodic law worked very well. According to Richter’s:
[Mendeleev], on the basis of the periodic system, predicted […] the existence of new, not yet known, elements which correspond to unoccupied, free gaps in the table. In fact, three such gaps have been filled by the discovery of gallium, scandium, and germanium; their properties have shown themselves to be perfectly accordant with those deduced from the periodic system.
Score three for the periodic law! (Side note: Useful scientific ideas are predictive; in addition to telling us something we do know, they predict things that we don’t. Then we can try to find out if the predictions are correct!)
But it was wrong.
I have two elements on the table above in bold: tellurium (Te) and iodine (I). According to the periodic law, as you go from group to group, the atomic weight should increase. But tellurium’s weight is larger than iodine’s. Not by much, but it is still larger.
This is a big problem for the periodic law, and demanded an answer. Some other elements had been out of order as well (and were redetermined). Atomic weight measurements weren’t as easy then as now, so it was natural for there to be some flux in the atomic weights.
The problem? Tellurium and iodine wouldn’t go away!
Richter’s makes a statement.
We are consequently justified, until we have more evidence to the contrary, in assuming that the determinations of the atomic weight of tellurium have placed that value too high. If it should finally be proved to be greater than that of iodine, then the periodic system would be seriously affected in its foundations; it would then lose its claim to being a natural law – for this would not tolerate an exception.
Science is always at the mercy of the data. Eventually, it was proved that tellurium had a greater atomic weight than iodine. The periodic law, for all its successes, was wrong, wrong, wrong! So why do we teach the periodic table in chemistry classes, when the periodic law was wrong? Because science revises itself. The periodic law was wrong, true, but it also contained a lot that was right. The properties of the elements do vary with a regular pattern. Where the periodic law had it wrong was assuming this variation was based on the weight. It’s actually based on something else – the number of positively charged particles (protons) in the nucleus of an atom. That has a relationship to the atomic weight, but it’s not the same thing. That insight, which required us to figure out what was inside an atom, didn’t come until much later.
But it might not have come at all if the periodic law wasn’t wrong! If we thought the periodic law completely described the nature of atoms, we might not have tried to find out more about them.
There are a few points to make about the periodic law and the progress of science. First, good scientific ideas are predictive. They not only explain what we know, but also predict some things we don’t (yet) know. The validity of a scientific idea rests on how well these predictions turn out – not only on whether it agrees with all data that’s already available. If it doesn’t predict anything, it’s not very useful for science.
Second, science works best when everything is out in the open. The problems with the periodic law were out in the open for everyone to see and work on. While the reference quoted here says that the problems were likely due to experimental error (even though they weren’t), there’s no attempt to hide the discrepancy. Think about that the next time you read something about vast conspiracies of scientists trying to hide the truth from the masses. Science doesn’t progress that way.