Eka–

He left room for them.

Mendeleev's rule was simple and brutal: line the known elements up by atomic weight, and a family likeness recurs at regular intervals — the periodic law. But a strict weight-line had bumps that broke the families, so rather than force a known element into the wrong column he left the square empty and predicted its tenant from the neighbours: interpolation from available neighbours and analogues, sometimes by averaging four neighbours. Tap a dashed gap below to read his predictions, with their dates stated, about an element that did not yet exist.

Predicted, then measured.

This is the whole point of a science, and the rarest thing a theory can do: say a number before anyone measures it, and be right. Below, for each of the three, Mendeleev's prediction sits beside the value found after discovery and analysis. The predictions date to 1871 except for gallium’s low melting point, whose dating is disputed below. The thin bar reads the error off the centre line: how far the world's answer fell from his guess. Switch between them. Seven of the nine numbers land within three percent; all nine within ten. The two loosest are named where they sit: scandium's oxide density (about nine percent out) and germanium's specific heat (about four).

The exceptions were the table telling the truth.

Mendeleev's ordering principle was atomic weight — and yet he broke it, on purpose, in a handful of places. Tellurium is heavier than iodine, but he placed tellurium first, because its chemistry belongs with oxygen and sulphur and iodine's with the halogens. He trusted the family over the scale. For forty years this looked like a fudge. Flip the strip below from order-by-weight to order-by-atomic number:

In 1913 a young physicist, Henry Moseley, fired electrons at a series of elements and measured the frequency of the X-rays they fired back. He found that the square root of that frequency climbs in a nearly straight line, following the element's position in the table, an integer he called the atomic number (which we now know counts the protons in the nucleus). The line below is real: each dot is an element, plotted by the square root of its characteristic X-ray frequency. Toggle the horizontal axis between weight and number and watch which one the points actually fall on.

So the table's true axis was a whole number all along — a count of protons no one could see in 1871. Mendeleev's "errors" weren't errors; they were the periodic law reaching past the weights he had to the numbers he couldn't, and getting there first. Moseley was killed at Gallipoli in 1915, twenty-seven years old.

He was not an oracle.

A page that only showed the hits would be lying by omission. Mendeleev predicted more than came true, and was wrong in ways worth naming. The method was powerful because it was periodicity, not prophecy — and periodicity by weight was itself only an approximation to the deeper law. A few of the misses and the caveats:

The honest summary: a real predictive theory, batting well above chance and far above any rival of its day — but a theory, with a real error bar and real failures, not a crystal ball. That it could be wrong in checkable ways is exactly what made being right mean something.