A Table That Forbade Things

Several people noticed periodic patterns among the elements in the 1860s. Newlands proposed a law of octaves and was asked at a meeting whether he had tried arranging them alphabetically. Others produced partial arrangements.

What Mendeleev did in 1869 that the others did not was trust the pattern more than the data.

Where an element's properties did not fit the sequence, he left a gap and said an undiscovered element belonged there. And he did not merely say so: he predicted the missing elements' atomic weights, densities, melting behaviour, the formulas of their oxides and chlorides. For the space below aluminium he described eka-aluminium in detail.

Gallium was isolated in 1875. Mendeleev wrote to the discoverer suggesting that the measured density was wrong. It was remeasured, and Mendeleev's prediction was closer than the original measurement.

Scandium and germanium followed, matching equally well. That is what turned a classification into a theory: it forbade certain things, made specific claims about what would be found, and survived the test.

He also had to swap a few pairs out of atomic weight order to make the chemistry work (tellurium before iodine, most conspicuously) which looked like fudging and was criticised as such.

Henry Moseley resolved this in 1913 by measuring the X-ray spectra of elements and finding that the correct ordering is by nuclear charge, the number of protons, which is not quite proportional to atomic weight. Every one of Mendeleev's swaps was correct, and the anomalies vanished. Moseley was killed at Gallipoli two years later at twenty-seven.

The table's shape then turned out to be a picture of something nobody had suspected.

Chemistry is almost entirely about an atom's outermost electrons, because those are what another atom encounters. Electrons occupy orbitals that hold fixed numbers (two in an s orbital, six across the p orbitals, ten in the d, fourteen in the f) and elements in a column share an outer-shell arrangement.

So the periods are two, eight, eight, eighteen, eighteen, thirty-two elements long, and the blocks of the table have exactly those widths. The shape is the electron-filling order drawn out. It was arrived at empirically, from reactions in flasks, decades before quantum mechanics existed to explain it, which is one of the better examples of a correct pattern preceding any understanding of why.

This also explains the group behaviours that otherwise have to be memorised. Lithium, sodium and potassium each have a single outer electron they readily lose, so all three react with water, increasingly violently down the group as that electron gets further from the nucleus and is held more loosely. Fluorine and chlorine each need one electron, so they are aggressive oxidisers.

And the noble gases have full outer shells. A filled shell is a low-energy arrangement with nothing to gain by reacting, which is why they are nearly inert, and why every other kind of bonding is an attempt to reach that same configuration by giving electrons away, taking them, or sharing.

The noble gases are also the table's best story about incompleteness. Mendeleev's original had no column for them; nobody suspected an entire family of unreactive gases was missing, because being unreactive makes you hard to notice. When argon was isolated in 1894 it did not fit anywhere, and the resolution was to add a whole new group, which the rest of the structure then accommodated without disturbance.

One caution for the bottom of the table, because it is where the neat picture stops working.

In very heavy elements the innermost electrons move at an appreciable fraction of the speed of light, and relativistic effects become significant. Some orbitals contract and others expand, which changes the energies involved in bonding.

This is not an obscure correction. It is why gold is yellow rather than silvery (the relativistic contraction shifts an absorption into the blue) and why mercury is liquid at room temperature, since its outer electrons are held too tightly to bond well with its neighbours. At the very bottom of the table these effects can make an element behave unlike its group entirely, and there is serious doubt whether oganesson, sitting below radon, is a noble gas in any useful sense.

The table tells you where to look first. It is not a guarantee.