Sorting the Invisible: How the Periodic Table Was Built

Sorting the Invisible

[1]Imagine organizing a closet full of objects you are not allowed to look at. You can weigh each one and test how it behaves, but you can never see its shape. That was the situation of chemists in the 1800s. They knew dozens of elements and could measure how much mass a typical atom of each one had compared with hydrogen. But no instrument could show them an atom. Any order they found would have to come from patterns in behavior and mass alone.

[2]The first clear pattern was spotted by a German chemist, Johann Döbereiner. In 1829 he described several sets of three elements that behaved alike: chlorine, bromine and iodine formed similar salts with metals; lithium, sodium and potassium were all soft metals that reacted with water. In each trio, the atomic mass of the middle element was close to the average of the other two: sodium's 23 Da sits almost exactly between lithium's 7 Da and potassium's 39 Da. The middle element's behavior was usually in between as well. Döbereiner's triads hinted that mass and behavior were linked, but they covered only a few elements.

[3]In 1864 and 1865 an English chemist, John Newlands, tried a bolder move. He lined up every known element in order of increasing atomic mass and noticed that properties seemed to repeat every eighth element, the way a musical note repeats every eighth key on a piano. He called this the law of octaves. The pattern worked for the lighter elements but broke down for the heavier ones, and when Newlands presented the idea to the Chemical Society in London in 1866, members laughed. One asked whether he had tried arranging the elements alphabetically.

[4]The breakthrough came in 1869 from a Russian chemist, Dmitri Mendeleev, who was writing a textbook and wanted a sensible order for the elements. He also listed them by increasing atomic mass, but he made two decisions that Newlands had not. First, he let the rows be different lengths, so that elements with similar behavior always landed in the same column. Second, when the next element in the mass order did not fit the column it would have landed in, he left the space empty. An empty space, he argued, was not a flaw in the table but a sign of an element nobody had found yet.

[5]Mendeleev then did something that turned his table from a filing system into a scientific model: he predicted the properties of the missing elements. Using the averages of the elements above, below and beside each gap, he estimated their atomic masses and described how they would look and react. In 1875 a French chemist, Paul-Émile Lecoq de Boisbaudran, discovered gallium, and its properties matched the prediction for the element below aluminum. When the first measurement of gallium's density came in lower than predicted, Mendeleev suggested re-measuring; the corrected value agreed with his table. Scandium followed in 1879 and germanium in 1886. Germanium's measured atomic mass, about 72.6 Da, was within one dalton of the 72 Da Mendeleev had predicted.

[6]A model that correctly predicts things before they are discovered earns trust, and chemists adopted Mendeleev's table. It was not perfect. A few pairs, such as tellurium and iodine, had to be swapped out of mass order so that their properties would fit their columns, and Mendeleev could not explain why. The reason was found in 1913, when the English physicist Henry Moseley showed that the true ordering number of an element is not its mass but the number of protons in each of its atoms, now called the atomic number. Ordered by atomic number, the swapped pairs fall into place.

[7]The table also grew. When argon was discovered in 1894, it fit no existing column, so a new column of unreactive gases was added. By 2016 the table held 118 elements, many made in laboratories. Through all of these changes, the core idea from 1869 has survived: arrange the elements in order, line up the ones that behave alike, and an element's position will tell you how it is likely to behave. That is why a chart first sketched for a textbook still hangs in nearly every science classroom in the world.