Mendeleev · Periodic table
Line up the elements — does a pattern appear?
That time, that place
Born in Siberia, he lost his father at fourteen; his mother took him thousands of kilometers to get him educated. He came to the problem while writing a textbook and needing to decide what order to present the elements in.
Why this question
Sixty-three elements were known. Others had already noticed that arranging them by weight makes similar properties recur at regular intervals.
The trouble was that it did not come out clean. At certain points the periodicity broke.
What was found
Mendeleev did two things.
First, where the periodicity broke he refused to force a fit and left the space empty — holding that an undiscovered element belonged there.
Second, he predicted what the missing element would be like: roughly what atomic weight, what density, what form its oxide would take.
Within fifteen years gallium, scandium, and germanium were found, and their properties matched closely. When gallium's discoverer measured a density at odds with the prediction, Mendeleev wrote asking him to measure again. On remeasurement, the prediction was right.
The table had proved to be not a filing system but an instrument of prediction.
The elements were a list of separate substances
New elements matching the properties predicted for the empty spaces
Why the periodicity exists he did not know. That waited for the twentieth century and electron shells