Mendeleev's Puzzle

Part of the Atomic Structure section of Coddy's Chemistry journey. Lesson 29 of 45.

In 1869 Dmitri Mendeleev laid out the 63 known elements on cards in order of atomic weight, starting a new row whenever their properties began to repeat. Elements that behave alike ended up in the same column.

Where no known element fitted, he left a gap and predicted the missing element. "Eka-aluminum" was found in 1875 as gallium, with the weight and properties he had described.

The first four periods of the periodic table, with the boxes of scandium, gallium and germanium left empty

The three boxes Mendeleev had to leave empty

He put properties before weight: tellurium (127.6) went before iodine (126.9). The modern table is ordered by atomic number, the number of protons, and that order needs no swaps.

Challenge

Twelve of Mendeleev's cards have fallen out of the table: Na, Mg, Al, Si, P, S, Cl, K, Ca, Zn, As and Se. Put each card back in its box. The boxes of gallium and germanium, between Zn and As, are the two he left empty in 1869.

Try it yourself

Periodic tablePut each card in its cell