The Periodic Table: The Chemist Who Left Blanks on Purpose

In 1869 Dmitri Mendeleev laid the known elements out in order and noticed the properties repeated in a pattern. That alone would have been a decent career. What made him famous was the next move: he left blank squares.

The pattern demanded elements that nobody had found. Rather than fudge the table, Mendeleev left holes and described, in advance, what the missing elements would be like — their weight, their density, how they would behave. Within about fifteen years, chemists found them, and they matched. That is a rare thing in science: a table that made testable predictions about things that did not yet exist on paper.

Why gold is Au

Several symbols look like they have nothing to do with the name, because they come from Latin. Gold is Au (aurum). Iron is Fe (ferrum). Lead is Pb (plumbum — the same root as "plumbing", because Roman pipes were lead). Sodium is Na (natrium) and potassium is K (kalium).

If a quiz asks why the symbol does not match the English word, "it is the Latin name" is nearly always the answer.

The element found on the Sun

Helium was detected in the spectrum of sunlight in 1868 — before anyone had found it on Earth. It is named after Helios, the Greek sun god. It remains the only element first identified somewhere other than this planet.

Names, and who gets one

• Marie Curie named polonium after Poland, her homeland, and also discovered radium. She remains the only person to win Nobel Prizes in two different sciences.
• Element 101 is mendelevium, after the man who left the blanks.
• Element 96, curium, honours Marie and Pierre Curie.

Two facts that always come up

Hydrogen is the most abundant element in the universe; oxygen is the most abundant in Earth's crust. Mercury is the only metal that is liquid at room temperature. And diamond and graphite — the hardest natural substance and the stuff in your pencil — are both nothing but carbon, differing only in how the atoms are arranged.

There are 118 confirmed elements. The heavy ones at the end exist for fractions of a second in a laboratory and then fall apart.