The Periodic Table: The Chemist Who Left Blanks on Purpose

In 1869 Dmitri Mendeleev laid the known elements out in order and noticed that their 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 Chemist Who Left Blanks on Purpose

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. He gave them placeholder names such as "eka-aluminium" and "eka-silicon".

Within about fifteen years, chemists found them: gallium in 1875, scandium in 1879 and germanium in 1886. They matched his predictions closely. 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 — Au (aurum)
• Silver — Ag (argentum). Argentina is named after silver.
• Iron — Fe (ferrum)
• Lead — Pb (plumbum) — the same root as "plumbing", because Roman pipes were lead.
• Tin — Sn (stannum)
• Sodium — Na (natrium) and potassium — K (kalium)

Tungsten is the odd one out: its symbol, W, comes from its German name, Wolfram. If a quiz asks why a 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 is the second most abundant element in the universe, yet it is rare on Earth because it is so light that it escapes into space.

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.
• Einsteinium (99) was first found in the debris of the first hydrogen bomb test in 1952.
• Oganesson (118) is named after the Russian physicist Yuri Oganessian — one of only two elements named after a living person at the time (the other is seaborgium).
• The village of Ytterby in Sweden gave its name to four elements: yttrium, ytterbium, terbium and erbium — all found in minerals from a nearby mine.
• Technetium (43) was the first element to be made artificially, in 1937. Its name comes from the Greek for "artificial".

Records and Oddities

• 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 bromine is the only liquid non-metal.
• Gallium melts at about 30°C — it will melt in your hand.
• 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.
• The noble gases — helium, neon, argon, krypton, xenon and radon — barely react with anything, which is why neon is used in signs and argon in light bulbs.

How the Table Is Organised

The modern periodic table orders elements by atomic number — the number of protons in each atom. Columns, called groups, contain elements with similar properties: the reactive alkali metals on the far left, the noble gases on the far right. Rows are called periods. Once you understand the layout, you can predict a lot about an element just from its position — exactly what Mendeleev did.

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