The periodic table of elements

What is truly elemental? Ancient Greek philosophers believed there were only four basic elements—fire, air, water, and earth. Modern chemists found more, and redefined elements as substances that cannot be further split, and from which all other matter is formed—the building blocks of the universe.

This definition requires a theoretical framework to understand the properties of each of these building blocks relative to one another. Scientists started out by throwing the known elements into a table. They arranged and rearranged the table as new elements were discovered and new patterns of properties emerged. The table didn’t just accommodate existing elements, but also guided researchers toward the discoveries of new ones, and even the creation of elements that don’t exist in nature.

Contemporary scientists, though, still see room for improvement. Pull up a chair.

63: Elements in the periodic table of elements Dmitri Mendeleev published in 1869

118: Elements in today’s periodic table

94: Elements found naturally on earth

24: Man-made, or synthetic, elements in the periodic table

30+: Elements that exist in human bodies—96% of our body is made of carbon, oxygen, hydrogen, and nitrogen

75: Elements that go into a smartphone

75%: Share of the universe that is hydrogen, the most abundant element

What does the periodic table do? Here are some of the basics.

Atomic number: The number of protons in the nucleus. Elements are arranged left to right, top to bottom based on it. A bigger atomic number generally means a bigger atomic mass.

Periods: Elements in each row contain the same number of electron shells.

Groups: Elements in each column (mostly) contain the same number of valence electrons.

Metallic character: Or, how easily an element loses an electron; increases right to left and top to bottom.

Atomic radius: Increases right to left and top to bottom.

Electronegativity: A measure of an atom’s propensity to attract the electrons in a bond; increases left to right and bottom to top.

There are elements that don’t quite fit the patterns, which is why there’s no one perfect periodic table (or, for that matter, definition of element). Some scientists want to build a better one.

Building on top of prior discoveries, Russian chemist Dmitri Mendeleev created a periodic table very similar to the one we see today while writing a chemistry textbook, arranging the elements by atomic weight and splitting them into groups of eight, so that elements of similar chemical properties appeared in the same column.

Just after Ernest Rutherford unlocked the structure of the atom, theoretical-physics enthusiast Anton van den Broek, suggested reordering the periodic table based on atomic charge, the number of protons in the nucleus, instead of atomic weight.

Henry Moseley, who had been a student of Rutherford, tested van den Broek’s hypothesis by blasting samples of the elements with x-rays. The frequencies emitted turned out to have a remarkably straightforward relationship to the atomic number. Rearranging the periodic table both predicted new elements and disproved hypothetical ones. But Moseley’s breakthrough coincided with the breakout of World War I, and he was killed in 1915.

“You could stack the top two rows without much trouble. The third row would burn you with fire. The fourth row would kill you with toxic smoke. The fifth row would do all that stuff PLUS give you a mild dose of radiation. The sixth row would explode violently, destroying the building in a cloud of radioactive, poisonous fire and dust. Do not build the seventh row.”

—Randall Munroe answers the question, “what if you made a periodic table out of cube-shaped bricks, where each brick was made of the corresponding element?”

1789: Antoine-Laurent Lavoisier presents the first table of simple chemical substances, containing 33 elements divided into four columns: gasses, non-metals, metals, and soil.

1818: Jöns Jacob Berzelius introduces today’s system of chemical symbols.

1862: Alexandre Béguyer de Chancourtois publishes a 3D arrangement of the elements, the first to use a periodic arrangement, showing that similar elements appear at periodic atomic weights.

1869: Dmitri Mendeleev uses cards to arrange 63 elements into a table that resembles today’s.

1913: Henry Moseley discovers that each element emits X-rays at a unique frequency, and uses that information to reorder the table by atomic number.

1945: Glenn Seaborg suggests that actinide elements should fill an F sub-level, a discovery that leads to a Nobel Prize in 1951.

2015: On Dec. 30, The International Union of Pure and Applied Chemistry (IUPAC) adds four new elements (number 113, 115, 117, and 118) and completes the seventh row in the periodic table.

It would cost several thousand dollars to purchase the entire table, minus the last few radioactive elements, like one displayed at the University of Iowa. You can though, purchase a version that includes tiny bits of 83 elements for less than $200.

The best way to memorize the periodic table of elements? Listen to this song, a classic that includes 102 elements, written and sung by Tom Lehrer.

The IUPAC gives the naming rights to the discoverers of elements. Elements can be named after a mythological concept or character; a mineral, or similar substance; a place or geographical region; a property of the element; or a scientist. The four elements (113, 115, 117 and 118) discovered in 2015 were named after Japan (nihonium), Moscow (moscovium), Tennessee (tennessine), and Russian nuclear physicist Yuri T. Oganessian (oganesson).