The Periodic Table

On the wall of nearly every chemistry classroom hangs a chart of boxes, each with a symbol and a number. It looks like a reference list. It is actually one of the great discoveries of science: a pattern in nature so reliable that its inventor used it to predict elements nobody had ever seen, and described their properties before they were found.

Mendeleev's game of patience

By the 1860s chemists knew about sixty elements, substances that could not be broken down into anything simpler, and had measured the weight of each kind of atom. They also knew that some elements behaved alike. Lithium, sodium and potassium were all soft metals that fizzed in water; chlorine, bromine and iodine were all reactive, bad-smelling and formed salts. But nobody could see why.

In 1869 a Russian professor named Dmitri Mendeleev wrote each element on a card and arranged them in order of atomic weight, then shuffled them into rows so that similar elements fell in the same column. To make the pattern work he had to leave gaps and, in a few places, swap neighbours. He declared that the gaps were elements not yet discovered and predicted their weights, densities and chemistry. Within fifteen years three of them, gallium, scandium and germanium, were found, matching his predictions almost exactly. The pattern was real.

Why there is a pattern

Mendeleev did not know what an atom was. The explanation came fifty years later. The elements are numbered not by weight but by the number of protons in the nucleus, from hydrogen with one to the heaviest known with 118, and that number also fixes how many electrons an atom has. The electrons arrange themselves in shells of fixed capacity: two in the first, eight in the next, and so on. An element's chemistry depends almost entirely on how many electrons sit in its outermost shell, and that number repeats as you go along the list, every time a shell fills and the next begins. That repetition is the periodicity. Each row of the table is one shell being filled. Each column holds elements with the same number of outer electrons, and so the same behaviour.

Reading the table

The columns are called groups. On the far left, group one, are the alkali metals, lithium, sodium and potassium among them, each with one lonely outer electron that they give up eagerly, which is why they react so violently. On the far right are the noble gases, helium, neon and argon, with full outer shells and no interest in reacting with anything; they are used in balloons, lights and welding precisely because they are so inert. Just to their left are the halogens, fluorine, chlorine and the rest, one electron short of a full shell and desperate to grab one, which makes them fiercely reactive and, in the body's hands, excellent disinfectants.

The great block in the middle holds the transition metals, iron, copper, gold, silver, platinum, chromium, nickel, with their strength, conductivity and shine. Two rows usually drawn separately at the bottom hold the rare earths and the heavy radioactive elements, including uranium and plutonium. Metals occupy the left and centre, about three quarters of the table; non-metals cluster at the top right; a stair-step boundary between them holds the semi-metals such as silicon, whose in-between behaviour makes modern electronics possible.

Reading down a column, atoms get bigger and their outer electrons are held more loosely, so the metals become more reactive and the non-metals less. Reading across a row, the atoms shrink and grip their electrons harder. With these rules a chemist can guess how an element she has never handled will behave.

Where the elements came from

Hydrogen and helium, the two lightest, were made in the first minutes after the Big Bang and still account for 98 percent of ordinary matter in the universe. Everything else was forged inside stars: carbon, oxygen and the elements up to iron by fusion in stellar cores, and the heavier ones, gold, silver, lead and uranium, in the violence of supernovae and collisions between neutron stars, then scattered through space to be swept up into new stars and planets. The calcium in your bones and the iron in your blood are star material.

The elements beyond uranium, number 92, do not survive in nature. They have been made one atom at a time by smashing lighter nuclei together, and the heaviest live for only thousandths of a second before falling apart. Whether there is an upper limit, and whether an "island of stability" of longer-lived superheavy elements exists beyond the current edge, is an open question.

Why it matters

The periodic table is a map of everything matter can be made of. It tells chemists what will react with what, engineers what materials are possible, and astronomers what stars are burning. It was discovered by looking for a pattern before anyone understood its cause, and it turned out to encode the deepest structure of the atom. Few single pages contain as much of the universe.