Everything you have ever touched, from the air in your lungs to the phone in your pocket, is built from fewer than a hundred kinds of natural building blocks. Rearrange them and you get rust, sugar, nylon or a living cell. Chemistry is the study of matter, its properties and how substances change into other substances. It sits between physics, which studies the forces that hold matter together, and biology, which studies the most intricate chemistry we know.

An old idea, a new test

Around 400 BC the Greek thinker Democritus argued that matter is made of tiny pieces that cannot be cut further. He called them atomos, "uncuttable." It was a brilliant guess, but nobody could test it, and for two thousand years most scholars followed Aristotle's view that everything is a blend of earth, water, air and fire. Alchemists spent centuries trying to turn cheap metals into gold. They failed at that goal, but they built furnaces, glassware and methods of distilling that chemists still use.

The turn toward modern chemistry came through careful weighing. In the 1770s and 1780s the French chemist Antoine Lavoisier heated metals and burned substances in sealed vessels and weighed everything before and after. His results, set out in his 1789 textbook, rest on the law of conservation of mass: in a chemical reaction matter is neither created nor destroyed, so the products weigh exactly as much as the starting substances. When wood burns and seems to vanish, its mass has gone into invisible gases. Lavoisier showed that burning is a combination with oxygen, and he defined an element as a substance that could not be broken down further by chemical means. He was guillotined in 1794 during the French Revolution, not for his science but because he had been a tax collector for the old regime.

Dalton's atoms

In 1808 John Dalton, a Quaker schoolteacher in Manchester, turned the old guess into a working theory. He proposed that each element is made of its own kind of atom, and compounds form when atoms of different elements combine in fixed, simple ratios. This explained a puzzle chemists had noticed: pure water always contains the same proportion of hydrogen and oxygen by mass, no matter where it comes from. If water is always two hydrogen atoms joined to one oxygen atom, the fixed proportion follows.

Inside the atom

Dalton's atoms were solid balls, but they turned out to have parts. In 1897 J. J. Thomson discovered the electron, a tiny particle with negative charge. Then, in experiments run by Hans Geiger and Ernest Marsden, Ernest Rutherford's team fired positively charged particles at a thin sheet of gold. Most passed straight through, but a few bounced almost straight back. Rutherford explained the result in 1911: the atom is mostly empty space, with a tiny, dense, positively charged nucleus at its center.

The nucleus holds protons, which carry positive charge, and neutrons, which carry none. Electrons occupy the space around it. The number of protons, called the atomic number, decides which element an atom is. Every atom with six protons is carbon, every atom with eight is oxygen, every atom with 79 is gold. The electrons are where chemistry happens: chemical reactions involve only the electrons, rearranging how atoms are joined, while the nucleus stays unchanged.

Isotopes, elements and compounds

Atoms of one element can still differ in weight. Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 and carbon-14 both have six protons and behave almost identically in reactions, but carbon-14 carries two extra neutrons and is radioactive. Scientists now know 118 elements. Roughly ninety occur in nature; the rest have been made briefly in laboratories.

Elements combine to make compounds, and here is one of chemistry's surprises. A compound has its elements chemically bonded in a fixed ratio and has properties unlike its ingredients, while a mixture can be separated by physical means such as filtering, sorting or boiling. Sodium is a soft metal that bursts into flame in water. Chlorine is a poisonous green gas. Bonded together, they make table salt, which you sprinkle on food. Salt dissolved in water, by contrast, is a mixture: boil the water off and the salt is left behind.

Worked example: why the alchemists never made gold

Lead has 82 protons in each nucleus. Gold has 79. To turn lead into gold you would need to remove three protons from every nucleus, and no amount of heating, dissolving, grinding or mixing can do that, because those processes only shuffle electrons. The alchemists, including Isaac Newton, who wrote over a million words on alchemy, were working with the wrong tools for the job. Their failure was not a lack of skill or patience. Their goal was simply out of reach of chemistry.

Changing a nucleus takes nuclear physics. In 1980 researchers at Berkeley did make a trace of gold, by bombarding bismuth in a particle accelerator. The amount was far too small to see, and the run cost vastly more than the gold was worth. The episode shows the line between chemistry and nuclear physics clearly: chemistry changes which atoms are joined to which, while nuclear reactions change what the atoms are.