Mostly empty space
Everything around you is made of an ATOM, or rather an unimaginable number of them. A single human hair is several hundred thousand carbon atoms across. Yet atoms are almost entirely empty. At the centre is a tiny nucleus made of the PROTON and NEUTRON, and around it move the much lighter ELECTRON particles. If an atom were the size of a football stadium, its nucleus would be about the size of a pea on the centre spot.
Nobody knew this until 1911. Ernest Rutherford's team fired tiny particles at a sheet of gold foil thinner than paper. Most went straight through, but a very few bounced almost straight back. Rutherford said it was as if you had fired a huge shell at a piece of tissue paper and it had come back and hit you. The only explanation was that nearly all an atom's mass is packed into a minute centre.
Atoms swapping partners
When atoms join together with a chemical BOND, they make a MOLECULE. A water molecule is two HYDROGEN atoms bonded to one OXYGEN atom, so its FORMULA is H2O. In a chemical REACTION the bonds break and the atoms rearrange into new substances. Rusting is a slow reaction between iron, oxygen and water; burning is a fast one that gives off heat and light.
In the 1770s the French chemist Antoine Lavoisier weighed things carefully before and after they reacted, even in sealed containers, and showed that nothing is ever lost: the atoms are only rearranged. A CATALYST speeds up a reaction without being used up. The enzymes in your body are catalysts, and so is the metal inside a car's exhaust that turns harmful gases into less harmful ones.
Why fizzy drinks go flat
A GAS can dissolve in a LIQUID the way sugar dissolves in tea, making a SOLUTION. More gas dissolves when the liquid is cold and under pressure, and less when it is warm, so a fizzy drink left in the sun goes flat faster than one kept in the fridge. Fish rely on the same thing: warm water holds less dissolved oxygen, which is one reason fish struggle in hot summers. Our drinks page covers how fizzy water was invented.
A MIXTURE, like sand stirred into water, can be separated again by simple means such as filtering, while a COMPOUND, like water itself, needs a chemical reaction to split it back into its elements. A SOLID, a liquid and a gas are the three everyday states of matter.
The most common elements
Hydrogen is by far the most common ELEMENT in the universe, making up about three quarters of all ordinary matter, because it formed in the Big Bang and fuels the stars. On Earth, oxygen is the most common element in the rocks of the crust, and it makes up about two thirds of your body's weight, mostly locked up in water. CARBON is special because it can link to itself in long chains and rings, and it is the backbone of every living thing. Diamond and the graphite in a pencil are both pure carbon, with the atoms arranged differently.
An ACID and a BASE are opposites that cancel each other out, and the PERIODIC table sorts every known element; both have their own pages. In the lab, chemists mix chemicals in a BEAKER or a FLASK, as our lab equipment page shows.