WJEC GCSE Chemistry

## Elements An **element** is a substance that cannot be broken down into anything simpler by chemical means, and every substance in the universe is built from the roughly 100 elements. It contains only one kind of atom: a piece of pure copper is copper atoms and nothing else, and a sample of oxygen gas is oxygen atoms, joined in pairs as $\ce{O2}$ molecules. Each element has a one or two letter symbol, with the first letter a capital and the second, if there is one, in lower case: C, O, Na, Cl. Some symbols come from older names, so sodium is Na and iron is Fe. Some elements exist as single atoms (helium, He), some as molecules made of a fixed number of atoms joined together ($\ce{H2}$, $\ce{N2}$, $\ce{O2}$, $\ce{Cl2}$, $\ce{Br2}$, $\ce{I2}$, $\ce{P4}$, $\ce{S8}$), and some as giant structures (carbon, silicon, metals). A chemical formula says how many atoms of each element are in one molecule or in the simplest unit of the substance. ## Compounds A **compound** contains two or more different kinds of atom that are chemically joined, in a fixed proportion. Water is always $\ce{H2O}$, two hydrogen atoms for every oxygen atom, and a compound with a different ratio would be a different compound (hydrogen peroxide, $\ce{H2O2}$, is an example). The properties of a compound are completely different from those of the elements in it. Sodium is a soft, reactive metal and chlorine is a poisonous green gas, yet sodium chloride is the harmless white solid in a salt cellar. A compound…