All matter is made from atoms, the smallest particles of an element that still have its properties. Atoms combine with other atoms to form molecules, from two atoms to thousands. A molecular formula is the recipe: it shows the kinds and numbers of atoms in one molecule, so H2O means two hydrogen atoms and one oxygen atom. Some solids, such as table salt, are not molecules but extended structures of repeating units. The same kinds of atoms in different numbers or arrangements make different substances, such as water and hydrogen peroxide.