The smallest indivisible unit of a substance is an atom. For example, if you have a sheet of carbon, you could cut the sheet in half and the two halves would still be carbon. You could continue cutting the sheet into smaller and smaller pieces until you get to a single atom. You could not break the atom apart and still have carbon. Three fundamental particles make up an atom; positively charged protons, negatively charged electrons and uncharged neutrons.
The number of protons in an atom--or atomic number--determines what element it is. The periodic table of the elements arranges all known elements by atomic number and groups them by chemical properties. Hydrogen, the simplest element, has one proton and is listed first on the periodic table. The highly unstable synthetic element ununoctium is the heaviest, with an atomic number of 118.
Most elements don't exist as single atoms in nature. Atoms of a single element bond to each other to form elemental molecules. Some---such as hydrogen, oxygen and nitrogen---pair up to form two-atom molecules, called diatomic. The chemical notation for these molecules is H2, O2 and N2, respectively. Atoms of other elements, such as carbon, can combine with each other in a number of different ways, forming molecular chains with different physical properties. Only the noble gasses--helium, neon, argon, krypton, xenon and radon--exist as single atoms.
Atoms of different elements can bond to form a compound molecule. The compound often has characteristics unlike the physical properties of either element. The two gaseous elements hydrogen and oxygen combine in a 2-1 ratio to form water (H2O), a liquid. Add another oxygen atom and you get the disinfectant hydrogen peroxide (H2O2), with very different properties. It is the combination of different elements, under various conditions, that forms all the substance of the universe.