All substances are made form atoms. A substance that is made of the only one sort of atom is called an element. There are about 100 different elements. Elements are shown in the periodic table. The groups contain elements with properties. Atoms of each are represented by a chemical symbol, eg O represents an atom of oxyegen, and Na represents an atom of sodium. Atoms have a small central nucleus, which is made up of protons and nuetrons and around which there are elements. The number of a particular elements have the sam enumber of protons. Atoms of different elements and neutrons in an atom is its mass number.