Magnesium is made up of atoms only, so it is an element. An element is a substance that is made up of one kind of atom only. Every element has a name and a symbol and is displayed on the Periodic Table. Element Symbol: Carbon C Hydrogen H Aluminium Al Sodium Na Potassium K Lithium Li Silver Ag Mercury Hg Lead Pb Elements can combine with each other to form compounds. A compound contains two or more kinds of atoms joined together. The symbol for a compound is called its formula. It is made up from the symbols of the elements it contains and also tells us about the % composition of each element within the compound. Example: H2O - A molecule of water contains 2 hydrogen atom for every oxygen atom.