Matter is present in the universe with different atomical structures. These different atomical structures are called elements. The symplest element in the universe is hydrogen which is made up of the symplest atom which consists of one neutron, one proton, orbited about by one electron. Atoms combine to form molecules. A molecule can be composed of different elements. For example, one atom of oxygen + two atoms of hydrogen combine together to form one molecule of water. For a better understanding each elements is designated with one or two letters to represent them; oxygen with O, and hydrogen with H. Thus a molecule of water can be represented as H2O.
Elements are divided into two groups, metalic, and non-metalic. Iron (Fe), for example, is a metalic element, whereas potassium (K) is a non-metalic element. The Russian chemist Dmitri Mendeleev devised a table in 1869 containing all the elements, metalic and non-metalic, each one with their atomic numbers and weights. It is called the periodic table of elements.