Elements are pure substances that cannot be broken down into simpler substances through chemical means. Elements are represented by one or two letters on the periodic table. When elements combines chemically they form chemical compounds and molecules. They may combine in definite proportions or ratios by mass to forms a chemical compund with constant properties

For example, Water (H2O ) is a compounds. Two atoms of hydrogen (H) combine to one atom of Oxygen (O) molecules, in which the properties and composition to H2O molecule is always the same.