Molecules of elements: The molecules of an element contain two or more similar atom chemically bonded together, for example oxygen gas has 2 oxygen atoms combined together whereas ozone gas has 3 oxygen atoms combined together, so ozone exists in the form of O3. In contrast to it, molecules of compounds contain two or more different types of atoms chemically bonded together. For example: the molecule sulphur dioxide (SO2) contain one atom of sulphur chemically bonded with two atom of oxygen.