Chemistry is the study of the composition, properties and transformations of substances. One has to distinguish between physical properties (shape, colour, texture, etc.) and chemical properties such as the fundamental transformations a substance, for example, wood, undergoes when tested. One of the simplest transformational experiments is to see what happens when they are heated. Chemists have systematically studied thousands of different types of materials using many different types of transformational experiments. To explain how elements combined to form compounds with very different properties than those of the component elements, a theory of atoms was gradually developed during the 1800s. In Rutherford’s model of the atom, atoms are made up of three types of subatomic particles of matter called protons, neutrons and electrons. Like most molecules made by living organisms, essential oil molecules are mainly made up of hydrogen and carbon atoms. Bonding is an interaction between two atoms. There are two main types of bonding: ionic and covalent.