Molecules are small objects not susceptible of direct observation even under the most powerful microscope. However, their properties can be deduced indirectly from experiments. These objects are different on the conceptual level as well. Classical physics can no longer offer a consistent description. It is replaced by quantum theory, which describes the objects only in term of probability, energy levels, and other quantum numbers. A well-defined orbit such as the path of a planet around the Sun becomes the probability of finding the object at a certain location in the microscopic world. Thus, all the illustrations related to these objects would be just a schematic diagram conveying some ideas, they should never be taken literally as the real thing. Historically, the quantum theory began with the attempt to account for the discrepancy between the theoretical and observational blackbody radiation. The classical theory of Rayleigh-Jeans failed to fit the observation of the radiation energy distribution from a blackbody at high frequency. In searching for a modification that would reduce the contribution of the high frequencies to the energy, Planck was led to make an assumption: The energy of the radiation with frequency is restricted to integral multiples of a basic unit hv (a quantum), i.e., E = nh where h = 6.625x10 -27 erg-sec is the Planck constant and n is an integer. With this assumption, Planck obtained an exact fit to the observed distribution of radiation energy. According to classical theory, electromagnetic radiation is a wave phenomenon. The Planck's assumption