As one of the last blog posts for today, we will be discussing the field of chemistry where quantum mechanics and essentially quantum theory has been derived from: physical chemistry. One of the first ideas that was proposed that started quantum theory was proposed by Planck which was the idea that energy was discontinuous. This idea originated from the observation of blackbody radiation. The definition of the blackbody is an object that has the property that no form of radiation is reflected over the body i.e. the blackbody is able to absorb all of the radiation. Logically, this type of object that can perfectly absorb all radiation can also perfectly emit all radiation. As a result, this type of object will be able to radiate the maximum amount of energy possible at a given temperature. Additionally, this type of object would be able to emit this type of energy infinitely. However, based on variety of observations and experiments, researchers saw that this blackbody object was not able to emit energy for an infinite amount of time. The reason for this explanation is the basis of quantum theory.
What happens in quantum mechanics is that energy is limited to a specific set of values. Each of these values is not continuous but the differences in these energy levels is small tiny jumps which are known as quantums. These quantums are equivalent to small energy packets, hence the small difference in energy in different energy levels. Because of this revelation and the assumption that any atom that exist on the surface of a heated solid vibrates at the given frequency, Planck created his most famous equation known as Planck’s equation:
where E is the energy, v is frequency, and h is planck’s constant which is equal to 6.62607*10^-34 J*s. However, when this was initially created, it was highly skeptical because his hypothesis could not be applied to anything other than blackbody radiation.
However, another phenomenon in chemistry came to light that fully supported Planck’s hypothesis about the existence of quantums. This phenomenon was known as the photoelectric effect which states that when light is shined on a substance, electrons are ejected from that substance. Through experiments, it has been shown that the frequency of light which is proportional to how much energy the light actually contains, must be above a certain limiter value in order to allow electrons to be fully emitted. In order to explain this effect, which contradicted normal classical chemistry because under the classical world, electrons would be ejected at any frequency as long as the light had enough intensity but this was proven completely false due to experimentation, Einstein proposed that any radiation also had particle like qualities. These particles of light were known as photons. Each of these photons has an energy level that is equal to quantum of energy and it is this value of energy that must be reached in order for an electron to leave a metallic surface. The quantum of energy is described by the following equation:
E = hv = 1/2*(m)*(u^2) + w
where the first part of the RHS is the kinetic energy while w = work.
The reason why this effect is so significant is because the relationship between radiation and a particle of matter allowed scientists to understand the wave theory of radiation wasn’t going to be enough to explain the majority of phenomena in the world. This led to the discovery of wave-particle duality, which states that light is both a particle and a wave. This was later shown through G.P. Thomson’s famous double slit experiment which showed the interference patterns that occurs when light enters 2 different slits. This can only occur if light is indeed a wave. All in all, quantum theory has started to allow us to describe many of the phenomena within the world of chemistry and hopefully one day, we can fully explain and understand every phenomena in chemistry through the world of quantum mechanics.