Section 3–2 Chemistry

(Theoretical chemistry / Inorganic chemistry / Organic chemistry)

Feynman states that the science which is perhaps the most deeply affected by physics is chemistry. In this section, the three main ideas discussed are theoretical chemistry, inorganic chemistry, and organic chemistry.

1. Theoretical chemistry:

“… All these rules were ultimately explained in principle by quantum mechanics, so that theoretical chemistry is in fact physics (Feynman et al., 1963, section 3.2 Chemistry).”

According to Feynman, the deepest part of theoretical chemistry must end up in quantum mechanics and in short, theoretical chemistry is physics. Furthermore, statistical mechanics, the science of the phenomena of heat or thermodynamics, was developed by both sciences. This branch of physics and chemistry adopts statistical methods in microscopic situations that are also based on mechanical laws. In general, chemical reactions involve a large number of atoms that are all jiggling around in a random and complicated manner. Nevertheless, it is practically impossible to follow in detail the motions of all molecules, and it is beyond the capacity of computers and the human mind to predict the motions of all molecules. The method for dealing with such complicated situations is also known as statistical thermodynamics in chemistry.

In a sense, chemistry is closely related to physics to the extent that Marie Sklodowska Curie was known to be a chemist and physicist for her works in radioactivity. In 1903, she was awarded Nobel Prize in Physics for her research on the radiation phenomena discovered by Henri Becquerel. In 1911, Marie was awarded Nobel Prize in Chemistry for the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of radium. In her Nobel Lecture, Curie (1911) mentions that “only 15 years after Becquerel's discovery, we are face to face with a whole world of new phenomena belonging to a field which, despite its close connexion with the fields of physics and chemistry, is particularly well-defined (p. 202).” She also calls this field of science as the chemistry of the imponderable.

2. Inorganic chemistry:

“Inorganic chemistry is, as a science, now reduced essentially to what are called physical chemistry and quantum chemistry … (Feynman et al., 1963, section 3.2 Chemistry).”

Inorganic chemistry can be described as the chemistry of substances which are not associated with living things. For example, it can be related to elements and how they form various relatively simple compounds that are found in rocks and earth. In addition, Feynman explains that inorganic chemistry can be further classified as physical chemistry and quantum chemistry: (1) physical chemistry studies the rates of chemical reactions and their detailed processes; (2) quantum chemistry helps us to understand what happens in terms of the physical laws. To be more precise, quantum chemistry focuses on applying principles of quantum mechanics in physical models and experiments of chemical systems.

It is worth mentioning that Ernest Rutherford was awarded Nobel Prize in Chemistry in 1908 for his work in the disintegration of the elements, and the chemistry of radioactive substances. Interestingly, Rutherford was startled and joked that the sudden “metamorphosis into a chemist” was quite unexpected. However, the Nobel Committees for Physics and Chemistry view that Rutherford’s research is more relevant to chemistry than physics. On the other hand, Walter Kohn was awarded Nobel Prize in Chemistry in 1998 for his work in the development of the density-functional theory. It helps to calculate quantum mechanical electronic structure by equations involving the electronic density instead of the many-body wavefunction. Kohn was conferred a Ph.D. degree in physics by Harvard University in 1948, where was supervised by Schwinger on the three-body scattering problem.

Note: John Pople was awarded Nobel Prize in Chemistry in 1998 for his work in the development of computational methods in quantum chemistry. He was conferred a Ph.D. in mathematics in 1951 for his research on lone pair electrons and considered himself more of a mathematician instead of a chemist. Pople also accepted positions as a professor of chemical physics at Carnegie Institute of Technology and head of the new Basic Physics Division at the National Physical Laboratory.

3. Organic chemistry:

“…The other branch of chemistry is organic chemistry, the chemistry of the substances which are associated with living things (Feynman et al., 1963, section 3.2 Chemistry).”

Organic chemistry is a branch of chemistry that studies the structure, properties, and reactions of organic compounds. Historically, the substances associated with living things were so complicated that scientists believed they could not be made by human beings from inorganic materials. This is not true because organic materials are similar to substances made in inorganic chemistry, but they are usually more complicated arrangements of atoms. In daily lives, organic chemistry has a very close relationship to the biology and industry, and more importantly, physical chemistry and quantum mechanics can be applied to both organic and inorganic compounds. Currently, organic chemistry involves the analysis and synthesis of the substances which are formed in biological systems, and thus, it leads to further branches such as biochemistry and molecular biology.

Recently, William Moerner was awarded Nobel Prize in Chemistry in 2014 for his work in the development of super-resolved fluorescence microscopy. Moerner is sometimes known as a physical chemist or chemical physicist that contributes in imaging of organic molecules and probing biological processes. He was conferred a Ph.D. degree in physics in 1982 for his research on vibrational relaxation dynamics of an IR-laser-excited molecular impurity mode in alkali halide lattices.

Questions for discussion:

1. Do you agree that theoretical chemistry is in fact physics?

2. Can inorganic chemistry be essentially reduced to physical chemistry and quantum chemistry?

3. What are the main differences between organic chemistry and inorganic chemistry?

The moral of the lesson: the fundamental rules of theoretical chemistry are ultimately explained in principle by physics and physicists could be awarded Nobel prizes in Chemistry. In other words, physicists help to build (or shake) the foundations of chemistry.

References:

1. Curie, M. (1911). Radium and the New Concepts in Chemistry. In Nobel Lectures in Chemistry 1901 – 1921. Singapore: World Scientific.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.