Marcellin Berthelot

Marcellin Berthelot was born in Paris, France on October 27, 1827. He is regarded as one of the greatest chemists of all time and was responsible for the Thomsen-Berthelot principle of thermochemistry and for disproving the theory of vitalisim by synthesizing many organic compounds from inorganic substances.

The fundamental conception that underlay all of Berthelot's work was that all chemical phenomena depend on the action of physical forces which can be determined and measured. When he began his career it was assumed that all organic chemistry depended upon vital forces to produce organic compounds. Berthelot opposed this idea and in order to disprove it he synthesized many organic molecules (including hydrocarbons, and natural fats and sugars) from inorganic starting materials, thus proving that the synthesis of these molecules did not depend of vital forces and that organic chemicals obeyed the same principals as inorganic compounds.

His other major contribution is the Thomsen-Berthelot principle of thermochemistry. Berthelot and Dutch chemist Julius Thomsen both independently came up with slightly different formulations for this principle which states that all chemical changes are accompanied by the production of heat and the processes which occur will be the ones in which the most heat is produced. This postulate led to the thermal theory of affinity which postulated that the true measure of chemical affinity was the amount of heat that was produced by a particular reaction. This was later disproved by Herman von Helmholtz, who discovered that the true affinity was not the amount of heat produced by a reaction, but the maximum amount of work or free energy produced when the reaction was carried out reversibly.

For his work in disproving the theory of vitalism and pioneering work in the field of thermochemistry Marcellin Berthelot is the Dead Scientist of the Week for the Week of October 25-31, 2009.

References:

Marcellin Berthelot Wikipedia Entry

Thomsen-Berthelot principle of thermochemistry Wikipedia Entry

Marcellin Berthelot NNDB Entry

James Chadwick

Born on October 20, 1891, James Chadwick won the Nobel Prize in Physics in 1935 for his discovery of the neutron.

In 1932 Chadwick discovered a nuclear particle that did not have any charge (one of his letters announcing the discovery can be found here). These chargeless particles, called neutrons, differ from the previously discovered protons in that they do not have any charge. Because they do not have any charge they can be combined with other nuclei without having to overcome electrostatic repulsion.

Atomic nuclei are composed of two different types of particles, protons and neutrons. They both have about the same mass (neutrons are slightly more massive than protons) but differ in the amount of electrostatic charge they carry. Protons carry a positive charge and neutrons have no charge. Because they are all positively charged protons repel each other and therefore it is difficult to add alpha particles (alpha particles have two protons) to a nucleus. A neutron has no charge and can be used to bombard a nucleus without having to overcome the electrostatic charge.

Chadwick also discovered that atomic number is determined by the number of protons that are found in a nucleus. This is now the definition of atomic number. What element a particular atom is is determined by the number of protons present in its nucleus. For example, 1 proton is hydrogen, two protons is helium, three protons is lithium, etc. A full list of atom:atomic number corespondences can be found on a periodic table.

Chadwick later work with particle accelerators contributed to the making of the atomic fission bomb.

For discovering the neutron James Chadwick is the Dead Scientist of the Week for the week of October 18-24, 2009.

References:

James Chadwick Wikipedia entry

James Chadwick Nobel biography

James Chadwick Answers. com biography

Arthur Harden

Born on October 12, 1885, Arthur Harden shared the 1929 Nobel Prize in chemistry for his work elucidating the glycolytic pathway. This is the pathway by which glucose and other sugars are broken down into smaller molecules and energy is extracted in living cells. Arthur Harden discovered posphorylated intermediates along this pathway and he characterized glucose phosphate and fructose diphosphate, two of the molecules along the pathway.

It has been known since ancient times that fruit juices and other sugar containing liquids, under the right conditions, undergo fermentation. Fermentation is the process by which microorganisms convert sugar into alcohol. Ancient cultures used knowledge of fermentation to produce wine and beer.

The glycolytic pathway is a number of steps in which a six-carbon long sugar molecule is broken down into two three-carbon pyruvate molecules (a really cool discussion of all the steps can be found here). This pathway occurs in nearly all biological organisms, both aerobic (those using oxygen) and anaerobic (those that don't use oxygen). Each sugar molecule that goes through the pathway produces two molecules of pyruvate, two molecules of hydrogenated nicotinamide adenine dinucleotide (NADH+) and two molecules of adenosine triphosphate (ATP). ATP is the primary molecule by which living cells store and transfer chemical energy.

In order to get the glycolytic pathway started two molecules of ATP are used to add phosphate to the six-carbon sugar molecule. The addition of the first phosphate produced glucose phosphate and the addition of the second phosphate produces fructose diphosphate (the two molecules that Arthur Harden characterised). This six-carbon sugar diphosphate is then broken down (through a number of steps) into two three carbon molecules, each of which will add phosphate to two molecules of adenosine diphosphate (ADP) to make two molecules of ATP. In summary two molecules of ATP are used to energize the process which will produce four molecules of ATP, giving a net production of two molecules of ATP for each molecule of sugar that is broken down.

For his work studying the glycolytic pathway Arthur Harden is the Dead Scientist of the Week for the week of October 11-18, 2009.

References:

Arthur Harden, Wikipedia Entry

Glycolysis, Wikipedia Entry

Moran, Laurence, a blog entry on Arthur Harden

Niels Bohr

Born on October 7, 1885, Niels Bohr won the Nobel Prize in physics in 1922 for proposing a structure for the atom and his work in quantum mechanics.

Before Bohr physicists knew that the structure of an atom consisted of a small dense nucleus orbited by electrons. Earnest Rutherford in 1911 published the results of an experiment in which alpha particles emitted by the decay of radium were used to bombard a thin piece of gold foil. The results of the experiment showed that a small amount of the alpha particles were deflected and did not penetrate the gold foil. Rutherford hypothesized that these alpha particles were deflected by the small, hard nuclei of the gold atoms. This led him to propose that atoms were composed of small nuclei surrounded by orbiting electrons that orbited the nuclei in a similar manner to the way planets orbit stars. This model is sometimes called the planetary model (wikipedia entry).

In 1913 Niels Bohr proposed a model for atomic structure where:

1. Electrons orbit nuclei only in certain orbits: orbits set at discrete distances from the nucleus.

2. Electrons can change orbitals, but in doing so they must either absorb (when moving to a higher orbital, further from the nucleus) or emit energy (when moving to a lower orbital, closer to the nucleus).

3. The frequency of the light emitted by an electron changing orbitals is related to the period of the orbital.

The emission of light by electrons falling back into lower orbitals can be seen in neon lights. A gas, sealed in a tube is electrified, causing electrons (normally staying in the lowest energy levels) to move to higher energy levels (higher orbits). When they fall back to their lower, ground state they emit light. This is how neon lights work, and each gas emits a different spectrum giving each gas a distinct color.

For his work determining the structure of atoms, Neils Bohr is our Dead Scientist of the Week for the week of October 4-10, 2009.

References:

Neils Bohr Wikipedia Entry

Bohr Model, Wikipedia Entry

Carpi, Anthony, Vision Learning, Atomic Structure II