20th century chemistry

The applications of electrochemistry were described in the early 1900s by Joseph Thompson (1856-1940) and Robert Millikan (1868-1953), who, amongst other things, noted that charged particles were not only deflected by magnets, but also by an electric field.  This lead to the discovery of the electron in 1891, which was built on by Philipp Lenard (1862-1947) when he proposed the Photoelectric Effect, the idea that electrons could be emitted from a metal.  


But perhaps the most fundamental discovery of the 20th century, the culmination of centuries worth of experimentation and theory, was that of the nuclear atom (see left).  Developing Marie Curie’s work on radioactivity, Rutherford concluded that an atom comprised a dense nucleus surrounded by empty space, in which electrons were found.  


Nuclear atom: image reproduced from http://www.edu.bham.ac.uk/images/pgce/chemistry.jpg



In 1920 Ernest Rutherford (1871-1937) (see right) identified and named the proton as a unit of positive charge, and the mass of a hydrogen atom.  In 1932 James Chadwick suggested the concept of a neutron, a particle of equal mass to the proton, but with no charge. 



Rutherford: image reproduced from http://dbhs.wvusd.k12.ca.us/gallery.html


The idea of a nuclear atom implied that during a chemical reaction, only the outer electrons would be affected.  This led to the valence theory and was the origin of our understanding of chemical bonds.


In 1913 Henry Moseley (1887-1915) (see left) identified the trend of decreasing wavelengths of emitted x-rays with increasing atomic weight of the element concerned, as a direct result of the magnitude of the positive charge in the nucleus.  This was already found to be reflected in the order of the periodic table as devised by Mendeleev in the previous century.


Moseley: image reproduced from http://dbhs.wvusd.k12.ca.us/gallery.html


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