Secondary Science

Chemistry 1913 – a centenary of chemistry

We have seen in previous articles that 1913 was an important year for developing the ideas that are taught in GCSE chemistry. We have seen how

  • Mosely showed that it was atomic number, the number of positive charges on the nucleus of the atom, that determines what the element is;
  • Bohr developed Rutherford’s nuclear atom, stating that the electrons occupied stable shells and thus explaining the chemical behaviour of elements and their arrangement in the Periodic Table
  • Soddy defined isotopes as atoms with differing mass but the same atomic number and that relative atomic mass is the average of the mass of all the atoms in a sample.

These weren’t the only important breakthroughs of the year.  In the summer of 1913 the father and son partnership of W.H. and W.L.Bragg used X-ray crystallography for the first time to determine the positions of atoms and ions in the structure of solids.  When they photographed X rays after they had been diffracted through a diamond they used the pattern of bright dots to calculate that the each carbon atom was at the centre of a tetrahedron of other carbon atoms.  Lawrence Bragg, the son, then used the same technique to determine the cubic arrangement of ions in sodium chloride, potassium chloride and potassium bromide.  For the first time scientists could practically “see” atoms and ions as they existed in crystals.  Just two years later the Braggs received the Nobel Prize and their work began the whole science of X-ray crystallography that lead to the structure of DNA in 1953 and many other discoveries.

Since his discovery of the electron in 1997 J.J.Thomson had been leading the Cavendish Laboratory at Cambridge University. His team had perhaps been overshadowed by Rutherford’s many discoveries. In 1913 Thomson came back to prominence with a demonstration of the possibilities of mass spectrometry.  In a similar method to how he discovered and measured the mass of the electron, Thomson used charged electric plates to accelerate positively charged ions and then a magnetic field to bend the path of the ions. Ions of different mass travelled along different paths. By varying the magnetic field Thomson was able bring each type of ion in turn to a detector and measure their mass.  In this way he showed that neon was made up of two isotopes of mass number 20 and 22.  He passed his work on to a colleague, Francis Aston, who developed it into the mass spectrograph.  This could be used to identify the mass and abundance of isotopes in a sample of an element or to identify molecules by the pattern of fragments they broke up into in the machine.  The mass spectrograph or mass spectrometer has become one of the most important tools scientists have to analyse samples.  Miniaturised mass spectrometers now travel on space probes to Mars to analyse samples of from the atmosphere or the soil.

We have seen that in its early years the Nobel Prize was often awarded very soon after a discovery was announced such as the Braggs work on X ray crystallography.  The 1913 Nobel Prize was however awarded to Alfred Werner for research he started over twenty years earlier, in 1889 when he was a 23 year old assistant to a professor. He studied the arrangement of atoms in molecules containing nitrogen.  In the following years he held various posts in Zurich during which he developed his theories of the ways that atoms were arranged in molecules. He developed the idea of valency or combining number of atoms and how this would result in particular patterns of atoms around a central atom in a molecule.  A common arrangement that Werner investigated is one in which 6 atoms or groups of atoms are arranged around a central atom in the shape of an octahedron.  This theory of valency and what became called coordination number explained the structure of thousands of compounds and gives rise to the diagrams we draw today of molecules such as ammonia, NH3 and methane, CH4.  The 1913 Nobel Prize was awarded for one of the most important insights into the way substances behave.

1913 was thus a glorious year for chemistry and one which still influences us today. The following year also had a great effect on science in a more disturbing way.  The outbreak of the First World War in the summer of 1914 saw scientists who had worked together take opposite sides (e.g. Hans Geiger and Ernest Rutherford), young researchers like Henry Mosely killed in battle and the scientists who spent their time exploring the nature of matter turning their attention to designing new weapons and methods of fighting war.  Science was never quite the same again.

Activities

  1. Find out more about the life and work of the scientists mentioned here – W.H. and W.L. Bragg, J.J. Thomson, Francis Aston and Alfred Werner.
  2. (a) Draw a part of the diamond structure worked out by the Braggs.  (b) Find out and sketch the arrangement of sodium and chloride ions in sodium chloride crystals and of potassium and chloride ions in potassium chloride crystals.
  3. Explain why X ray crystallography was such an important new tool for chemists.
  4. What difference is there between the atoms of neon-20 and neon-22?
  5. Find out about the uses of mass spectrometers today.
  6. Draw 3-D diagrams showing the position of the atoms in  (a) ammonia, NH3;  (b) methane, CH4;  (c) sulphur hexafluoride, SF6
  7. Why is knowing the shape of molecules important to chemists?
  8. What do you think was the most important chemical discovery of 1913?  Give your reasons for your choice.

Peter Ellis

Collins Secondary

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