Scientists and Theories of Atomic Structure

   Dalton used his own symbols to represent atoms of different elements and developed the first table of atomic masses.  This hypothesis remains the foundation of modern chemistry.

   Although Dalton’s theory was a breakthrough as to how atoms worked, it didn’t quite expand on the actual structure of the atom itself. The ‘indivisible’ part of the theory was later proved wrong in 1897 by a man named J.J. Thomson who provided the first hint that an atom is made up of even smaller particles and from this he produced the ‘plum pudding model’. The reason he compared the atom to a plum pudding was due to the comparison of scattered electrons in the atom to raisons in a plum pudding.  

  Thomson’s discovery of the ‘electron’ was through a series of experiments using cathode rays. He studied the passage of an electric current through a gas. In doing so, Thomson discovered that cathode rays were a stream of negatively charged particles and concluded that they must have come from within the atoms of the electrodes themselves. Therefore he was able to show that atoms were made up of smaller particles thus disproving the ideas proposed by the ancient Greeks and Dalton. The gas used was neutral and atoms had no charge so he knew there must be positive charge within the atom. From this he came to a conclusion that an atom should be thought of as being made of up negative electrons’ moving around in a ‘sea’ of positive charge.

   Around the same time of Thomson, a lady called Marie Curie also came to the same conclusion that some atoms (at least the one she was studying) are divisible. Her contribution to the development of the atomic structure was not direct but still mattered because it was a revolutionary discovery disproving those that came before her.

   She discovered and isolated radium, a new element which spontaneously disintegrated into other elements- She called this process ‘radioactivity’. Through continued systematic studies of the different chemical compounds gave the surprising result that the strength of the radiation didn’t depend on the compound that was being studied. Marie came to the conclusion that the ability to radiate must be linked to the interior of the atom itself as oppose to the arrangement of the atoms in a molecule. This proved that the atoms of one element at least were not indivisible and therefore disproving Dalton’s theory. However Thomson had already discovered sub-atomic particles so in all fairness Marie Curie didn’t really change anything – she just added to this idea of atoms being divisible.

   Nevertheless the ‘plum pudding model’ was later disproved by Ernest Rutherford who is known as the ‘father of nuclear physics’ due to pioneering of the orbital theory of the atom with his famous much anticipated gold foil experiment. Rutherford and two of his students carried out an experiment which led to the downfall of Thomson’s theory. They directed a beam of alpha particles towards a sheet of very thin gold and measured any deflection of the particles however the result they got was not what they expected. Most positive charges passed through the foil, a small percentage of particles were deflected significantly indicating a small concentrated positive charge. He stated that the model of the nuclear atom is as follows: the positive charge of an atom and most of its mass is concentrated in a small dense nucleus, at the centre; Negative electrons orbit the nucleus as oppose to being ‘scattered’ in the model because the alpha particles passing through shows that most of the atom is ‘open space’; The volume of the atom is contained in this ‘open space’ between the small nucleus and the orbiting electrons.

    A chemist who worked under Rutherford, named Henry Moseley discovered that the energy of x-rays released by the elements increased in a linear fashion with each successive element in the periodic table.  In 1913, he proposed that the relationship was a function of the positive charge on the nucleus.  After discovering this he then rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements.  This new table left additional holes for elements that would soon be discovered. 

   At this point the structure of the atom was beginning to come together bit by bit through the participation of many different people. In 1913 Niels Bohr, a Danish physicist altered Rutherford’s model to allow electrons to follow only certain paths otherwise electrons would spiral into the nucleus. These were specific energy levels located at certain distances from the nucleus, a structure known as the ‘planetary’ atom, in which electrons orbited a central nuclear ‘sun’ in ‘shells’. However instead of gravity, the force providing attraction between the nucleus and the orbiting electrons was called electrostatic forces.

     Although it is unsure of who exactly made the discovery of the proton, the credit would most likely go to Rutherford because only through his discovery of the electron was it possible for the proton to be discovered. The negatively charged electron and the overall neutral charge of an element would mean that something balanced out the charges so there must be a positive charge within the atom.

   In 1918 Rutherford discovered the ‘proton’ whilst performing his various experiments in the field of radioactivity, bombarding nitrogen gas with alpha particles - he noticed that one of the experimental results was a flow of hydrogen. Him and others carried out a number of experiments, transmuting one atom into another by striking it with high energy α particles and each time hydrogen nuclei were emitted. He correctly deduced that the hydrogen atoms must have come from within the nitrogen atoms themselves, which would mean that there was something within all of these atoms which was divisible, the amount of which would determine the charge of the nucleus in the atom. This integer was the number of hydrogen nuclei or ‘protons’ so therefore the particle Rutherford isolated was the proton.

   Finally, in 1932 an English physicist named James Chadwick bombarded light elements with high energy alpha particles. He found that a new type of radiation was emitted much more penetrating than any that had been detected before. He showed that this new type of radiation was made up of uncharged particles with approximately the same mass as a proton, known as neutrons. This discovery took place roughly after 20 years