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1a. History Of The Atom (2)


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1a. History Of The Atom (2)

  1. 1. Atomic Structure <ul><li>Principle protagonists </li></ul><ul><li>Empedocles (Greek) </li></ul><ul><li>Leucippe of Milet (Greek) </li></ul><ul><li>Democritus (Greek) </li></ul><ul><li>Robert Boyle </li></ul><ul><li>John Dalton) </li></ul><ul><li>Henri Becquerel </li></ul><ul><li>Sir J J Thomson </li></ul><ul><li>Ernest Rutherford </li></ul><ul><li>Niels Bohr </li></ul><ul><li>James Chadwick </li></ul><ul><li>X </li></ul>
  2. 2. The Ancient Greek Philosophers <ul><li>Empedocles (c.492-432 BC). Divided matter into four elements: Earth, air, fire, water. Developed the idea that these elements were bound together by love and forced apart by conflict </li></ul><ul><li>Leucippe of Milet (420 BC). Had the idea that matter is composed of particles. </li></ul><ul><li>Democritus (460–370 BC). Follower of Leucippe. Explained that matter is composed of particles in perpetual motion. </li></ul>
  3. 3. Democritus’ view of the atom <ul><li>Atoms are invisible because of their small size. </li></ul><ul><li>Indivisible. (Greek word, ‘atomos’ means indivisible). </li></ul><ul><li>Solid (no void inside) </li></ul><ul><li>Eternal because they’re perfect. </li></ul><ul><li>Surrounded by empty space (explains movement and changes in density). </li></ul><ul><li>Different atoms have different shapes, (to explain the diversity observed in nature) </li></ul><ul><li>Surprisingly modern view! </li></ul>
  4. 4. Robert Boyle (1627-1691) <ul><li>In 1661 he published his Sceptical Chymist in which he distinguished between chemical elements and compounds, and adopted an atomic theory to explain chemical changes. </li></ul>
  5. 5. John Dalton (1766-1844) <ul><li>Manchester schoolmaster. </li></ul><ul><li>Between 1802 and 1808 put forward his ideas in a series of essays and papers: </li></ul><ul><li>A few basic kinds of atom. that… </li></ul><ul><li>Maintain their identity through all physical and chemical changes. </li></ul><ul><li>All atoms of a given element are identical in mass </li></ul><ul><li>Atoms of different elements have different masses. </li></ul><ul><li>Atoms only combine together in certain small, whole-number ratios. </li></ul><ul><li>Atoms can be represented as solid spheres (‘Billiard ball model’). </li></ul><ul><li>He was the first to work out the masses of atoms from chemical experiments. </li></ul><ul><li>x </li></ul>
  6. 6. Henri Becquerel (1852-1908) <ul><li>In 1896, discovers radioactivity. This showed that particles could be emitted from within the atom. Therefore the atom was not indivisible. </li></ul>
  7. 7. Sir J J Thomson (1856-1940) <ul><li>In 1897, showed that cathode rays produced in a discharge tube consist of identical negatively-charged, very small particles… </li></ul><ul><li>He had discovered the electron. </li></ul><ul><li>i.e. the atom is not indivisible – it is made up of smaller parts. </li></ul><ul><li>In 1904, he proposed a model in which the atom is a sphere of electrically positive substance mixed with negative electrons – the so-called ‘plum pudding model’. </li></ul><ul><li>X </li></ul>
  8. 8. Ernest Rutherford (1871-1937) <ul><li>In 1912, fired alpha particles (helium nuclei – positively-charged) into a thin film of gold. </li></ul><ul><li>Expected most of the particles to pass straight through the foil. </li></ul><ul><li>Perhaps a few might be deflected as a result of repulsion or attraction within the ‘plum pudding’ model. </li></ul><ul><li>Astounded that a small number came straight back in the direction from which they’d come. </li></ul><ul><li>X </li></ul>
  9. 9. Rutherford’s Comment <ul><li>“ It was quite the most incredible thing that has ever happened to me in my life. It was almost as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you”. </li></ul>
  10. 10. Interpretation <ul><li>The helium nuclei must have rebounded from something inside the atom that was small, dense and positively charged. </li></ul><ul><li>Most of the particles had passed through the outer part of the atom. </li></ul><ul><li>Rutherford had discovered the atomic nucleus. </li></ul><ul><li>Developed the ‘planetary model ’… </li></ul><ul><li>The positive electrical charge and most of the mass are concentrated in an almost point-sized nucleus . </li></ul><ul><li>Proposed that the electrons move around the nucleus like planets around the Sun. </li></ul><ul><li>X </li></ul>
  11. 11. Sir James Chadwick (1891- <ul><li>Working at Cambridge </li></ul><ul><li>Noticed that when a sheet of beryllium was placed between an alpha source and a charged particle counter, no reading was registered… </li></ul><ul><li>But when a piece of paraffin wax was inserted between the beryllium and the detector, a signal was observed. </li></ul><ul><li>Surmised that alpha particles were stopped by beryllium foil… </li></ul><ul><li>But charged particles were leaving the paraffin wax. </li></ul><ul><li>Attributed this to alpha particles displacing neutral particles from the beryllium, which were not detectable. </li></ul><ul><li>When these neutral particles hit the paraffin wax they displaced positively-charged protons that were detected. </li></ul><ul><li>Chadwick had discovered that the nucleus of the atom also contains neutral particles. He had discovered the neutron. </li></ul>
  12. 12. Niels Bohr (1885-1962) <ul><li>Danish physicist. </li></ul><ul><li>1913. The modern picture of the atom. </li></ul><ul><li>Electron orbits are grouped into definite ‘shells’. </li></ul><ul><li>Each shell lies at a definite radius – the Bohr radius . </li></ul><ul><li>The electrons in each shell are at the same energy. </li></ul><ul><li>i.e. Only certain specific energies are allowed – the energies are quantised. </li></ul><ul><li>Each shell can contain only a fixed maximum number of electrons… </li></ul><ul><li>The inner-most shell can hold only 2… </li></ul><ul><li>The next 8, and so on. </li></ul><ul><li>The total number of electrons allowed in a shell is given by 2n 2 ,where n = ‘shell number’. </li></ul><ul><li>X </li></ul>
  13. 13. Isotopes <ul><li>That is not the end of the story… </li></ul><ul><li>Even atoms of the same element can have different numbers of neutrons. </li></ul><ul><li>Such atoms have the same atomic number but… </li></ul><ul><li>Different atomic mass . </li></ul><ul><li>Called isotopes . </li></ul><ul><li>X </li></ul>