A Brief History of Atomic Theory PDF

A (Brief) History of Atomic Theory Scientist Description of Contribution Model Democritus  Matter is made of atomos (“indivisible”) n/a ~400 BC...

A (Brief) History of Atomic Theory Scientist Description of Contribution Model Democritus  Matter is made of atomos (“indivisible”) n/a ~400 BC Billiard Ball Model John Dalton  Developed the Atomic Theory (elements consist of atoms which 1803 cannot be divided, created, or destroyed)  Discovered the electron (matter is divisible) Blueberry Muffin  Was able to separate a negatively charged beam of particles from Model electrically neutral matter (Cathode Ray Tube Experiment)  Estimated mass of the particles (mass-to-charge ratio) based on deflection in a magnetic field J.J. Thomson 1897  Discovered that certain elements are radioactive Marie Curie &  Radioactive substances emit positively charged particles (alpha Henri particles), negatively charged particles (beta particles), and Becquerel electromagnetic radiation (gamma rays)… collectively “nuclear 1896 radiation” Planetary or Nuclear  Directed the Gold Foil Experiment (“Geiger-Marsden Experiment”) Model  This involved firing heavy, high-energy alpha particles through very thin sheets of gold foil Ernest Rutherford 1913  Rutherford’s results were not in agreement with Thomson’s model  His conclusions lead to a revised model of the atom  Rutherford could not, however, explain why the electrons did not spiral into the nucleus, as predicted by early 20th century physics James  Discovered the neutron by studying particle collisions. Chadwick  This particle was added to the nucleus of current atomic models. 1932  Was trying to explain why objects glow when heated Max Planck  Proposed that energy is not emitted in a 1900 continuous stream, but instead in discrete quantities, or quanta   Applied Planck’s idea of quantized energy to explain the Photoelectric Effect – a puzzling observation which could not be Albert explained using the current model of light. Einstein  Proposed the idea that light consists of packets of energy or called 1905 photons (“quantized” particles of light)  When matter interacts with photons, the photons are absorbed in an “all-or-none” manner (matter cannot absorb a part of a photon).  Bohr was aware of the work of Planck, Einsten, as well as Bohr Model observations of Atomic Emission Spectra (expplained below)  White light is composed of all of the colours, so when it is dispersed through a prism, a continuous spectrum of colours is produced  Coloured light however, is not composed of all colours, so when Niels Bohr passed through a prism the spectrum is not continuous 1915  For example, when neon gas is excited by running an electric current through it, coloured light is produced. When dispersed through a prism, a non-continuous emission spectrum or line spectrum is produced.  Each element has its own charactertic emission spectrum Bohr combined all of these ideas to propose the following atomic model:  Electrons exist in circular orbits  Electrons can only exist in a series of “allowed” orbits or energy levels  Electrons in specific orbits have specific amounts of energy associated with them (further from the nucleus = more energy)  Since they can only exist in certain orbits, and each orbit has a certain energy associated with it, electrons in atoms have quantized energy levels.  Electrons can “jump” between orbits (energy levels) by absorbing or emitting photons carrying an amount of energy equal to the difference in the energy levels they are moving between. Bohr was able to use the observations of the hydrogen emission spectrum, Planck’s concepts, and basic laws of physics to successfully calculate the radius of the allowed orbits of a hydrogen atom Unfortunately, his model did not work for atoms that had more than one electron, but his idea of quantized energy levels laid the foundation for the quantum mechanical model of the atom.