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History of Atomic Models Democritus (460-360 BC) Democritus All things are composed of minute, invisible and indivisible particles, atoma, (indivisibles) in constant motion through otherwise empty space, kenon. Aristotle Argued against atomic theory and believed everything was composed of the 5 elements. Highly influential for 2000 years. Dalton 1800s Chemicals are composed of elements combined together in certain proportions. billiard ball model of atoms Sir Joseph John Thomson 1900 Analyzed the nature of cathode rays. He noticed that they had particle like properties and were identical regardless of the material producing the rays. Plum-Pudding Model Lord Kelvin and JJ Thomson concluded that the atom was composed of tiny negatively charged electrons and positive charge mixed together. This was called the Plum-Pudding Model. Plum-Pudding Model Ernest Rutherford Rutherford Gold Foil (Geiger Muller) Experiment 1911 Ernest Rutherford proposed shooting alpha particles at extremely thin gold foil. Most went right through with little deflection, but some bounced back. This result was thought to be like cannon balls bouncing off tissue paper! Alpha particles should be undeflected but observed to bounce back Rutherford Atomic Model The alpha particles must have bounced off an area of the atom that is positively charged and extremely dense. Rutherford concluded that the atom has a dense positive nucleus with electrons orbiting around. Rutherford Model of the Atom Niels Bohr Bohr Model 1913 Light (electromagnetic radiation) is emitted from matter at particular energies, or quanta (from Planck). Light is produced when electrons move from high energy states to low energy states. E=hf Electrons exist in particular orbits. Bohr Energy E=-13.6eV/n2 What is the energy of the emitted photon when an electron drops from the 3rd to the first orbital? What is the wavelength? is it visible? De Broglie said that electrons act like waves. Waves resonate if the frequency is just right for the container (musical instrument). The orbital is the container for the electron and only certain orbitals will work for electrons of a certain energy. the wavelength of a particle l=h/p plank’s constant divided by its momentum. De Broglie derived this relationship from E=hf, the energy of a photon and p=E/c =h/l the momentum of a photon so that the waveparticle duality that Einstein applied to photons also applied to particles. Modern Atomic Model Quantum Mechanics shows how there is a limit to our ability to determine the motion and position of particles. (Heisenberg) The motion of objects is described using a wave equation, y , where y2 is the probability distribution of the object. Graphing y2 for an electron around the hydrogen atom we obtain orbital shapes. (Schrödinger) Inside the Nucleus Chadwick 1931 discovered that the nucleus contains protons and neutrons. Protons and neutrons are composed of quarks (1964 Gell-Mann and Zweig). The strong force between the quarks is what holds the nucleus together and produces massive amounts of energy in a nuclear reaction. Homework p1049 q 46-48, 54,