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Atomic Structure The History of the Atomic Model John Dalton (1766 – 1844) Dalton proposed the Atomic Theory in 1805 which stated that 1. All matter was composed of small indivisible particles termed atoms 2. Atoms of a given element possess unique characteristics and weight 3. Three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules) Dalton's theory identified chemical elements as a specific type of atom. John Dalton (1766 – 1844) Dalton inferred proportions of elements in compounds by taking ratios of the weights of reactants, setting the atomic weight of hydrogen to be identically one. Dalton’s view of the atom is that of a solid sphere, similar to a billiard ball. J. J. Thomson (1856 – 1940) Thomson discovered the electron, and showed that it was part of the structure of the atom. He did this using Gas Discharge tubes. The gas discharge tube is a glass tube that has most of the air pumped out, and two electrodes at either end. Gas Discharge Tube J. J. Thomson (1856 – 1940) In 1887, Thomson determined the rays were due to negatively charged particles that have mass. He called them ‘corpuscles’, later called electrons. Thomson received the Nobel Prize in 1906 for his discovery. J. J. Thomson (1856 – 1940) He theorized that the electrons were much like raisins in pudding, where the pudding is the positive particle. Ernest Rutherford (1871-1937) Henri Becquerel discovered radioactivity in 1896. Ernest Rutherford discovered that radiation could be split into 3 types of beams with magnetic fields. Alpha particles (positive particle) Beta particles (negative particle) Gamma rays (neutral wave) Ernest Rutherford (1871-1937) Rutherford became interested in the atomic structure. He theorized that using Thomson’s model, alpha particles should pass through atoms unaffected. Alpha particles would pass directly through the gold atoms according to the Thomson model of the atom since it is “a sea of positive charge embedded with negative charges.” Ernest Rutherford (1871-1937) Rutherford devised an experiment to test his theory. Alpha particles can be detected as a flash of light when they strike zinc sulfide. A sheet of gold is hammered into a very thin sheet of foil. Flashes of light should form only opposite the emitter. Ernest Rutherford (1871-1937) Rutherford was shocked that the a particles bounced backwards. He concluded that the atom contained a tiny, dense core that was positively charged. The atom is mostly empty space since the majority of alpha particles pass through unaffected, but there is a tiny positively charged core that deflected the positively charged alpha particle. Anatomy of Rutherford’s Model The Electron Problem The nucleus contains positively charged particles called protons. Electrons are 1/2000 the mass of protons. Opposites attract! Why don’t the electrons crash into the nucleus? James Maxwell (1831-1879) “[The work of Maxwell] ... the most profound and the most fruitful that physics has experienced since the time of Newton.” – Albert Einstein At the beginning of the 19th century, there were two theories regarding the nature of light. James Maxwell (1831-1879) Newton proposed that light was a particle. Huygen proposed the light was a wave. James Maxwell proposed that light is an electromagnetic wave. The frequency of electromagnetic waves is continuous, and visible light is just one part of the range of frequencies possible. The range of frequencies is known as the Electromagnetic Spectrum. James Maxwell (1831-1879) Max Planck (1858-1947) Max Planck became interested in studying black bodies that emit electromagnetic radiation. When a solid is heated, it begins to glow. The hotter the object, the higher the frequency of electromagnetic radiation emitted. The problem was that if light is a continuous wave, then the math predicts a curve that is not experimentally observed. Max Planck (1858-1947) Actual curve Predicted curve Max Planck (1858-1947) Planck was able to find a mathematical formula that fit the empirical data IF he treated light as if it was discrete, NOT continuous. He treated the energy from light in discrete amounts, or packets he called a quantum of energy. This was the first indication of light acting as a particle instead of a wave. Max Planck (1858-1947) “But even if the radiation formula should prove to be absolutely accurate it would after all be only an interpolation formula found by happy guesswork, and would thus leave one rather unsatisfied. I was, therefore, from the day of its origination, occupied with the task of giving it a real physical meaning …” (Max Planck, 1919 Nobel Prize address, 'The Origin and Development of the Quantum Theory') Albert Einstein (1879-1955) In 1887, Heinrich Hertz discovered the Photoelectric effect accidentally. This occurs when electromagnetic radiation (light) is shone on a metal plate, which results in flowing electrons. Photoelectric Effect Albert Einstein (1879-1955) The classical theory of light (as a wave) predicted that the brighter the light, the greater the number of liberated electrons. However, experimental evidence showed that brightness of light does not increase electron flow. However, a higher frequency of light does! Albert Einstein (1879-1955) In the first of 4 of Einstein’s major papers, he proposed an explanation for the Photoelectric effect. Einstein used Planck’s quantum theory to say that light consisted of streams of Planck’s quanta, and called them photons. It is the collision of photons with electrons that breaks them free from the atom. The higher the energy/frequency, the greater the chance the collision will break an electron free. Albert Einstein (1879-1955) Um, what about those electrons? Now that we understand the quantum nature of light, we can now return to atomic theory, and consider how Neils Bohr used the idea of light as discrete energy packets to explain the nature of electrons. Questions – Page 173 #1, 3, 5-7 Answers to homework 1. The first experimental observation was that the electromagnetic radiation emitted by black bodies did not agree with the classical theory of light as a wave. Planck deduced that the amount of energy released could be predicted if light were treated as packets of energy, or quanta. The second experimental observation was that the amount of electrons liberated by the photoelectric effect only increased in response to increased frequency, not brightness. Einstein used Planck’s quantum theory to explain the photoelectric effect as due to photons colliding with electrons. Answers to homework 3. The photoelectric effect occurs when electromagnetic radiation (e.g. visible light, ultraviolet, etc) is directed at a metal. This results in the release of electrons from the metal (i.e. current) due to collisions between photons and electrons. Answers to homework 6. E = hf = 6.6x10-34 J/Hz x 5.5 x1014 Hz = 3.63x10-19 J b. 7 a. UV = 9.9x10-19 J, IR = 2.178x10-19 J b. UV:IR = 4.5:1 c. Visible light photons are between UV and IR in energy, and X-rays have more energy than all of them.
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