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History of the Atomic Model Seeing the Invisible 1 A Big Debate Can matter be divided into smaller and smaller pieces forever? YES! NO! 2 Aristotle’s Claim • The universe consists of the following – Earth – Water – Fire – Wind – Void 3 Atomic Models: Greek Democritus (460-370 B.C.) Matter can not be divided forever • Smallest piece = “atom” (Greek “atomos” = “not to be cut”) • He had no way of knowing what atoms looked like!4 The word “atom” comes from a Greek word that means “unable to be cut” … and you kept Imagine you on cutting the had a piece leftover piece in of gold that half… you then cut in half… …and then you cut one of these smaller pieces in half… GoGold ld 5 The word “atom” comes from a Greek word that means “unable to be cut” …and kept …and going…kept going… An atom of gold Eventually you would have 1 …and keptcut it piece of gold left. If you going… have in half, you wouldn’t gold any more – you’d have something else. This tiny, tiny single piece of gold is called an atom of gold. An atom is the smallest particle of an element that acts like the element. 6 Democritus He hypothesized that atoms were: You can’t really touch atoms. The – Small & Hard negative electrons repel you! – Diff in shape & size There are only so many atoms in the – Infinite universe they MUST be recycled. Carbon, water, nitrogen cycles, etc. – Always moving – Capable of joining 7 Who Was Right? • Greek society was slave based • No actual experiments – It was all a thought game • Settled disagreements by verbal debate • Aristotle was more famous so he usually won even if he was wrong. • His ideas carried through to the middle ages. (2000 years later)…until the 8 Enlightenment John Dalton (Late 1700’s) • • • School teacher in England Based his conclusions on experimentation and observations (Law of Conservation). Combined ideas of elements & matter with that of atoms 9 Dalton’s Atomic Theory 1. All elements are composed of submicroscopic indivisible parts called atoms. 2. Atoms of the same element are identical, those of different atoms are different. 3. Atoms of different elements combine in whole number ratios to form compounds. 4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed. 10 Dalton’s Theory • Most of Dalton’s theory is still accepted today. • Except the part about atoms being indivisible…and he was wrong about all elements of the same type being identical • These are called isotopes. • U-238 seen below. 11 Isotopes • Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912 • Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. • Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials. 12 J.J. Thomson and the Cathode Ray Tube 1897 English physicist Provided the first evidence that atoms are made of even smaller particles Description of a cathode ray tube and a short video of how it works: http://www.chem.uiuc.edu/clcwebsite/cathode.html 13 Thomson’s Experiment - + • How it works… • Cathode/Anode in a vacuum. 14 Thomson’s Experiment - + 15 Thomson’s Experiment - + 16 Thomson’s Experiment - + • Passing an electric current makes a beam appear to move from the negative to the positive end. 17 Thomson’s Experiment - + • Passing an electric current makes a beam appear to move from the negative to the positive end. 18 Thomson’s Experiment • By adding an electric or magnetic field 19 Thompson’s Experiment • By adding an electric or magnetic field 20 Thompson’s Experiment • By adding an electric or magnetic field 21 Thompson’s Experiment • By changing the electric field he found the moving particles were negative • This is still how all CRT TV’s work. • Don’t try this at home! MAGNET LINK 22 Thompson’s Model • Found the electron • Concluded that there must be a small positive charge everywhere since atom was neutral • Atom was like plum pudding – A bunch of positive stuff, with electrons able to be removed (as in his experiment, static, etc.) 23 Ernest Rutherford • Former student of J.J. Thomson – Believed in plum pudding model 1871-1937 • Wanted to find out how big they (electrons) are • Fired positively charged radiation called alpha particles at a piece of gold foil, which can be made a few atoms thick 24 Rutherford’s Experiment • When alpha particles hit a fluorescent screen it will glow. • Here’s what the setup looked like … 25 26 • What he expected to see: No change in the path.. • This would have confirmed his atomic model 27 – Rutherford thought alpha particles should pass through without a change in direction – Positive charges in this model are spread out evenly. Alone they should not be enough to stop an alpha particle Wrong! 28 • He was wrong… Rutherford animation http://micro.magnet.fsu.edu/electromag/java/rutherford/ 29 • How he explained it 1. Atom is mostly empty 2. Small dense, positive piece at the center: he found the nucleus! 3. Alpha particles are deflected if they get close enough to positive center 30 Niels Bohr (1885-1862) • Electrons have orbits about the nucleus (planetary model) • Electrons could only exist at given energy levels • An energy level is where an electron is likely to be moving • Energy levels were like steps on a ladder – An electron can only be at any given step at any given time 31 De Broglie and Schroedingers (1924-25 Wave Model AKA Quantum Mechanical Model The Wave Model a.k.a. The Quantum Mechanical Model • Today’s atomic model is based on the principles of wave mechanics. • Electrons behave almost like light. do not move about an atom in a definite path, like the planets around the sun. The Wave Model a.k.a. The Quantum Mechanical Model (this is the closest to reality but we can’t really use it precisely…) • In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. • According to the modern atomic model, at atom still has small positively charged nucleus surrounded by a large electron cloud region in which there are enough electrons to make an atom neutral. • This cloud is really Most of the volume of an atom • Cloud is a ‘Probability region’ where electron may be found at any given time. • This structure only possible if electrons behave both like electromagnetic waves (light) and matter and are both ..until we observe them then they ‘become’ one or other • Schrödinger …no likey. Schrödinger's Cat Confusion Schrödinger thought it was dumb to have two states wave/matter and DISAGREED with the idea of both being simultaneous possible. No really! 35 Isotopes • Atoms of the same element can have different numbers of neutrons • Different mass numbers called isotopes • For example H1 H2 H3 (all have one proton and are hydrogen with only one proton but are also called protium deuterium, tritium) • Hydrogen was responsible for the explosion in Japans Nuclear Power Plant (not a nuclear explosion!). Still issues today 36 Atomic Models Indivisibl e Democritus X Dalton X Electron Nucleus/Proton Orbit Thomson X Rutherford X X Bohr X X Schroedinger X X DeBroglie Wave or Quantum Mechanical Model Electron Cloud ‘orbital’ X X