History of the Atom: From Greek Philosophers to Quantum Mechanics

1. Chpt. 2: The Atom

2. History of the Atom • Greek Philosophers (400BC): • - first proposed that matter was composed of minute particles • - believed that the tiny particles of which all matter was composed were so small that nothing smaller was possible • ‘Atomos’ Greek word for indivisible - ATOM

3. 2. John Dalton (1808): Dalton’s Atomic Theory • - All matter is made up of • very small particles called • atoms • All atoms are indivisible. They cannot be broken down into simpler particles • Atoms cannot be created or destroyed

4. What is inside the atom???

5. Discovery of the Electron 3. William Crookes – cathode ray tube (1875): • - passed electric current through gases • at low pressure • invisible radiation that caused the • glass to glow came from *cathode (-) • called cathode rays • showed existence of this radiation by • placing Maltese Cross inside the tube • *Note: • Cathode = plate connected to negative end of battery • Anode = plate connected to positive end of battery ,

7. 4. J.J Thomson – cathode ray tube experiments (1897): • devised experiment to • investigate if cathode rays • consisted of charged • particles • cathode rays attracted up • towards positive plate • (anode) => consisted of • negatively charged particles • hence cathodes are streams • of negatively charged • particles called electrons Definition: cathode rays are streams of negatively charged particles called electrons

8. Voltage source Thomson’s Experiment + ------- • Passing an electric current makes a beam appear to move from the negative to the positive end

9. Voltage source Thomson’s Experiment + - • By adding an electric field he found that the moving pieces were negative

10. Further experiment: • - he found electrons were also deflected in magnetic • field • found ratio of charge to mass of the electron (e/m): (electrical charge of electron) (mass of electron) = 1.76 x 108 coulombs = 1 gram of electrons • *Note: In 1891 George Stoney proposed that the smallest amount of electric charge be called an electron.

12. Thomson's ‘Plum Pudding Model’ of the Atom (1898): • Proposed that since atoms are neutral each one consists of: • - a sphere of positive • charge • - electrons embedded • randomly Dough = positive charge Raisins = electrons

13. 5. Robert Millikan (1909): - Experiment to measure size of charge on electron – Oil Drop Experiment - Charge of one electron = 1.6 x 10-19 coulomb THUS…. Mass of e- = 9.11 x 10-31g

14. Discovery of radiation led to the use of alpha particles in experiments Alpha particles are positively charged particles produced by certain radioactive substances

15. 6. Ernest Rutherford (1909): Rutherford discovered the nucleus and the proton

16. Rutherford's Gold Foil Experiment - bombarded tin foils of gold with alpha (α) particles - If plum pudding model was correct he expected: The alpha particles to pass through without changing direction very much Fluorescent Screen Gold Foil Lead block Uranium

17. What He Got!!!!! *Note: Detector flashes - of light produced when α particles strike zinc/sulphite screen

18. Rutherford’s Gold Foil Experiment • Results • Most alpha particles • passed straight through • undeflected • Some were deflected at • wide angles • Few deflected back • along own path • Explanation/Conclusion • Atom mainly empty space • occupied by electrons (negative) • Both the mass and positive • charge were concentrated in • a small dense core which he • called the nucleus

19. Rutherford – discovery of protons (1924): • Light atoms (oxygen, nitrogen) were bombarded with • alpha particles - small POSITIVE charged particles • were given off • This did not occur with heavier metals e.g. gold • Explanation – alpha particles were breaking up the • nuclei of the lighter atoms to release positively • charged particles • referred to these small positive particles as protons

20. 7. James Chadwick (1932): • Search for a neutral particle • to cement the nucleus • Bombarded beryllium with • alpha particles • Produced neutral particles – • neutron

21. Properties of Sub-Atomic Particles

22. Dalton Model of the Atom • Small, indivisible spheres

23. J.J. Thompson’s Model of Atom • Plum Pudding Model, 1896 • Thought an atom was like plum pudding

24. Rutherford’s Model of the Atom • Rutherford Model, 1911 • Thought atom was mostly empty space: • - Nucleus • - Electrons (negatively • charged) revolving • around nucleus

25. Bohr’s Model of the Atom • Neils Bohr, 1913 • Similar to Rutherford’s model • Thought atom was mostly empty space: • - Nucleus in center is dense, positively charge • - Electrons in orbits around nucleus

26. (Modern) Quantum Mechanical Model of the Atom • Heisenberg, Schrodinger, many others, ~1926 • Think atom is mostly empty space: • Nucleus in center is dense, positively charge • - Electrons cannot locate

27. Evidence for the existence of small particles!!! Why is it possible to smell the perfume that someone is wearing from several metres away?

28. Diffusion • The process by which molecules of a substance • spread through a solid, liquid or gas. • Some examples which can be demonstrated in the • lab:- Gas Jar full of air

29. Demonstration • Diffusion of ink in water • Diffusion of NH3 and HCl • Diffusion of smoke in air

30. Diffusion of NH3 and HCl

31. Diffusion of NH3 and HCl Word Equation: Ammonia + Hydrogen chloride = Ammonium chloride (Gas) (Gas) (White powdered ring) Chemical Equation: NH3 + HCl = NH4Cl