1. Unit 2 2.1 – History of the Atomic Model

2. Atomic Structure www.lab-initio.com

3. Democritus 400 B.C. A.He was a Greek philosopher of science. B.First to use the term “atom” to describe the basic particle of nature. 1.“atom” means “indivisible” 2.Atom – the smallest particle of an element that still retains the chemical properties of that element.

4. John Dalton 1808 He was an English schoolteacher. He was the first to propose an “Atomic theory” that states the 5 following statements:

5. John Dalton 1808 1.All matter is composed of extremely small particles called “atoms”. 2.Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. a.This has since been modified based on Isotopes and ions. 3.Atoms cannot be subdivided, created, or destroyed. a.This has since been modified based upon current studies in quantum physics. b.Such examples include muons and quarks.

6. John Dalton 1808 4.Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5.In chemical reactions, atoms are combined, separated, or rearranged.

7. Dalton’s view of atoms of elements Can you see 1-4 in this picture?

8. J.J. Thomson (1897) with the Cathode Ray Tube

9. A.He was an English Physicist. B.He worked with glass gas-filled tubes referred to as Cathode-Ray tubes. 1.The glass tubes were filled with a gaseous element under low pressure. 2.He then passed an electrical current using a battery and wires.

10. J.J. Thomson (1897) with the Cathode Ray Tube a.The electrical current caused the gas within the tube to glow with a beam (“ray”) within the tube to intensely glow within the tube. i.Magnets could make the “ray” move/deflect in various directions. ii.The ray is being deflected by the negative charge of the magnet. iii.Negative charge repels/deflects like negative charges. iv.The ray is made of negatively charged particles, Thompson called electrons (since they were associated with the electrical current.) The symbol for an electron is: e -

11. J.J. Thomson (1897) with the Cathode Ray Tube b.The electrical current came into the chamber (by a wire) at the cathode end. (The end where electricity enters the tube.) c.The electrical current left the tube on the anode end. (The end where the electricity goes back into the wire.) d.Hence the term Cathode Ray tubes.

12. Discovery of the Electron C.Further investigations using different elements in Cathode-Ray tubes confirmed that every elements atoms possess electrons.

13. Actual Cathode Ray in action

14. Thomson’s Atomic Model D.He proposes the “Plum Pudding” model of atoms. 1.It states that negatively charged electrons are evenly placed inside a positively charged mass. 2.Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

15. Robert A. Millken 1909 A.He was an American Physicist. B.He was the first to measure the charge and mass of an electron.

16. Millikan’s Oil Drop Experiment Helps to calculate the mass & charge of an electron

17. How it worked inside

18. Robert A. Millken 1909 1.Electron charge = 1.602 x 10 -19 Coulombs. a.This is an extremely small quantity of energy. 2.Electron mass = 9.11 x 10 -31 kg a.Electrons are 1/1837 th the mass of a single proton or neutron. b.This is a very, very, very small amount and size.

19. Robert A. Millken 1909 C.Milliken’s experiments allow for 2 inferences (conclusions based upon evidence and reasoning) to be made: 1.Because atoms, in the natural state, are electrically neutral, they must also contain an equal amount of positively charged particles. 2.Because electrons have so little mass, atoms must contain other particles with much greater mass (protons & neutrons).

20. Ernest Rutherford (1911) on New Zealand Money

21. Hans Geiger (1911)

22. Ernest Marsden (1911)

23. Rutherford’s Gold Foil Experiment A.Geiger and Marsden were students of the New Zealand Physicist. B.They performed the Gold Foil Experiment. C.They used high-energy alpha particle radiation( 2 protons & 2 neutrons ejected from a decomposing, radioactive element) to bombard a piece of gold foil that is surrounded by a fluorescent screen.

24. Rutherford’s Gold Foil Experiment 1.As alpha particles struck the fluorescent screen, they would produce a small detectable burst of light. 2.As the experiment was running, they detected light burst mainly behind the gold foil, but also occasionally all around the ring.

25. Rutherford’s Gold Foil Experiment a.These bursts of light around the ring were because of the positively charged alpha particles been deflected by positively charged particles in the atoms of the foil. b.The particles became known as protons. i.Just as with the electrons, positive charges repel/deflect like positive charges.

26. Rutherford’s Gold Foil Experiment a.As most of the bursts of light occurred behind the gold foil, they concluded that the majority of space in an atom is “empty space”. The alpha particles travelled through and never hit anything. b.Rutherford proposes the idea of the neutrally charged neutron particle in 1920.

27. Rutherford’s Gold Foil Experiment  Alpha (  ) particles are helium nuclei  Particles were fired at a thin sheet of gold foil  Particle hits on the detecting screen (film) are recorded

28. Rutherford’s Findings  The nucleus is small  The nucleus is dense  The nucleus is positively charged  Most of the particles passed right through  A few particles were deflected  VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions:

29. Atomic Particles

30. Niels Bohr 1913 A.He was also a student of Rutherford’s. B.He proposed the Bohr model of an atom. 1.The electrons move in a circular pattern around the positively charged center. a.Much like the planets revolve around the sun.

31. Bohr Model of an Atom

32. Erwin Schrödinger 1926 He was an Austrian Physicist. He helped develop the Quantum Theory of Atoms – This Theory tries to describe, by mathematics, the wave-like properties of electrons & other very small particles.

33. Schrödinger This re-enforces that electrons travel in orbitals. Orbital is defined as a 3- Dimensional region around a nucleus that indicates the probable location of a single electron within an orbital

34. Werner Heisenberg 1927 He was a German Physicist. He calculated that electrons and photons have about the same amounts of energy. – Photons are used to help detect the presence of electrons. – When they collide, the electron is deflected in a random direction.

35. Heisenberg’s Uncertainty Principle This states that it is impossible to determine exactly both position and velocity simultaneously of an electron in an orbit.

36. Dmitri Ivanenko (1930)

37. Victor Ambartsumian (1930)

38. Ivanenko & Ambartsumian 1930 A.These gentlemen were Russian Physicists. B.They proposed a model of the nucleus of an atom being composed of positively charged protons and neutral charged particles (neutrons).

39. Ivanenko & Ambartssumian Model of a Nucleus

40. James Chadwick 1932 A.He was an English Physicist. B.He also studied under Rutherford. C.He proved that the nucleus is definitely composed of protons and neutrons through his experiments with alpha particle radiation.

41. His Experiment using alpha particle radiation