1. The History of the Atom By Mrs. Palaski

2. Introduction What does an atom look like? It is so small that it cannot be seen. Yet we know there are particles like protons, neutrons and electrons that make up the atom. How did scientists discover these subatomic particles? This presentation will take you through the scientists who contributed to the discovery of the make-up of the atom.

3. The Early Chemists

4. Greeks Approximately 400 B.C., the Greeks classified matter into four substances.  Earth  Fire  Air  Water

5. Democritus  Greek philosopher  Matter consists of small, indivisible particles called atoms.

6. Aristotle  Greek philosopher who opposed Democritus’ beliefs  Believed that all matter was continuous and not made up of smaller particles  His ideas were accepted until the 17 th century

7. Alchemists  Mystics and fakes who claimed that they could turn cheap metals into gold  Dominated the next 2000 years of chemical history  Discovered the elements mercury, antimony, and sulfur  Learned to prepare the mineral acids hydrochloric, nitric, and sulfuric acids

8. Robert Boyle  The first person to perform truly quantitative, physical experiments using gases  Defined element – something that could not be broken down into two or more simpler substances

9. Antoine Lavoisier  Father of Modern Chemistry  Explained that combustion involved oxygen  Law of Conservation of Mass

10. Law of Conservation of Mass  Matter is neither destroyed or created during ordinary chemical reactions or physical changes.  Mass of reactants (starting materials) = mass of products (end materials)

11. John Dalton  English school teacher who was fascinated with science with an intense interest in astronomy, which led to an interest in the gases of the air and their ultimate components, atoms

12. John Dalton  First to recognize that atoms could explain the laws of: conservation of mass, definite composition and multiple proportions  Proposed the Atomic Theory in 1803

13. Dalton’s Atomic Theory 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.

14. Dalton’s Atomic Theory, cont. 3.Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements can combine in simple, whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged.

15. Modern Atomic Theory  Dalton’s atomic theory was accepted and became the basis for further experimentation.  There are exceptions to the theory: Atoms CAN be subdivided. Atoms of a given element may have different masses, they are called isotopes.

16. Dalton’s Model of the Atom  Dalton developed a model of the atom based on his atomic theory. He felt the atom was an extremely small, indivisible particle.  His model of the atom is called the homogeneous spheres or the Billiard Ball Model.

17. Early Experiments to Characterize the Atom

18. The Electron

19. Sir William Crookes  Developed the Crooke’s tube – a tube that produces cathode rays  Noticed that the rays moved from the cathode to the anode  Found that rays traveled in straight lines and possessed energy  Concluded that the stream of light was made up of particles

20. J.J. Thomson  Experimented with a CRT (cathode ray tube)  A CRT is an evacuated glass bulb containing two ends: the cathode and the anode.

21. Cathode Ray Tube  An electrical current passes through the tube from the cathode (negative end) to the anode (positive end).

22. Cathode Ray Tube  A magnet was applied to these rays and always with the same results: Negative end of magnet repelled cathode ray; Positive end of magnet attracted cathode rays.  Thomson noticed that the cathode rays were the same regardless of the element or metal used to make-up the cathode.

23. Discovery from CRT  Thomson concluded that the cathode ray was a stream of negatively charged particles, which he called electrons.  Determined ratio of particle’s electrical charge to mass (e/m).

24. Thomson’s Conclusion  Thomson concluded that all atoms of all substances contain the same kind of negative particles, and all electrons are identical.  Thomson discovered the ELECTRONS.  Plum Pudding model is Thomson’s model of the atom. It is called the plum pudding model because he pictured the electrons dispersed like raisins in a pudding.

25. Plum Pudding Model  Plum Pudding model is Thomson’s model of the atom.  The atom consists of a diffuse cloud of positive charge with negative electrons embedded randomly in it.  It is called the plum pudding model because he pictured the electrons dispersed like raisins in a pudding.

26. Plum Pudding Model + + + + + + + Negative electrons

27. Robert Millikan  Performed the Oil Drop Experiment (1909)  Work contained excellent precision  Determined the exact charge and exact mass of an electron

28. Oil Drop Experiment

29. How the Oil Drop Experiment Worked  A fine mist of oil is sprayed into the chamber.  A few oil drops will fall through the hole in the positively charged plate at the top.  As the oil drops fall due to gravity, they acquire extra electrons which are dislodged from gases in the air by X rays.  As the charged oil drops descend, the electrically charged plates are turned on.

30. How the Oil Drop Experiment Worked  The oil drops now have two forces acting on them. Gravity and electrical charge.  Using the microscope to observe the oil drops, Millikan could determine the charge needed to suspend the drops in mid-air.  Millikan calculated the: exact mass (9.109 x 10 -31 kilograms) and charge (-1.6 x 10 -19 coulombs) of an electron. exact mass (9.109 x 10 -31 kilograms) and charge (-1.6 x 10 -19 coulombs) of an electron.

31. Results of CRT and Oil Drop Experiment 1.Proved that atoms are divisible. 2.Atoms are electrically neutral therefore they must have a positive charge equal to the negative charge. 3.Since electrons have such a small mass, atoms must have additional particles to account for most of their mass.

32. Oil Drop Experiment http://physics.wku.edu/~womble/phys2 60/millikan.html http://physics.wku.edu/~womble/phys2 60/millikan.html

33. Eugene Goldstein  In 1885, Goldstein used a canal ray tube filled with hydrogen to discover protons.  He called them “canal rays.”

34. Ernest Rutherford  Thought that the atom was all empty space  In 1908, he carried out an experiment to test Thomson’s plum pudding model.

35. Gold Foil Experiment  Used alpha particles as bullets  Shot the alpha particles at a thin sheet of gold foil  Predicted the alpha particles would pass straight through

36. Results of Gold Foil Experiment  Almost all of the particles passed straight through.  Some were deflected. (1 in 10,000)  1 in 20,000 were reflected straight back  Results caused Rutherford to formulate a new model of the atom.

37. Gold Foil Experiment  Rutherford said it was “as if you had fired a 15-inch (artillery) shell at a piece of tissue paper and it came back and hit you.”  Why did this happen?  Rutherford reasoned that the fast-moving particles must be repelled by some powerful force within the atom. Also, whatever caused this repulsion must occupy a very small amount of space since only a very few particles ran into it.  Results caused Rutherford to formulate a new model of the atom.

38. The Nucleus

39. So how small is the nucleus?  How large is an atom’s volume compared to its nucleus?  Think of a football field and place a dime in the center of the 50 yard line.

40. Rutherford’s Model of the Atom  Rutherford’s model of the atom is the nuclear model.  There is a very small, dense center of concentrated charge, called the nucleus.  Rutherford concluded that the atom is mostly empty space.  Electrons must be present in the empty space about the nucleus to maintain the neutrality of matter.

41. Nuclear Model of the Atom Nucleus - - - - - - - - - - Nucleus is positively charged. - electrons

42. A Puzzle  Electrons must be present in the empty space about the nucleus to maintain the neutrality of matter.  If an atom has a positive center and the negative electrons are on the outside of the atom, why don’t the electrons fall into the center?  Centripetal force

43. Niels Bohr  Studied under Ernest Rutherford  In 1913, he developed his own structure of the atom based on Rutherford’s theory.  Rutherford had shown that the atom consisted of a positively charged nucleus with negative electrons in orbit around it.

44. Bohr  Bohr expanded this theory by suggesting that electrons travel in specific successively larger orbits or paths around the nucleus which he called energy levels. These energy levels are designated distances from the nucleus in which electrons may be found.  He suggested that outer orbits could hold more electrons than the inner ones, and that these outer orbits determine the atom’s chemical properties.

45. Planetary Model of the Atom Nucleus e -

46. James Chadwick  Discovered the neutron in 1932  The neutron is a particle in the nucleus that has about the same mass as a proton, but has no charge.