Chapter 4: Atomic Structure

1. Chapter 4: Atomic Structure History of the Atom -Atomic History song

2. Size of Atom Video • Atoms and elements

3. 400 B.C.- Democritus • Believed matter was made of empty space and tiny particles he called “atoms” • He said that different things were made of diff. types of atoms • Leuccippus & Epicurus agreed w/ him

4. 335 B.C. - Aristotle • Said matter was continuous, what you see is what you get • Called it “hyle” • Identified 4 elements: earth, wind, fire, water

5. Late 1500’s - Galileo • 1st person to make extensive use of the experimental method to study natural phenomena • 1st to disagree w/ Aristotle • Agreed w/ Copernicus (sun center of universe)

6. Late 1600’s – Sir Issac Newton • Brilliant English scientist • In his 23-24th year he was stuck at home during the Bubonic plague • During this time he: • Invented calculus • Did 100’s studies on light • Developed laws of motion • Developed laws of gravity • Atom idea resurfaced- said there were small particles (offered no proof)

7. 1789- Antoine Lavoisier • French scientist who developed the Law of Conservation of Mass

8. 1799- Joseph Proust • French scientist who developed the Law of Definite Proportions

9. 1803- John Dalton • English scientist • Published series of 3 papers, based on the findings of Proust and Lavoisier

10. Dalton’s Atomic Theory • All matter is composed of small particles called atoms that can’t be broken down • Atoms of the same element have identical properties (same mass,etc) • Atoms of different elements have different properties • Atoms unite in simple ratios to form compounds

11. Developed Law of Multiple Proportions • Dalton’s Model of the Atom • Used solid wooden balls to show matter was made of indivisible particles

12. 1876- Eugen Goldstein • German scientist who introduced the name “cathode ray” and its positive counterpart “canal rays”

13. 1895 – Wilhelm Roentgen • German scientist who discovered X-rays

14. 1896 – Antoine Henri Becquerel • French scientist who discovered radioactivity • Radioactivity: the phenomenon in which the unstable nuclei of certain elements spontaneously emit particles or energies

15. Forms of Radiation • Alpha particle ( ) [Rutherford] -helium nucleus (2 neutrons & 2 protons) - can be stopped by sheet of paper

16. 2. Beta Particle (ß) [Rutherford] - high speed electron formed at the moment of decay -can be stopped by sheet of aluminum or layers of clothing

17. Gamma Rays ( ) [Villard] -high energy x-ray -stopped by few feet of concrete or few inches of lead

18. 1897- J.J. Thomson • Did research on cathode rays, built a CRT (cathode ray tube), now found in TV’s and computer screens (originally built by William Crookes) • CRT is a glass tube filled w/ a gas and it has 2 electrodes (anode=positive electrode, cathode=negative electrode)

19. Passed a voltage between the electrodes, saw a light beam called a cathode ray • Subjected the beam to electric and magnetic field, found the beam to be negatively charged • Called those negative particles -electrons

20. Thomson’s Model of the Atom • Called the “plum pudding model” • Disproves Dalton’s theory (atom can’t be broken down)

21. 1911- Robert Millikan • American physicist • Did the “oil drop experiment”

22. In this experiment he had oil drops that were charged pass thru charged plates, he offset the voltage on the plates to get the drop to suspend briefly, wrote down the voltage • He found the exact charge on an electron(e-) to be (-1) • Also using Thomson’s work he found the mass of the e- to be 1/1897th of a Hydrogen atom

23. 1911-1912 –Lord Ernest Rutherford • New Zealand physicist • Worked w/ Bohr, Geiger and Marsden on the “Gold Foil Experiment” • Credited w/ the discovery of the atomic nucleus

24. Gold Foil Experiment Set-Up

25. Gold Foil Movie

26. Gold Foil Experiment Set-Up • Lead box w/ pin hole • In box- piece of radioactive material which gave off alpha particles (positive particles) • Used a very thin sheet of gold foil as a target (few atoms thick) • Surrounded the foil w/ a screen coated in zinc sulfide that sparked when alpha particles hit it

27. Observations

28. Observations • 99% of particles went straight thru the foil like it wasn’t there • .9% went thru foil but passed thru at a slight angle • .1 % hit the gold foil like a brick wall & bounced right back

29. Conclusions • Atoms made of mostly empty space • Some sort of (+) charge is concentrated in the nucleus to repel the alpha particles (must have a charge of (+1) to offset the electron-because atoms are neutral) • All the mass of the atom is concentrated in one place

30. Rutherford’s Model of the Atom • Atoms consist of a small, dense, positively charged center (the nucleus)-surrounded by empty space • Electrons travel in that empty space orbiting around the positive nucleus

31. 1914- Discovery of Proton • Thomson predicted the existence of a positive particle w/ CRT experiments • Rutherford coined term “proton” in 1919 (he is credited w/ the discovery w/ Gold Foil experiment) • Found positive particle, that is larger and the exact opposite charge of the electron, 1800 times heavier than the electron

32. 1914- Atomic Number • Henry Mosley • Studied x-rays produced by different metals (used the metals as anodes) • Saw that each metal produced a different wavelength- because each had a different number of protons

33. Atomic Number = # of protons # protons = # electrons # of protons determines the identity of the element

34. 1922- Bohr’s Model of the Atom • Niels Bohr worked on gold foil • Had electrons in specific orbits or shells

35. 1932 – Discovery of the Neutron • James Chadwick found high energy particles w/ no charge and a mass equal to a proton • Call neutrons

36. Nucleons • Particles in nucleus • Also known as hadrons • Consist of protons and neutrons • Each proton is made of 3 quarks (2 ups and 1 down), each neutron is made of 3 quarks (2 downs and 1 up)

38. Mass Number =# of protons + # of neutrons # of neutrons = mass # - atomic # Ex. Carbon Mass #= 12 Atomic # = 6 # protons =# e- = 6 # of neutrons = 6

39. Atomic notation Mass # 222Rn 86 Atomic# (#p=#e-)

40. #protons = 86 # electrons = 86 #neutrons = 136

41. Isotopes • JJ Thomson discovered • Atoms of the same element that have a different mass due to the fact that have different # of neutrons • Disproves Dalton’s theory that atoms of the same element are the same

42. Average Atomic Mass (# on the periodic table) • Scientists found the masses of all the isotopes and their percent abundance with a mass spectrometer • A mass spectrometer compares each element to Carbon-12 atom • Measured in atomic mass unit (amu) • The average atomic mass was calculated using those numbers (get a decimal #, not a whole #, because it’s an average)

44. Ex. Find average atomic mass (from isotopes) Cu-63 62.930 amu 69.17% in the sample Cu-65 64.928 amu 30.83% in the sample

45. Find mass contribution of each isotope (mass) x ( %) Cu-63  62.930 x .6917 = 43.53 Cu–65  64.928 x .3083 = 20.02

46. Find average mass by adding the isotopes mass contributions together Average atomic mass = 43.53 + 20.02 = 63.55 amu

47. History of the atom- part 1 • History of the atom- part 2