Chapter 4:

1. Chapter 4: Structure of the Atom

2. 4.1 Early theories and 4.2 Defining the atom • Historical Background: • Models of the Atom: -see reference chart • (On Pg. 10 of your packet)

3. 4.1 Early theories and 4.2 Defining the atom

4. 4.1 Early theories and 4.2 Defining the atom • 1. Atomists and Democritus • Greeks approx 2,500 years ago • Matter was made up of atoms “atomos” or “Indivisible” particles • Seashell experiment—broken into smaller & smaller pieces

5. 4.1 Early theories and 4.2 Defining the atom • 2. John Dalton • 1766-1844; returned to theory of atoms • Atoms are like billiard balls (solid spheres) which cannot be broken down further • 4 major postulates • 1) All elements are composed of atoms • 2) Atoms of the same element are identical • 3) Atoms can physically mix or chemically combine in simple whole number ratios • 4) Reactions occur when atoms separate, join, or rearrange

6. 4.1 Early theories and 4.2 Defining the atom • 3. William Crookes • developed Crookes tube (CRT) in 1870’s • first evidence for existence of electrons because you could “see” electrons flow and confirm their existence. • tube is precursor to today’s TV picture tubes

7. 4.1 Early theories and 4.2 Defining the atom • 4. J.J. Thomson • discovered electron in 1897 • discovered positively charged particles surrounded by electrons • found the ratio of the charge of an electron to its mass to be 1/1837

8. 4.1 Early theories and 4.2 Defining the atom • 4. J.J. Thomson

9. 4.1 Early theories and 4.2 Defining the atom • 4. J.J. Thomson • cathode ray tube experiments – advancement of Crookes tube • “plum-pudding model”

10. 4.1 Early theories and 4.2 Defining the atom • 5. Ernest Rutherford • Discovered nucleus (dense core of atom) in 1911 • Gold foil experiments • Quote from E.R.’s Lab Notebook • “It is about as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” -ER

11. 4.1 Early theories and 4.2 Defining the atom Video Clip: Rutherford Gold Foil Experiment

12. 4.1 Early theories and 4.2 Defining the atom • 6. Robert Milliken • Oil drop experiment • determined the charge and mass of an electron Video Clip: Milliken Oil Drop Experiment

13. 4.1 Early theories and 4.2 Defining the atom • 7. James Chadwick • discovered the neutron (no charge, but same mass as proton) • Neutrons help disperse the strong repulsion of positive charges • Relative Sizes • Nucleus diameter = 10-5 nm • Atom diameter = 10-1 nm • Nucleus = basketball --> Atom = 6 miles wide!

14. 4.1 Early theories and 4.2 Defining the atom • 8. Niels Bohr • improved on Rutherford’s work • “planetary model”- positive center is surrounded by electrons in defined orbits circling the center

15. 4.1 Early theories and 4.2 Defining the atom • defined the following: • energy level – the location where an electron is found at a set distance from the nucleus dependent on the amount of energy it has • ground state – the typical energy level where an electron is found; lowest energy • excited state – an energy level higher than the ground state for an electron; temporary condition

16. 4.1 Early theories and 4.2 Defining the atom • 9. Quantum Mechanical Model • Erwin Schroedinger; Mathematical model • Electron locations are based on probability • Electrons are not particles, but waves! • http://phet.colorado.edu/en/simulation/hydrogen-atom Defined: • Orbital – region where an electron is likely to be found 90% of the time

17. 4.3 How atoms differ • Atoms – vocabulary and classifications • Atom – the smallest particle of matter that retains its properties. • can “see” individual atoms with a scanning tunneling microscope.

18. 4.3 How atoms differ • Subatomic particles – the component parts of an atom: proton, neutron, and electron

19. 4.3 How atoms differ • Ion- atom with the same number of protons but a different number of electrons. • If the atom has a (+) charge it has fewer electrons than protons, If the atom has a (-) charge it has more electrons than protons.

20. 4.3 How atoms differ

21. 4.3 How atoms differ • Calculations involving Subatomic Particles: • atomic number = # of protons • mass number = # of protons + # of neutrons • (neutral atom): # of protons = # of electrons • (charged ion): charge = #p+ - #e-

22. 4.3 How atoms differ • Isotopes and Calculations: • Isotope – atoms of the same element with different numbers of neutrons • Atomic mass – weighted average of the masses of all the isotopes of an element

23. 4.3 How atoms differ • Isotope (Isotopic Notation) Mass # Z X Atomic Symbol A Atomic # Example: Uranium-238

24. 4.3 How atoms differ • Isotope Problems: • Multiply the mass number of the isotope by the decimal value of the percent for that isotope • Add the relative masses of all of the isotopes to get the atomic mass of the element

25. 4.3 How atoms differ • Example: • If 90% of the Beryllium in the world has a mass number of 9 and only 10% has a mass number of 10, what is the atomic mass of Beryllium?

26. 4.4 Unstable Nuclei and Radioactive Decay • Vocabulary • Radioactivity-the spontaneous emission of radiation from substances • Nuclear reactions- changes in an atom’s nucleus • Radiation-rays and/or particles emitted from radioactive material

27. 4.4 Unstable Nuclei and Radioactive Decay • Types of Radiation Alpha radiation -stream of high energy alpha particles • alpha particles consist of 2 protons and 2 neutrons and are identical to helium-4 nucleus. • symbol 4He 2+ 2 • not much penetrating power, travel a few centimeters, stopped by paper, no health hazard

28. 4.4 Unstable Nuclei and Radioactive Decay • mass number decreases by 4 atomic number by 2 • alpha decay: 226 Ra 222Ra + 4 He 88 86 2 • Example: Uranium-238

29. 4.4 Unstable Nuclei and Radioactive Decay • Beta radiation • high speed electrons • To form beta radiation a neutron splits into a proton and an electron • The proton stays in nucleus and the electron propels out at high speed. • Symbol 0e-0e-0B -1 -1 -1 • 100 times more penetrating then alpha, pass through clothing to damage skin

30. 4.4 Unstable Nuclei and Radioactive Decay • Beta decay: 131I 131 Xe + 0B 53 54 -1 • Example: Astatine-220

31. 4.4 Unstable Nuclei and Radioactive Decay • Gamma radiation • similar to X rays • doesn’t consist of particles • symbol: 0 0 • penetrates deeply into solid material, body tissue, stopped by Pb or concrete, dangerous • usually emitted with alpha and beta radiation • no mass or electrical charge • emission of gamma rays by themselves cannot result in the formation of a new atom

32. 4.4 Unstable Nuclei and Radioactive Decay • Practice: • What is the alpha decay of plutonium-250?

33. 4.4 Unstable Nuclei and Radioactive Decay • 2. What is the beta decay of Carbon-14?

34. 4.1 Early theories and 4.2 Defining the atom

35. End of Chapter 4!