Atomic Structure

1. Atomic Structure

2. What is a theory? • a well-substantiated explanation of some aspect of the natural world; • an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena; • "theories can incorporate facts and laws and tested hypotheses"

3. "If I have seen further it is by standing on the shoulders of Giants." --Isaac Newton

4. Early Theories • Democritus: 4 B.C.: “atom” • Believed there were 4 elements: • Fire, Air, Water, Earth

5. Dalton: 1766-1844 >All elements composed of tiny particles called atoms >Atoms of same element are identical; atoms of different elements are different >Atoms of different elements can physically mix together or chemically combine to form compounds >Chemical reactions cannot change atoms of one type of element to another

6. Thomson: 1856-1940 • >discovered electrons in 1897 • >used a cathode ray tube • >the ray produced was deflected by an electrical field (showed that atoms had particles with (-) charge)

7. Cathode Ray Tubes • A cathode ray tube or CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. • TVs, PCs, ATMs, video games, video cameras, and monitors all contain cathode-ray tubes. • Displays millions of colors.

8. Rutherford: 1871-1937 >Gold Foil Experiment >Discovered the nucleus

9. Rutherford’s Gold Foil Experiment Experiment Shot positively charged alpha particles at gold foil • Results • Most particles passed through the foil • A few were deflected

10. Demonstration of Gold Foil Experiment

11. Rutherford’s Gold Foil Experiment Conclusions • small, dense, positively charged core (nucleus) • the rest of the atom is empty space

12. Modern Theories Bohr  planetary model • electrons arranged in concentric circular patterns • paths or orbits around nucleus (energy level) Wave-Mechanical Model  Electron Cloud Model • based on the ideas that orbitals are the area of highest probability where an electron will be found. • Orbitals have a variety of shapes and names (s, p, d, f)

13. Example: Wave Mechanical Model • Ψ2 (psi2) is a calculation that can predict the probability of finding an electron in a given area.

14. Summary- Atomic Models Dalton’s Cannonball

15. Thomson’s Plum Pudding

16. Rutherford’s Nuclear Model

17. Bohr’s Planetary Model

18. Wave-Mechanical Model

19. 1 amu = 1/12th mass of a carbon-12 atom Subatomic Particles **Note: amu = atomic mass unit

20. Atomic Number • Equal to the number of protons • Every element has its own atomic number • See Periodic Table C 6

21. Mass Number • Equal to the sum of the protons and the neutrons (whole number) • Can be written as carbon-12 C 12

22. To find: # of protons  look up atomic number on Periodic Table

23. To find: # of electrons  in a neutral atom, it is equal to the number of protons

24. To find: # of neutrons  if protons + neutrons = mass then, # of neutrons = mass # - # protons

25. Practice 20 40 20 20 20 12 24 12 12 12 11 23 11 12 11 2 4 2 2 2

26. Ions • Defined as “charged particles” • Ions are formed when the number of electrons changes. • If a (+) ion is formed, electrons are lost (called cations). • If a (-) ion is formed, electrons are gained (called anions).

27. Examples • Ca2+ A Ca atom has 20 protons and 20 electrons. A Ca2+ ion has lost two electrons to have 18.

28. Examples • Cl- A Cl atom has 17 protons and 17 electrons. A Cl-ion has gained one electron to have 18.

29. Practice 30 30 65 30 35 26 56 26 30 23 9 19 9 10 9 127 53 74 54 53 3 4 3 7 2

30. Isotopes • Definition: elements that have the same atomic number but different mass (different # of neutrons)

31. Isotopic Symbols • Must write isotopic symbol to show mass • Same atomic #, different mass # X Mass # Atomic #

32. Write the isotopic symbol for: • Carbon-14 C 14 6

33. Write the isotopic symbol for: • Oxygen-17 O 17 8

34. Write the isotopic symbol for: • Chlorine-37 Cl 37 17

35. Common Isotopes of Hydrogen 1 1 2 1 3 1

36. Why is atomic mass not a whole number? • The atomic mass on the periodic table is a weighted average of the isotopes of the elements. • The weighted atomic mass takes into account the relative abundances of all the naturally occurring isotopes.

37. Example of a general weighted average • Your grade in chemistry • 60% exams 85 • 15% quizzes 100 • 15% labs 95 • 10% HW/CW 80 88.25 (0.60)85 + (0.15)100 + (0.15)95 + (0.10)80 =

38. Example 1: • Determine weighted atomic mass • Boron-10 19.78% 10.013 amu • Boron-11 80.22% 11.009 amu 10.812 amu (0.1978) 10.013 + (.8022) 11.009 =

39. Example 2 • Determine weighted atomic mass • Potassium-39 93.12% 38.964 amu • Potassium-41 6.88% 40.962 amu 39.101 amu (0.9312) 38.964 + (0.0688) 40.962 =

40. Bohr models How do electrons “orbit” the nucleus? Each principal energy level … • is a fixed distance from the nucleus • can hold a specific number of electrons • has a definite amount of energy

41. The greater the distance from the nucleus…the greater the energy of the electrons in it. • The orbits are called principal energy levels or shells.

42. Energy levels or shells energy level number of e- 1 2 2 8 3 18 4 32 Increasing distance from nucleus Increasing energy

43. Bohr models: examples -energy levels and total number of electrons 8 e- P+ n0 nucleus--- # protons And neutrons 2 e- Electron configuration: element’s symbol and number of electrons in each orbit; LOOK UNDER ATOMIC NUMBER ON PT of E

44. TRY THESE 12 p+ 12 n0 Mg 2 e- 8 e- 2 e- Electron configuration (bottom left corner on PT): Mg 2-8-2 H Na F C

45. answers H Na H 1 Na 2-8-1 F C F 2-7 C 2-4 11 p+ 12 n0 1p+ 1 e- 2 e- 8e-1e- 9 p+ 10 n0 6 p+ 6 n0 2 e- 7 e- 2 e- 4 e-

46. Lewis Dot Diagrams Valence shell: outer most shell of an atom that contains electrons Valence electrons: electrons that occupy the valence shell (last number in electron configuration) Electron dot diagrams or Lewis dot diagrams: show only the valence shell of the atom Ex: Lewis dot for nitrogen: N

47. TRY THESE O F C Ne I K

48. Ions For ions: remember that ions have gained or lost electrons. (+)  indicate charge (-)  use brackets and charge

49. Ca  Ca+2 Cl  [ Cl ]-1

50. Ground State vs. Excited State • When all electrons in an atom occupy the lowest available orbitals, it is said to be in the ground state. • When electron(s) absorb energy, they have the ability to jump to higher energy levels. • The excited state is when electrons have absorbed energy and no longer occupy the lowest available energy levels.