Chapter 2 Atoms, Molecules and Ions

1. Chapter 2Atoms, Molecules and Ions Preview: • Fundamental Chemical Laws and Atom. • Modern View of Atomic Structure, Molecules, and Ions. • Periodic Table. • Naming Simple compounds, Ionic compounds, Formula from names.

2. The Early History of Chemistry • Before 16th Century • Alchemy: Attempts (scientific or otherwise) to change cheap metals into gold • 17th Century • Robert Boyle: First “chemist” to perform quantitative experiments • 18th Century • George Stahl: Phlogiston flows out of a burning material. • Joseph Priestley: Discovers oxygen gas, “dephlogisticated air.”

3. Law of Conservation of Mass • Discovered by Antoine Lavoisier • Mass is neither created nor destroyed • Combustion involves oxygen, not phlogiston

4. Other Fundamental Chemical Laws Law of Definite Proportion • A given compound always contains exactly the same proportion of elements by mass. • Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine: CCl4

5. Other Fundamental Chemical Laws Law of Multiple Proportions • When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers. • The ratio of the masses of oxygen in H2O and H2O2 will be a small whole number (“2”).

6. 2 Law of Multiple Proportions 2.1

7. Mass of Nitrogen That Combines With 1 g Oxygen Compound A 1.750 g Compound B 0.8750 g Compound C 0.4375 g A/B = 1.750/0.8750 = 2/1 B/C = 0.875/0.4375 = 2/1 A/C = 1.750/0.4375 = 4/1 i.e. amount of nitrogen in A is twice that in B, etc.

10. Dalton’s Atomic Theory (1808) • Each element is made up of tiny particles called atoms. • The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

11. Dalton’s Atomic Theory(continued) • Chemical compounds are formed when atoms combine with each other.A given compound always has the same relative numbers and types of atoms. • Chemical reactions involve reorganization of the atoms- changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

12. Dalton's theory lead to: • 1gm hydrogen + 8gm of oxygen water • he assumed that water formula is "OH" and the mass of hydrogen is "1" and of oxygen is "8". • Using the same concepts, Dalton's proposed the first table of atomic masses. It has been proved later that Dalton's table contain incorrect. • Gay-lussac (1778-1850) found experimentally that:

13. Figure 2.4: A representation of some of Gay-Lussac's experimental results on combining gas volumes. Interpreted in 1811 by Avogadro

14. Avogadro’s Hypothesis (1811) At the same temperature and pressure, equal volumes of different gasescontain the same number of particles. • 5 liters of oxygen • 5 liters of nitrogen • Same number of particles!

15. Figure 2.5: A representation of combining gases at the molecular level. The spheres represent atoms in the molecules.

16. Early Experiments to Characterize the Atom • J. J. Thomson- postulated the existence of electrons using cathode ray tubes. • Measured charge /mass of e- • Received 1906 Nobel Prize in Physics • e = -1.76 x 108 C/g • m

17. Figure 2.7: A cathode-ray tube. The fast-moving electrons excite the gas in the tube, causing a glow between the electrodes.

18. 2.2

19. (Uranium compound)

20. Figure 2.9: Thomson plum pudding model of the atom.

21. Ernest Rutherford - explained the nuclear atom, containing a dense nucleus with electrons traveling around the nucleus at a large distance.

22. Figure 2.12: Rutherford's experiment on -particle bombardment of metal foil.

23. Figure 2.13: (a) The expected results of the metal foil experiment if Thomson's model were correct. (b) Actual results.

24. Millikan oil drop experiment • Millikan did another experiment to determine the mass of the –ve particles (electrons). The experiment used mainly to determine the magnitude of the electron charge and using e/m to get m- value.

25. The Modern View of Atomic Structure The atom contains: • electrons • protons: found in the nucleus, they have a positive charge equal in magnitude to the electron’s negative charge. • neutrons: found in the nucleus, virtually same mass as a proton but no charge.

26. Figure 2.14: A nuclear atom viewed in cross section. Note that this drawing is not to scale.

27. Figure 2.15: Two isotopes of sodium. Both have eleven protons and eleven electrons, but they differ in the number of neutrons in their nuclei.

28. The Mass and Charge of the Electron, Proton, and Neutron

29. Summary • J.J. Thompson (1897) “cathode rays are electrons” (e–) and finds e/m ratio • Robert Millikan (1909) measures e and hence melectron known at 9.1110-31 kg • E. Rutherford (1906) bounces  (He2+) off Au tissue proving protons (p+) in nucleus • F.A. Aston (1919) “weighs” atomic ions • J. Chadwick (1939) observes neutrons (no charge) by decomposition (to p+, e–, and ).

30. The Chemists’ Shorthand: Atomic Symbols 39 Mass number  K  Element Symbol 19 Atomic number 

32. Q6. In Rutherford’s experiment, most of the alpha particles directed at the thin metal foil, A) passed directly through the foil without being deflected. B) were reflected directly back from the foil. C) were deflected at a 90º angle. D) were absorbed by the foil. E) converted into high-energy gamma particles.

33. Chemical Bonds • The forces that hold atoms together in compounds. Covalent bondsresult from atoms sharing electrons. • Molecule: a collection of covalently-bonded atoms.

34. The Chemists’ Shorthand:Formulas • Chemical Formula: • Symbols = types of atoms • Subscripts = relative numbers of atoms CO2 • Structural Formula: • Individual bonds are shown by lines. O=C=O

35. Figure 2.16: The structural formula for methane.

36. Figure 2.17: Space-filling model of methane. This type of model shows both the relative sizes of the atoms in the molecule and their spatial relationships.

37. Figure 2.18: Ball-and-stick model of methane.

39. Ions Cation: A positive ion Mg2+, NH4+ Anion: A negative ion Cl, SO42 Ionic Bonding: Force of attraction between oppositely charged ions.

40. Periodic Table Elements classified by: properties atomic number Groups (vertical) 1A = alkali metals 2A = alkaline earth metals 7A = halogens 8A = noble gases Periods (horizontal)

41. Earth’s Most Abundant Elements Oxygen 0 46% Silicon Si 27% Aluminum Al 8% Iron Fe 5% Calcium Ca 3% Sodium Na 2.8% Potassium K 2.5% Magnesium Mg 2.0%

42. Figure 2.21: The Periodic Table.http://center.acs.org/periodic/tools/pt.html

43. http://www.elements-of-life.org/eol_index_flash.html

44. Naming Compounds Binary Ionic Compounds: 1. Cation first, then anion 2. Monatomic cation = name of the element Ca2+ = calcium ion 3. Monatomic anion = root + -ide Cl = chloride CaCl2 = calciumchloride HI = hydrogen iodide

45. Figure 2.19: Sodium metal reacts with chlorine gas to form solid sodium chloride.

46. Naming Compounds(continued) Binary Ionic Compounds (Type II):  metal forms more than one cation  use Roman numeralin name PbCl2 Pb2+is cation PbCl2 = lead (II) chloride

47. iron(II) chloride FeCl2 2 Cl- -2 so Fe is +2 FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide

48. Figure 2.22: The common cations and anions

49. Naming Compounds(continued) Binary compounds (Type III):  Compounds between two nonmetals First elementin the formula is named first. Second elementis named as if it were an anion.  Use prefixes  Never use mono- P2O5 = diphosphorus pentoxide

50. TOXIC! Laughing Gas Molecular Compounds NF3 nitrogen trifluoride SO2 sulfur dioxide N2Cl4 dinitrogen tetrachloride NO2 nitrogen dioxide N2O dinitrogen monoxide 2.7