Chemical Bonds Continued…

1. Chemical Bonds Continued… Unit 4 - MoLECULES

2. NaCl vs. CO2 • What do you already know? • Imagine! • Close your eyes and picture a piece of salt. Now zoom in…..what does it look like at the atomic level? • Now imagine carbon dioxide – what does it look like at the atomic level? • How are these two compounds different?

3. How do non-metals bond with each other? • Recall non-metals have______(high or low) ionization energy when compared to metals. • Reason: • Result: • What is electronegativity? Why does it exhibit a distinct periodic trend? • How do nonmetals bond?

4. Electronegativity - Linus Pauling

5. Covalent bonding Atoms are “bonded” because both nuclei (+) are attracted to the same electrons (-).

6. Covalent vs Ionic

7. Using electronegativities • Electronegativity Difference /Bond Character • > 1.7 and above / ionic • 0.4 - 1.7 / polar covalent • 0 -0.4 /nonpolar covalent

8. What is a molecule? • Two or more atoms covalently bonded to make a neutral particle is a unit called a molecule. • A polyatomic ion (i.e. NO3-) is very similar to a molecule, except that it has a charge. • Covalent compounds AND most non-metal elements are composed of molecules. H2O CO2 O2 P4

9. MORE KEY TERMS • All diatomic molecules are NONPOLAR COVALENT • Practice lewis dot structure • Electronegativity – tendency of an atom to attract shared electrons to itself! (greatest attraction for electrons)

10. What is a Lewis symbol? • Lewis symbols… • are simple pictures of atoms • are used to represents covalent bonds Lewis Symbol Ne Each dot is a valence electron Gilbert N. Lewis

12. Molecular Representations

14. Diatomic nonpolar molecules

16. Polar covalent vs. nonpolar covalent

17. What determines the structure of molecule? • “Octet rule” - atoms usually form covalent bonds with each other resulting in a total of 8 valence electrons around each atom. • What is special about eight e-? • Why usually but not always 8? • What’s the actual REASON that non-metals share electrons?

18. Drawing Lewis Dot Structure • 1. Write symbols of the elements. • - if 3 or more elements figure out which one is the central one ( typically C , N, P, S – sometimes O) If all of these are present usually C is the central one. • 2. Determine total # of valence electrons • Don’t forget about adding or subtracting an electron/s if you are dealing with the polyatomic ions • 3. Use a single bond to connect each atom together -> then fill in the remaining electrons around the atoms to complete the octet rule - if total # of electrons won’t fulfill octet rule – double or triple bonds are necessary

24. Example: Lewis structure for water • Formula of water is H2O • Total valence electrons: • Lewis structure to obey octet rule: • Practice on your own: PCl3 and SF2

25. Lewis structures have limitations • What does a Lewis structure show us about a molecule? • What does it NOT show?

30. CF4

31. CF4

32. NH3

33. NH3

34. What about the ammonium ion? NH4+

37. N2

38. N2

39. nomenclature for molecules • Rules for naming covalent molecules • 1. Use prefixes to indicate the # of atoms present • EXCEPTION: • Never use mono for the first element in the molecule • 2. Same rules for –ide ending for the last element in molecule • Prefixes: • Mono -1 hexa -6 • Di- 2 hepta -7 • Tri -3 octa -8 • Tetra- 4 nona -9 • Penta- 5 deca -10

40. Practice! • Name these compounds: • PCl3 • H2O • N2O4 • SF6 • Write formulas for these compounds: • Diphosphoruspentoxide • Carbon tetrachloride

41. VSEPR theory

42. Linear shape (2/0)

43. Trigonal planar 3/0

44. Tetrahedral 4/0

46. Trigonal Pyramidal(3/1)

47. Bent: 2/2 or 2/1

50. Molecular Structures and the Periodic table