Chapter 8 “Covalent Bonding”

1. Chapter 8“Covalent Bonding”

2. Covalent Bonds • covalent • co- (Latin - “together”) • valere - “to be strong” • 2 e- shared - strength holding 2 atoms together • Smallest particle - “molecule”

3. Molecules • Some elements in nature are molecules: • neutral group of atomscovalently bonded • Ex. - air contains O molecules, 2 O atoms joined covalently • Called “diatomic molecule” (O2)

4. + + + + How does H2 form? (diatomic hydrogen molecule) • The nuclei repel each other, (both have + charge)

5. + + How does H2 form? • nuclei attraction to e-’s stronger than repulsion of nuclei • e-’s shared • covalent bond • Only NONMETALS!

6. Covalent bonds • Nonmetals • don’t lose e-’s • still want NGC • share valence e- w/ each other = covalent bonding • both atomscount shared e- for NGC Covalent bonding w/ Fluorine atoms

7. Covalent bonding • Fluorine has 7 valence e- • A second atom also has seven • By sharing electrons… • …both end with full orbitals F F 8 Valence electrons

8. Covalent bonding • Fluorine has 7 valence e- • A second atom also has 7 • By sharing electrons… • …both full orbitals F F 8 Valence electrons single covalent bond between 2 H atoms

9. Molecular Formulas • molecular compounds - Compounds bonded covalently • Molecular compounds have: • lower melting/boiling pts • Generally stronger bond than ionic • gases or liquids @ RT • molecular formula: • Shows # atoms of each element in molecule

10. Reminder from Ch. 7 • No “molecule” of sodium chloride • Ionic cmpds exist as collection of + & - charged ions arranged in repeating 3D patterns. • Formula unit

11. Molecular Formulas • water H2O • Subscript “2” means 2 atoms of H • subscript 1 omitted • Molecular formulas do not give info about structure (arrangement of atoms)

12. - Page 215 3. The ball and stick model is BEST, because it shows 3D arrangement. These are some of the different ways to represent ammonia: 1. The molecular formula shows how many atoms of each element are present 2. The structural formula ALSO shows the arrangement of these atoms!

13. Section 8.2The Nature of Covalent Bonding

14. A Single Covalent Bond is... • sharing 2 valence e- • Only nonmetals and H • Different from ionic bond b/c they actually form molecules. • Two specific atoms joined • In an ionic solid, you can’t tell which atom e-’s moved from or to

15. How to show the formation… • It’s like a jigsaw puzzle. • You put the pieces together to end up with the right formula. • C ….. special ex. - can it really share 4 electrons: 1s22s22p2? 2p 1s 2s C • Yes, due to e- promotion!

16. How to show the formation… • It’s like a jigsaw puzzle. • You put the pieces together to end up with the right formula. • Carbon is a special example - can it really share 4 electrons: 1s22s22p2? 2p 1s 2s

17. H O Water Another example: water formed w/ covalent bonds, by using e- dot structure • Each H has 1 valence e- - Each H wants 1 • O has 6 valence e- - wants 2 more • Sharing completes each octet Single bonds

18. O Water • Put the pieces together • The first H is happy • The O still needs 1 more H

19. O Water • a 2nd H attaches • Every atom has full energy levels Note the two “unshared” pairs of electrons H H

20. This is what happens when you miss Chemistry class Do you know this kid?

21. How many bonds do I make? #bonds = # valence e- needed - # valence e- present / 2 Check out my webpage for a detailed list of rules for electron dot/structural formula rules for covalent bonds

22. Examples: • Conceptual Problem 8.1 on page 220 • We’ll do #7 & 8

23. Double and Triple Covalent Bonds • Sometimes atoms share more thanone pair of valence e-’s • double bond: atoms share 2 pairs of e-’s (4 total) • triple bond: atoms share 3 pairs of e-’s (6 total) • Table 8.1, p.222 - Know these 7 elements as diatomic: Br2 I2 N2 Cl2 H2 O2 F2

24. O Dot diagram for Carbon dioxide • CO2 - Carbon is central atom( more metallic ) • C - 4 valence e- • Wants 4 more • O- 6 valence e- • Wants 2 more C The chemistry of CO2 6:44

25. O Carbon dioxide • Attaching 1 oxygen leaves oxygen 1 e- short, and carbon 3 short C

26. O O Carbon dioxide • Attaching 2nd O • both O 1 e- short • C 2 e- short C

27. O O Carbon dioxide • only solution  share more C

28. O O Carbon dioxide • The only solution is to share more C

29. O Carbon dioxide • The only solution is to share more O C

30. O Carbon dioxide • The only solution is to share more O C

31. O Carbon dioxide • The only solution is to share more O C

32. Carbon dioxide • The only solution is to share more O C O

33. Carbon dioxide • The only solution is to share more • Requires 2 double bonds • Each atom counts all e- in bond O C O

34. Carbon dioxide • The only solution is to share more • Requires two double bonds • Each atom counts all e- in the bond 8 valence electrons O C O

35. Carbon dioxide • The only solution is to share more • Requires two double bonds • Each atom counts all e- in the bond 8 valence electrons O C O

36. Carbon dioxide • The only solution is to share more • Requires two double bonds • Each atom counts all e- in the bond 8 valence electrons O C O How covalent bonds form - Mark Rosengarden

37. How to draw them? • Use the handout guidelines: • Add up all valence e- • Count total e- needed to make all atoms happy (stable) • Subtract; Divide by 2 (tells you how many bonds to draw) • Central atom (least electronegative) • Start w/ most electronegative atom, fill in remaining valence e- to fill atoms up

38. Examples • NH3, ( ammonia ) • N – central atom; 5 valence e- (wants 8) • H - has 1 (x3) valence electrons, wants 2 (x3) • NH3 has 5+3 = 8 • NH3 wants 8+6 = 14 • (14-8)/2= 3 bonds • 4 atoms with 3 bonds N H

39. Examples • Draw in the bonds; start with singles • All 8 e- accounted for • Everything full – DONE! H H N H

40. Example: HCN • HCN: C is central atom • N - has 5 valence electrons, wants 8 • C - has 4 valence electrons, wants 8 • H - has 1 valence electron, wants 2 • HCNhas 5+4+1 = 10 • HCNwants 8+8+2 = 18 • (18-10)/2= 4 bonds • 3 atoms with 4 bonds – this will require multiple bonds - not to H however

41. HCN • Put single bond between each atom • Need to add 2 more bonds • Must go between C and N (Hydrogen is full) H C N

42. HCN • Put in single bonds • Needs 2 more bonds • Must go between C and N, not the H • Uses 8 electrons – need 2 more to equal the 10 it has H C N

43. HCN • Put in single bonds • Need 2 more bonds • Must go between C and N • Uses 8 electrons - 2 more to add • Must go on the N to fill its octet H C N

44. Another way of indicating bonds • Often use a line to indicate a bond • structural formula • Each line = 2 valence e- H O H H O H =

45. Other Structural Examples H C N H C O H

46. C O A Coordinate Covalent Bond... • When one atom donates both electrons in a covalent bond. • Carbon monoxide (CO) is a good example: Both the carbon and oxygen give another single electron to share

47. Coordinate Covalent Bond • When one atom donates both e- • i.e. Carbon monoxide (CO) Oxygen gives both of these electrons, since it has no more singles to share. This carbon electron moves to make a pair with the other single. C O

48. Coordinate Covalent Bond • When one atom donates both e- • Carbon monoxide (CO) The coordinate covalent bond is shown with an arrow as: C O C O

49. Coordinate covalent bond • Most polyatomic cations & anionscontain covalent & coordinate covalent bonds • Table 8.2, p.224 • Sample Problem 8.2, p.225 • Ammonium ion (NH4+)can be shown as another example

50. Bond Dissociation Energies... • Total energy required to break bond btwn 2 covalently bonded atoms • High dissociation energy usually means compound relatively unreactive, b/c high energy needed to break bond