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Chapter 8 “Covalent Bonding”

Chapter 8 “Covalent Bonding”. Covalent Bonds. covalent combination of prefix co- (Latin - “together”), valere , - “to be strong” 2 e-’s shared have strength to hold 2 atoms together particle called a “ molecule ”. Molecules. Some elements in nature are molecules :

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Chapter 8 “Covalent Bonding”

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  1. Chapter 8“Covalent Bonding”

  2. Covalent Bonds • covalent combination of prefix co- (Latin - “together”), valere, - “to be strong” • 2 e-’s shared have strength to hold 2 atoms together • particle called a “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 hold valence e-’s • don’t give away e-’s • still want NGC • share valence e-’s with each other = covalent bonding • both atomscount e-’s for NGC Covalent bonding w/ Fluorine atoms

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

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

  9. Molecular Compounds • Compounds bonded covalently called molecular compounds • Molecular compounds have • lower melting and boiling points • Weaker bond than ionic • gases or liquids at room temperature • a molecular formula: • Shows how many atoms of each element a molecule contains

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

  11. Molecular Compounds • The formula for water is written as H2O • The subscript “2” behind hydrogen means 2 atoms of hydrogen • subscript 1 omitted • Molecular formulas do not tell any information about structure (arrangement of various 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-’s • Only nonmetals and hydrogen. • 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. • Carbon is a special example - can it really share 4 electrons: 1s22s22p2? 2p 1s 2s C • Yes, due to electron 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 is formed with covalent bonds, by using an electron dot diagram • Each hydrogen has 1 valence electron - Each hydrogen wants 1 more • The oxygen has 6 valence electrons - The oxygen wants 2 more • They share to make each other complete

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

  19. O Water • So, a second hydrogen attaches • Every atom has full energy levels Note the two “unshared” pairs of electrons H H

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

  21. 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

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

  23. O Carbon dioxide • Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short C

  24. O O Carbon dioxide • Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short C

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

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

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

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

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

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

  31. Carbon dioxide • The only solution is to share more • Requires 2 double bonds • Each atom can count all the electrons in the bond O C O

  32. Carbon dioxide • The only solution is to share more • Requires two double bonds • Each atom can count all the electrons in the bond 8 valence electrons O C O

  33. Carbon dioxide • The only solution is to share more • Requires two double bonds • Each atom can count all the electrons in the bond 8 valence electrons O C O

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

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

  36. Examples • NH3, which is ammonia • N – central atom; has 5 valence electrons, 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

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

  38. 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

  39. 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

  40. 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

  41. 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

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

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

  44. 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

  45. Coordinate Covalent Bond • When one atom donates both electrons in a covalent bond. • Carbon monoxide (CO) is a good example: 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

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

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

  48. Bond Dissociation Energies... • Total energy required to break bond btwn 2 covalently bonded atoms • High dissociation energy usually means compound relatively unreactive, b/c it takes hi energy to break bond

  49. Resonance is... • When more than one valid dot diagram is possible. • Consider the two ways to draw ozone (O3) • Which one is it? Does it go back and forth? • It’s hybrid of both, shown by double-headed arrow • found in double-bond structures!

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