Covalent Bonding: Molecular Compounds and their Structure

1. Chapter 8“Covalent Bonding” Ball-and-stick model Pre-AP Chemistry Charles Page High School Stephen L. Cotton

2. Section 8.1Molecular Compounds • OBJECTIVES: • Distinguish between the melting points and boiling points of molecular compounds and ionic compounds.

3. Section 8.1Molecular Compounds • OBJECTIVES: • Describe the information provided by a molecular formula.

4. Bonds are… • Forces that hold groups of atoms together and make them function as a unit. Two types: Ionic bonds – transfer of electrons (gained or lost; makes formula unit) Covalent bonds – sharing of electrons. The resulting particle is called a “molecule”

5. Covalent Bonds • The word covalent is a combination of the prefix co- (from Latin com, meaning “with” or “together”), and the verb valere, meaning “to be strong”. • Two electrons shared together have the strength to hold two atoms together in a bond.

6. Molecules • Many elements found in nature are in the form of molecules: • a neutral group of atoms joined together by covalent bonds. • For example, air contains oxygen molecules, consisting of two oxygen atoms joined covalently • Called a “diatomic molecule” (O2)

7. + + + + How does H2 form? • The nuclei repel each other, since they both have a positive charge (like charges repel). (diatomic hydrogen molecule)

8. + + How does H2 form? • But, the nuclei are attracted to the electrons • They share the electrons, and this is called a “covalent bond”, and involves only NONMETALS!

9. Covalent bonds • Nonmetals hold on to their valence electrons. • They can’t give away electrons to bond. • But still want noble gas configuration. • Get it by sharing valence electrons with each other = covalent bonding • By sharing, both atoms get to count the electrons toward a noble gas configuration.

10. F Covalent bonding • Fluorine has seven valence electrons (but would like to have 8)

11. F F Covalent bonding • Fluorine has seven valence electrons • A second atom also has seven

12. F F Covalent bonding • Fluorine has seven valence electrons • A second atom also has seven • By sharing electrons…

13. F F Covalent bonding • Fluorine has seven valence electrons • A second atom also has seven • By sharing electrons…

14. F F Covalent bonding • Fluorine has seven valence electrons • A second atom also has seven • By sharing electrons…

15. F F Covalent bonding • Fluorine has seven valence electrons • A second atom also has seven • By sharing electrons…

16. F F Covalent bonding • Fluorine has seven valence electrons • A second atom also has seven • By sharing electrons…

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

18. 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

19. 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

21. Molecular Compounds • Compounds that are bonded covalently (like in water, or carbon dioxide) are called molecular compounds • Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds – this is not as strong a bond as ionic

22. Molecular Compounds • Thus, molecular compounds tend to be gases or liquids at room temperature • Ionic compounds were solids • A molecular compound has a molecular formula: • Shows how many atoms of each element a molecule contains

23. Molecular Compounds • The formula for water is written as H2O • The subscript “2” behind hydrogen means there are 2 atoms of hydrogen; if there is only one atom, the subscript 1 is omitted • Molecular formulas do not tell any information about the structure (the arrangement of the various atoms).

24. - Page 215 3. The ball and stick model is the BEST, because it shows a 3-dimensional 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!

25. Section 8.2The Nature of Covalent Bonding • OBJECTIVES: • Describehow electrons are shared to form covalent bonds, and identify exceptions to the octet rule.

26. Section 8.2The Nature of Covalent Bonding • OBJECTIVES: • Demonstrate how electron dot structures represent shared electrons.

27. Section 8.2The Nature of Covalent Bonding • OBJECTIVES: • Describe how atoms form double or triple covalent bonds.

28. Section 8.2The Nature of Covalent Bonding • OBJECTIVES: • Distinguish between a covalent bond and a coordinate covalent bond, and describe how the strength of a covalent bond is related to its bond dissociation energy.

29. Section 8.2The Nature of Covalent Bonding • OBJECTIVES: • Describe how oxygen atoms are bonded in ozone.

30. A Single Covalent Bond is... • A sharing of two valence electrons. • Only nonmetals and hydrogen. • Different from an ionic bond because they actually form molecules. • Two specific atoms are joined. • In an ionic solid, you can’t tell which atom the electrons moved from or to

31. Sodium Chloride Crystal Lattice • Ionic compounds organize in a characteristic crystal lattice of alternating positive and negative ions, repeated over and over.

32. 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? • Yes, due to electron promotion! • Another example: lets show how water is formed with covalent bonds, by using an electron dot diagram

33. H O Water • 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

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

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

36. Examples: • Conceptual Problem 8.1 on page 220 • Do PCl3

37. Multiple Bonds • Sometimes atoms share more thanone pair of valence electrons. • A double bond is when atoms share two pairs of electrons (4 total) • A triple bond is when atoms share three pairs of electrons (6 total) • Table 8.1, p.222 - Know these 7 elements as diatomic: Br2 I2 N2 Cl2 H2 O2 F2 What’s the deal with the oxygen dot diagram?

38. O Dot diagram for Carbon dioxide • CO2 - Carbon is central atom( more metallic ) • Carbon has 4 valence electrons • Wants 4 more • Oxygen has 6 valence electrons • Wants 2 more C

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

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

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

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

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

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

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

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

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

48. 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

49. 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

50. 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