1 Chapter 8 “Covalent Bonding” Ball-and-stick model

2 Section 8.2 The Nature of Covalent Bonding l OBJECTIVES: –Describe how electrons are shared to form covalent bonds, and identify exceptions to the octet rule.

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

4 How to show the formation… l It’s like a jigsaw puzzle. l You put the pieces together to end up with the right formula. l Carbon is a special example - can it really share 4 electrons: 1s 2 2s 2 2p 2 ? –Yes, due to electron promotion! (just know that there are exceptions) l Another example: lets show how water is formed with covalent bonds, by using an electron dot diagram

5 Water H O 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

6 Water l Put the pieces together l The first hydrogen is happy l The oxygen still needs one more H O

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

8 Examples: 1. Conceptual Problem 8.1 on page Do PCl 3

9 Multiple Bonds l Sometimes atoms share more than one pair of valence electrons. l A double bond is when atoms share two pairs of electrons (4 total) l A triple bond is when atoms share three pairs of electrons (6 total) l Table 8.1, p Know these 7 elements as diatomic: Br 2 I 2 N 2 Cl 2 H 2 O 2 F 2

10 Dot diagram for Carbon dioxide l CO 2 - Carbon is central atom ( more metallic ) l Carbon has 4 valence electrons l Wants 4 more l Oxygen has 6 valence electrons l Wants 2 more O C

11 Carbon dioxide l Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short O C

12 Carbon dioxide l Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short O C O

13 Carbon dioxide l The only solution is to share more O C O

14 Carbon dioxide l The only solution is to share more O C O

15 Carbon dioxide l The only solution is to share more O CO

16 Carbon dioxide l The only solution is to share more O CO

17 Carbon dioxide l The only solution is to share more O CO

18 Carbon dioxide l The only solution is to share more O CO

19 Carbon dioxide l The only solution is to share more l Requires two double bonds l Each atom can count all the electrons in the bond O CO

20 Carbon dioxide l The only solution is to share more l Requires two double bonds l Each atom can count all the electrons in the bond O CO 8 valence electrons

21 Carbon dioxide l The only solution is to share more l Requires two double bonds l Each atom can count all the electrons in the bond O CO 8 valence electrons

22 Carbon dioxide l The only solution is to share more l Requires two double bonds l Each atom can count all the electrons in the bond O CO 8 valence electrons

23 How to draw them? 1)Add up all the valence electrons separately for each atom. 2)Count up the total number of electrons to make all atoms happy. 3)Subtract the electrons the molecule wants from the electrons the molecule has; then divide by 2 4)Answer tells you how many bonds to draw 5)Fill in the rest of the valence electrons to fill atoms up.

24 Example l NH 3, which is ammonia l N – central atom; has 5 valence electrons, wants 8 l H - has 1 (x3) valence = 3 electrons, wants 2 (x3) = 6 l NH 3 has 5+3 = 8 l NH 3 wants 8+6 = 14 l (14-8)/2= 3 bonds l 4 atoms with 3 bonds N H

25 NHH H Examples l Draw in the bonds; start with singles l All 8 electrons are accounted for l Everything is full – done with this one.

26 Example: HCN l HCN (hydrogen cyanide): C is central atom l N - has 5 valence electrons, wants 8 l C - has 4 valence electrons, wants 8 l H - has 1 valence electron, wants 2 l HCN has = 10 l HCN wants = 18 l (18-10)/2= 4 bonds l 3 atoms with 4 bonds – this will require multiple bonds - not to H however

27 HCN l Put single bond between each atom l Need to add 2 more bonds l Must go between C and N (Hydrogen is full) NHC

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

29 HCN l Put in single bonds l Need 2 more bonds l Must go between C and N l Uses 8 electrons - 2 more to add l Must go on the N to fill its octet NHC

30 Another way of indicating bonds l Often use a line to indicate a bond l Called a structural formula l Each line is 2 valence electrons HHO = HHO

31 Other Structural Examples H CN C O H H

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

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

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

35 Coordinate covalent bond l Most polyatomic cations and anions contain covalent and coordinate covalent bonds l Table 8.2, p.224 l Sample Problem 8.2, p.225 l The ammonium ion (NH 4 1+ ) can be shown as another example

36 Bond Dissociation Energies... l The total energy required to break the bond between 2 covalently bonded atoms l High dissociation energy usually means the chemical is relatively unreactive, because it takes a lot of energy to break it down.

37 Resonance is... l When more than one valid dot diagram is possible. l Consider the two ways to draw ozone (O 3 ) l Which one is it? Does it go back and forth? l It is a hybrid of both, like a mule; and shown by a double-headed arrow l found in double-bond structures!

38 Resonance in Ozone Neither structure is correct, it is actually a hybrid of the two. To show it, draw all varieties possible, and join them with a double-headed arrow. Note the different location of the double bond

39 Resonance Occurs when more than one valid Lewis structure can be written for a particular molecule (due to position of double bond) These are resonance structures of benzene. The actual structure is an average (or hybrid) of these structures.

40 Resonance in a carbonate ion (CO 3 2- ): Resonance in an acetate ion (C 2 H 3 O 2 1- ): Polyatomic ions – note the different positions of the double bond.

41 The 3 Exceptions to Octet rule l For some molecules, it is impossible to satisfy the octet rule #1. usually when there is an odd number of valence electrons –NO 2 has 17 valence electrons, because the N has 5, and each O contributes 6. Note “N” page 228 l It is impossible to satisfy octet rule, yet the stable molecule does exist

42 Exceptions to Octet rule Another exception: Boron Page 228 shows boron trifluoride, and note that one of the fluorides might be able to make a coordinate covalent bond to fulfill the boron #2 -But fluorine has a high electronegativity (it is greedy), so this coordinate bond does not form #3 -Top page 229 examples exist because they are in period 3 or beyond