Sharing of Electron Pairs Non-metal with Non-metal Atoms Covalent Bonding Sharing of Electron Pairs Non-metal with Non-metal Atoms

Covalent Bonding Like ionic bonds, covalent bonds are +/- attractions. Closed-shell considerations often explain the patterns of bonding, i.e., the formulas -- just as for ionic systems

H2 H e- H e- Neutral atoms Push ‘em together...

H2 e- H H : H The pair of shared electrons Equivalent to: Positive nuclei Negative shared electrons

H + C 6C 2e- 4e- 1H 1e- Equivalent to: · C H· Outer e- only shown

H + C H· · C H· H· H· Outer e- only shown

:H H + C : : H H: C H 2 e- at each H 8 e- at carbon Positive nuclei H 2 e- at each H : H: :H C : Negative electrons H 8 e- at carbon all atoms closed shell Outer e- only shown

H + N N · : H· H· H· Outer e- only shown

:H H + N : : H: N H 2 e- at each H 8 e- at nitrogen all atoms closed shell Outer e- only shown

H + O O : · : H· H· Outer e- only shown

: H + O : : H: O H 2 e- at each H 8 e- at oxygen all atoms closed shell Outer e- only shown

H + F : F : · : H· Outer e- only shown

: : H + F : : F H 8 e- at fluorine all atoms closed shell 2 e- at H Outer e- only shown

Patterns for Major Elements CH4 C = 4 bonds; all electrons shared NH3 N = 3 bonds; one lone pair H2O O = 2 bonds; two lone pairs HF F = 1 bond; three lone pairs

4 Bonds All e- shared Carbon, group IV 3 Bonds 1 Lone Pair Nitrogen, Group V 2 Bonds 2 Lone Pairs Oxygen, Group VI 1 Bond 3 Lone pairs Fluorine, Group VII

Other Compounds Have Same Pattern C = 4 bonds; Cl (like F) = 1 bond P (like N = 3 bonds; Br (like F) = 1 bond

More Similarities: S (like Oxygen) = 2 bonds, 2 lone pairs C = 4 bonds; O = 2 bonds, 2 lone pairs

Multiple Bonds Atoms may share more than one pair of electrons a DOUBLE BOND forms when atoms share two pairs of electrons (4 e-) a TRIPLE BOND forms when atoms share three pairs of electrons (6 e-) Total number of bonds per atom unchanged

Multiple Bonds C2H4 Valence e- = 12 H2CO Valence e- = 12 4 Bonds, each carbon H2CO Valence e- = 12 Two bonds at oxygen

Multiple Bonds Total valence e- = 16 (Oxygen = 6 e-, each; Carbon = 4 e-. Four bonds/C; 2 bonds oxygen Total valence e- = 10 (H=1, C=4, N=5) Bonds: C=4, N=3, H=1 Total valence e- = 18 (O=6, Cl = 7, P = 5) Bonds: Cl=1, P=3, O=2

Predicting Dot Diagrams How Do We Proceed? Determine total number of valence electrons Based on usual bond nos., identify reasonable layout for atoms Place bonding electrons between atoms to make usual no. of bonds Place remaining electrons as lone pairs around atoms still lacking an octet

Total number of Valence e- Sum of group numbers for atoms of representative elements Eg., C = 4e- N = 5e- O = 6e- H = 1e- CCl4 = 4e- + (4 x 7e-) = 32e- SO2 = 6e- + (2 x 6e-) = 18e- N2H4 = (2 x 5e-) + (4 x 1e-) = 14e- C3H8 = (3 x 4e-) + (8 x 1e-) = 20e-

Based on Covalence (Usual Bonds) Nos. Reasonable Layouts Based on Covalence (Usual Bonds) Nos. H2S -- 1 Bond each H; 2 Bonds for S H-H=S Incorrect - One H has 3 bonds H-S-H Correct - All atoms with usual bond numbers (H=1; S=2) PCl3 Cl=P-Cl-Cl Incorrect - One Cl has two bonds Cl-P-Cl Correct -- (Cl=1; P=3) | Cl

Practice OF2

Layout: O-F-F

Layout: O-F-F Incorrect -- One F with two bonds; O only one bond

Layout: O=F-F

Layout: O=F-F Incorrect -- One F with three bonds; (O does have two bonds)

Layout: F-O-F

Layout: F-O-F Correct -- F = 1 bond, each; O = 2 bonds)

Practice Place bonding electrons for connected atoms

Practice Place bonding electrons for connected atoms

Practice Add lone pairs for atoms needing “octet”

Practice Add lone pairs for atoms needing “octet”

Practice Similarly, add lone pairs for fluorines:

Practice Check Results! Correct total e- (O = 6; F = 7, ea) = 20e- Usual bond nos. (O=2; F=1, ea) Closed shell (octets) O=8; F=8 each

Now You Do One: CH2Cl2

Total e- C = 4 e- H = 1e-, each, 2 total Cl = 7e-, each = 14 more Grand total = 20e-

Layout: H-H-C-Cl-Cl

Layout: H-H-C-Cl-Cl Incorrect

Layout: H-C-Cl-Cl-H Incorrect

Layout: H-H-Cl=C=Cl Incorrect

Layout: C H Cl Correct!

Check Results: Total e- Usual no. Bonds Closed Shells

And Another OCS

C = 4e- O = 6e- S = 6e- Total = 16e- Number e- C = 4e- O = 6e- S = 6e- Total = 16e-

Layout? O C S or C S O S O C ??

2 Bonds for O; 4 Bonds for C; 2 Bonds for S Layout O=C=S 2 Bonds for O; 4 Bonds for C; 2 Bonds for S

Great!