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

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

H2H2 H e-e- H e-e- Neutral atoms Push ‘em together...

H2H2 H e-e- H e-e- Equivalent to: H : H The pair of shared electrons Negative shared electrons Positive nuclei

H + C 1H1H 1e - 6C6C 2e - 4e - Equivalent to: H·H· · C · · · Outer e - only shown

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

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

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

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

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

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

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

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

Patterns for Major Elements u u CH 4 C = 4 bonds; all electrons shared u u NH 3 N = 3 bonds; one lone pair u u H 2 OO = 2 bonds; two lone pairs u u HFF = 1 bond; three lone pairs

4 Bonds All e - shared 3 Bonds 1 Lone Pair 2 Bonds 2 Lone Pairs 1 Bond 3 Lone pairs Carbon, group IV Nitrogen, Group V Oxygen, Group VI 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 u u Atoms may share more than one pair of electrons u u a DOUBLE BOND forms when atoms share two pairs of electrons (4 e - ) u u a TRIPLE BOND forms when atoms share three pairs of electrons (6 e - ) u u Total number of bonds per atom unchanged

Multiple Bonds 4 Bonds, each carbon Two bonds at oxygen C 2 H 4 Valence e - = 12 H 2 CO Valence e - = 12

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 - u u Sum of group numbers for atoms of representative elements u u Eg., C = 4e - N = 5e - O = 6e - H = 1e - u u CCl 4 = 4e - + (4 x 7e - ) = 32e - u u SO 2 = 6e - + (2 x 6e - ) = 18e - u u N 2 H 4 = (2 x 5e - ) + (4 x 1e - ) = 14e - u u C 3 H 8 = (3 x 4e - ) + (8 x 1e - ) = 20e -

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

Practice OF 2

Layout: O-F-F

O-F-F O-F-FIncorrect -- One F with two bonds; O only one bond

Layout: O=F-F

O=F-F O=F-FIncorrect -- One F with three bonds; (O does have two bonds)

Layout: F-O-F

F-O-F F-O-FCorrect -- 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: CH 2 Cl 2

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

Layout: u H-H-C-Cl-Cl

Layout: u H-H-C-Cl-ClIncorrect

Layout: u H-C-Cl-Cl-HIncorrect

Layout: u H-H-Cl=C=ClIncorrect

Layout: u Correct! C H H Cl

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

And Another OCS

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

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

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

Great!