Covalent Bonding 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

He-

He-

Neutral atomsPush ‘em together...

H2

He-

He-

Equivalent to:

H : H

The pair of shared electrons

Negative shared electronsPositive nuclei

H + C

1H1e-

6C2e-

4e-Equivalent to:

H· ·C ··

·Outer e- only shown

H + C

H· ·C ··

·

Outer e- only shown

H·

H·

H·

H + C

C

Outer e- only shown

H

:HH: ::H 2 e- at each H

8 e- at carbonall atoms closed shell

Positive nuclei

Negative electrons

H + N

H·

Outer e- only shown

H·

H·N ··

·:

H + N

N

Outer e- only shown

H

:HH: :: 2 e- at each H

8 e- at nitrogenall atoms closed shell

H + O

H·

Outer e- only shown

H·

O :·

·:

H + O

O

Outer e- only shown

H

:H: :: 2 e- at each H

8 e- at oxygenall atoms closed shell

H + F

Outer e- only shown

H·

: F :·

:

H + F

F

Outer e- only shown

H

: : ::

2 e- at H

8 e- at fluorineall atoms closed shell

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 BondsAll e-

shared

3 Bonds1 Lone Pair

2 Bonds2 Lone Pairs

1 Bond3 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

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

4 Bonds, each carbon

Two bonds at oxygen

•C2H4 Valence e- = 12

•H2CO 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 DiagramsHow Do We Proceed?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-

Reasonable LayoutsBased on Covalence (Usual Bonds) Nos.Based on Covalence (Usual Bonds) Nos.

•H2S -- 1 Bond each H; 2 Bonds for SH-H=S Incorrect - One H has 3 bondsH-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

OFOF22

Layout:

O-F-FO-F-F

Layout:

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

Layout:

O=F-FO=F-F

Layout:

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

Layout:

F-O-FF-O-F

Layout:

F-O-FF-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:

CHCH22ClCl22

Total e-

C = 4 e-

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

Layout:

H-H-C-Cl-ClH-H-C-Cl-Cl

Layout:

H-H-C-Cl-ClH-H-C-Cl-Cl IncorrectIncorrect

Layout:

H-C-Cl-Cl-HH-C-Cl-Cl-H IncorrectIncorrect

Layout:

H-H-Cl=C=ClH-H-Cl=C=Cl IncorrectIncorrect

Layout:

Correct!Correct!C

H

H Cl

Cl

Check Results:

• Total eTotal e--

• Usual no. BondsUsual no. Bonds• Closed ShellsClosed Shells

And Another

OCSOCS

Number e-

C = 4eC = 4e--

O = 6eO = 6e--

S = 6eS = 6e--

Total = 16eTotal = 16e--

Layout?

O C SO C S

oror

C S OC S O

oror

S O CS O C

????

Layout

O=C=S

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

Great!Great!