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-
Equivalent to:
H : H
The pair of shared electrons
Negative shared electronsPositive nuclei
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
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
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
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
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