Chemistry / Unit 4: Chemical Bonding
4.2 Covalent Bonds and Lewis Structures
Notes
A covalent bond forms when two nonmetal atoms share a pair of electrons. The result is a molecule.
- Single bond: 1 shared pair. Double bond: 2 pairs. Triple bond: 3 pairs (as in $\text{N}_2$). More shared pairs make shorter, stronger bonds.
- Unshared pairs are lone pairs.
- Drawing a Lewis structure: (1) add up all valence electrons, (2) put the least electronegative atom in the center (never H), (3) connect atoms with single bonds, (4) complete octets on outer atoms, (5) if the center lacks an octet, turn lone pairs into double or triple bonds.
- Example: $\text{CO}_2$ has $4 + 2(6) = 16$ valence electrons and the structure O=C=O, with two lone pairs on each O.
- Hydrogen needs only 2 electrons (a duet).
Practice
How many valence electrons are in a Lewis structure of $\text{NH}_3$?
N has 5 and each H has 1: $5 + 3(1) = 8$.
How many lone pairs are on the nitrogen atom in $\text{NH}_3$?
Three bonds use 6 of the 8 electrons; the remaining 2 form one lone pair on N.
Which molecule contains a triple bond?
Each N has 5 valence electrons; sharing 3 pairs gives both atoms an octet.
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