How to draw Lewis dot structures for

covalent compounds

Text section 12.3

Review

• Valence e-’s = only outside shell (s & p orbitals only)

• Lewis structures - show only valence e-’s

How to draw a shared pair of electrons

(representation)

More than one way to share electrons.

H2 F2 O2 N2

H-H or

H H= a single bond

More than one way to share electrons:

H2 F2 O2 N2

H-H F-F O=O NΞN

single double triple

H-H F-F O=O NΞN

Bond length and bond strength

• More electrons shared between nuclei =o Shorter (bonds)o Stronger (bonds)

• Double & triple bonds don’t affect molecular shape.

Steps for drawing Lewis dot structures

for covalent compounds

1. Find the sum of the valence electrons for the entire molecule• ( if a cation, subtract one electron from the total)• ( if an anion, add one electron to the total)

2. Divide the sum of valence electrons by 2 to find the number of electron pairs

3. Begin placing electron pairs between the central atom and the ligands

4. Continue placing the electron pairs around each ligand until the octet rule has been satisfied (or duet in Hydrogen’s case)

5. Place any leftover electron pairs on the central atom ( which causes the molecule to bend further)

6. Check to make sure all atoms are satisfied (octet/duet), if not, try a multiple bond.

Example 1

NH3• Valence electrons = 8

• Divide by 2 to find # of pairs 8/2 = 4 pairs e-

• Place pairs between the central and all ligands

• Give the ligands an octet of e- or a duet

• Leftover pairs go back on the central

• Check for duets or octets for all atoms

• If not, use multiple bonds

Example 2

H2S

Example 3

CO2

Example 4

HCN

Example 5

NBr3

Example 6

SO3 2-

Example 7

NH4 +

Valence Shell Electron Pair

Replusion=

VSEPR

Resonance

• 2 or more Lewis structures: Real = average

• See nitrate on page 420 (for example)

• For shape: any Lewis structure will do.

Homework• Text pg. 421 a-i

• Complete VSEPR prelab (if we decide to do it…)