Lewis Structures

H + Cl ---> H-Cl

Procedure

1. Write the skeletal structure (CO₃^2-)

a.

b.

c.

2. Count the valence electrons & adjust the total for ions.

3. Use the valence electrons to connect atoms with single bonds. Use the remaining electrons to give lone pairs to the terminal atoms in order to satisfy the octet rule. Use any remaining electrons to give the central atom one or more lone pairs. If necessary, form multiple bonds to satisfy the octet rule (most common with C, N, O, & S)

4. Draw any resonance forms.

Isosurfaces show this delocalization better than Lewis structures:

Carbonate ion (CO₃^2-)

solid (electon density = 0.08) ca. covalent radius

mesh (electron density = 0.002) ca. Van der Waals radius

We can also see that the charge is delocalized:

Electrostatic potential mapped onto an electron density isosurface (0.002)

Another view:

Electrostatic potential mapped onto "bonding" electron density isosurface (0.08)

5. Reasonable structures have:

a.

b.

c.

examples:

SO₄^2-

N₂O

Formal Charge vs Oxidation State

Formal charge assumes __________________ of electrons.

Oxidation state assumes _______________ of electrons to the more electronegative atom.

CO₂

Electron density isosurfaces of CO₂.

Electrostatic potential mapped onto 0.002 isosurface of CO₂.

(blackboard & Hyperchem)

Formal charges:

• good for _______________________________
• good for _______________________________

Oxidation numbers:

• good for _______________________________
• good for _______________________________
• good for _______________________________

Comments about the Octet Rule

1. The octet rule is always obeyed for _________________

2. H, Be, & B may have _______________________

Are both of these Lewis structures okay?

Electrostatic potential map for BF₃.

BeCl₂ & BF₃ are _______________: they have empty orbitals which can ____________ electron pairs.

(blackboard)

3. Third row & heavier elements can have ______________________.

PF₅

This is a hypervalent compound. Hypervalency is due to the size of atoms and the availability of d orbitals.

4. Transition metal complexes do not obey the octet rule, but may _______________________.

Cr(CO)₆