Covalent Bonding: Formal Charges and

Resonance Structures

What happens when a molecule does not

seem to have enough valence electrons?

Main goal is to satisfy octet rule for every atom in chemical compound Atom will share more than one electron pair if

necessary (create multiple bonds)

Ex. Formaldehyde (H2CO)

Ex. Ozone (O3)

Formal Charges

Enables us to determine atoms likely to have a negative or positive charge

Difference between valence electrons in a NEUTRAL atom and valence electrons found in an atom WITHIN A CHEMICAL BOND. More valence electrons-----negative charge Less valence electrons------positive charge

Example 1:

Formal Charges (cont.)

Difference between # of valence electrons present in an atom NOT in chemical bond and # of valence electrons present in an atom WITH a chemical bond

Not actual charges in covalent molecule Neutral molecules = 0 formal charge Polyatomic/charged molecule = net charge

Valence Electrons in atom involved in chemical bond Lone-pair electrons around atom ½ electrons in a chemical bond

Formal Charges (cont.)

Formal Charge =

#valence electrons in free atom - #lone pair electrons around bound atom – ½ (# of electrons present in bond with atom)

Example 2:

Which Lewis structure is more likely??? Best Lewis Structure for a

compound is where atoms have 0 or smallest formal charge and negative formal charges are found in the electronegative atom.

Resonance

Deals with chemical compounds having double/triple bonds.

SOOO more than one Lewis Structure is possible? So which is correct? Neither

Resonance (cont.)

Some molecules or ions have more than one possible Lewis Structure

Structures ONLY vary in electron distribution

Resonance hybrid A mix/hybrid of all possible Lewis structures—resembles

true molecular structure Same atom arrangement, different electron arrangement

so multiple bonds change location in structures

**Draw all resonance structures and connect with double arrow—resonance hybrid is a blend **

Resonance (cont.)

Delocalized Electrons Electrons are not restricted to where they can “hang

out” Electrons distribute among several atoms Seen with resonance as electron distribution varies

Localized Electrons Electron distribution does NOT vary Electrons stay in a specific region between atoms No resonance

Example 1: acetate ion

Example 2:

Draw 3 Lewis structures for SO3 and describe how the resonance hybrid relates to all 3.

Example 3:

Draw 3 Lewis structures for NO3-

and describe how the resonance hybrid relates to all 3.

Homework

Read pp. 357-360

P. 381 #45-46