Resonance

A molecule or polyatomic ion for which 2 or more dot formulas with the same arrangement of atoms can be drawn is said to exhibit RESONANCE.

Resonance Example

CO32-

3 resonance structures can be drawn for CO32-

the relationship among them is indicated by the double arrow.

the true structure is an average of the 3.

Resonance Example

CO32-

3 resonance structures can be drawn for CO32-

the relationship among them is indicated by the double arrow.

the true structure is an average of the 3.

Resonance Example

CO32-

3 resonance structures can be drawn for CO32-

the relationship among them is indicated by the double arrow.

the true structure is an average of the 3

Resonance Structures

Another way to represent this is by delocalization of bonding electrons:

(the dashed lines indicate the 4 pairs of bonding electrons are equally distributed among 3 C-O bonds; unshared electron pairs are not shown) See p. 256

VSEPR

valence shell electron pair repulsion

Molecular Shape

Lewis structures (electron dot structures) show the structure of molecules…but only in 2 dimensions (flat).

BUT, molecules are 3 dimensional! for example, CH4 is:

Molecular Shape Lewis structures (electron dot structures)

show the structure of molecules…but only in 2 dimensions (flat).

BUT, molecules are 3 dimensional! but in 3D it is:

a tetrahedron!= coming out of

page= going into page= flat on page

Why do molecules take on 3D shapes instead of being flat?

Valence Shell Electron Pair Repulsion theory

“because electron pairs repel one another, molecules adjust their shapes so that the valence electron pairs are as far apart from another as possible.”

Why do molecules take on 3D shapes instead of being flat?

Valence Shell Electron Pair Repulsion theory

Remember: both shared and unshared electron pairs will repel one another.

H—N — H—

H

Non-BondingPairs

BondingPairs

5 Basic Molecule Shapes

Linear

Example: CO2

5 Basic Molecule Shapes

Bent or angular

Example: H2O Notice electron pair repulsion

5 Basic Molecule Shapes

tetrahedral

example: CH4

5 Basic Molecule Shapes

Pyramidal

Example: NH3 (note: unshared pair of electron

repels, but is not considered part of overall shape; no atom there to contribute to the shape)

5 Basic Molecule Shapes

Trigonal planar or planar triangular

Example: BF3

Geometry and polarity

Three shapes will cancel out polarity.

Shape One: Linear

Geometry and polarity

Three shapes will cancel out polarity.

Planar triangles

120º

Geometry and polarity

Three shapes will cancel out polarity.

Tetrahedral

Geometry and polarity

Others don’t cancel Bent

Geometry and polarity

Others don’t cancel Trigonal Pyramidal