Chem 221

Lecture 21- 11/12

Electron Density Distributions

Ions vs Neutral Atoms

Li vs Li⁺

• What two effects important?

Be vs Be²⁺

• Trend in size?

 Be²⁺ vs Li ⁺

Isodensity Surfaces:

Connect all points with a particular value of electron density

• Molecule Size: density = 0.002 electrons/Å.

LiH H₂ HF

• Electron cloud about H?
• HF: acting like H+ (hydrogen ion)
• LiH: acting like H- (hydride)

Higher electron density surfaces

Li

LiH H₂ HF

• Conclusion?

Fluorine wants electrons more than H

• What about Li?

Electron density concentrated around H

Electrostatic Potentials

Simplify electrostatic information:

• Use colors:
• most repulsive region: red (negative)
• most attractive region: blue (positive)

LiH H₂ HF

What does the model show:

charge separation

• H_2: uniform color

equal distribution of density

• HF: H blue, F red

Fluorine negative - hugging electrons

Conclusion?

• LiH: electrostatic attraction - ionic bonding
• H₂: shared electrons - covalent bonding
• HF: somewhere between - polar covalent

Ionic vs Covalent:

CH₄ NaCl H₂CO KF

ELECTRONEGATIVITY:

Scale that gives a measure of how much atom wants e-

H₂O CO₂

• Characterize bonding?

Electrons shared unequally

• Where is the negative end of the molecule?

Structure and Polarity

Electric dipole:

• Dipole allows us to quantify the charge separation
• One has NET DIPOLE, which one?
• Water has a dipole moment !
• What do we know about polar and non polar substances?

Polarity

This is illustrated with chromatography

• Separation is based on interactions
• between substance and paper
• between substance and solvent

·Simplify by looking only at solvents:

• H₂O and ethyl alcohol
• Both are polar
• Which substance moves with solvent?