Polarity of MoleculesPolarity of Molecules

ElectronegativityElectronegativity

A numerical value that describes the ability A numerical value that describes the ability of atoms to attract shared electrons to itself of atoms to attract shared electrons to itself within a chemical bondwithin a chemical bond

Nonmetals are far more electronegative Nonmetals are far more electronegative than metals, as you might imaginethan metals, as you might imagine

Electronegativity increases towards the Electronegativity increases towards the upper right of the periodic table (excluding upper right of the periodic table (excluding noble gases)noble gases)

Look at chart on p. 263Look at chart on p. 263

Polar Covalent BondsPolar Covalent Bonds

Most covalent bonds, even though electrons are Most covalent bonds, even though electrons are shared, still contain some difference in charge.shared, still contain some difference in charge.

Example: When Oxygen bonds with Hydrogen, Example: When Oxygen bonds with Hydrogen, there is a difference in electronegativity between there is a difference in electronegativity between Oxygen and Hydrogen which results in more of Oxygen and Hydrogen which results in more of the electrons surrounding the Oxygen more the electrons surrounding the Oxygen more frequently.frequently.

This causes the Oxygen end of the bond to be This causes the Oxygen end of the bond to be more negative and the Hydrogen end of the bond more negative and the Hydrogen end of the bond to be more positive. to be more positive.

Polar Covalent BondsPolar Covalent Bonds

δ- δ- O-H O-H δ+δ+

lower case Greek letter delta indicates chargelower case Greek letter delta indicates charge

This bond contains a dipole, which merely This bond contains a dipole, which merely means a positive and a negative end.means a positive and a negative end.

Electronegativity differenceElectronegativity difference

In order to determine the polarity of a bond, In order to determine the polarity of a bond, calculate the difference in electronegativitycalculate the difference in electronegativity

Electronegativity Electronegativity differencedifference

Type of BondType of Bond

>1.7>1.7 IonicIonic

1.7-0.81.7-0.8 Polar CovalentPolar Covalent

0.8-0.30.8-0.3 Weakly polarWeakly polar

<0.3<0.3 nonpolarnonpolar

Polar moleculesPolar molecules

Bonds within a molecule can be polarBonds within a molecule can be polar Molecules themselves can be polar or nonpolarMolecules themselves can be polar or nonpolar If the center of negative charge overlaps with the If the center of negative charge overlaps with the

center of positive charge within the molecule, the center of positive charge within the molecule, the molecule itself is nonpolar, e.g., methane (CHmolecule itself is nonpolar, e.g., methane (CH44), ),

COCO22..

If the centers of charge do not overlap, the If the centers of charge do not overlap, the molecule contains a dipole, and is considered molecule contains a dipole, and is considered polar, e.g, Hpolar, e.g, H22O, ammonia (NHO, ammonia (NH33). ).

Polarity ModelPolarity Model

δ-δ-O-HO-Hδ+δ+ lower case Greek letter delta lower case Greek letter delta

indicatesindicates charge charge

↓ ↓ center of negative chargecenter of negative chargeδ-δ-OO H Hδ+δ+

x ← center of positive chargex ← center of positive charge

HHδ+ δ+

The molecule has a dipole.The molecule has a dipole.

Polarity and SolubilityPolarity and Solubility

““Like Dissolves Like”Like Dissolves Like” Polar solvents like water can dissolve ionic Polar solvents like water can dissolve ionic

and polar compoundsand polar compounds The negative parts of the water molecule The negative parts of the water molecule

attract the positive ions.attract the positive ions. Positive part of water can attract negative Positive part of water can attract negative

ionsions Ionic solids are pulled apart and dissolvedIonic solids are pulled apart and dissolved

Polarity and Solubility IIPolarity and Solubility II

Polarity explains why oil and water don’t Polarity explains why oil and water don’t mix.mix.

An intermediate solvent like some alcohols An intermediate solvent like some alcohols will mix with both water and nonpolar will mix with both water and nonpolar solvents like gasolinesolvents like gasoline

Polar solvents have positive and negative Polar solvents have positive and negative parts that do not overlapparts that do not overlap

Nonpolar solvents do Nonpolar solvents do