Polarity of Molecules

Electronegativity A numerical value that describes the ability of atoms to attract shared electrons to itself within a chemical bond Nonmetals are far more electronegative than metals, as you might imagine Electronegativity increases towards the upper right of the periodic table (excluding noble gases) Look at chart on p. 263

Polar Covalent Bonds Most covalent bonds, even though electrons are shared, still contain some difference in charge. Example: When Oxygen bonds with Hydrogen, there is a difference in electronegativity between Oxygen and Hydrogen which results in more of the electrons surrounding the Oxygen more frequently. This causes the Oxygen end of the bond to be more negative and the Hydrogen end of the bond to be more positive.

δ- O-H δ+ Polar Covalent Bonds lower case Greek letter delta indicates charge This bond contains a dipole, which merely means a positive and a negative end.

Electronegativity difference In order to determine the polarity of a bond, calculate the difference in electronegativity Electronegativity difference Type of Bond >1.7 Ionic 1.7-0.8 Polar Covalent 0.8-0.3 Weakly polar <0.3 nonpolar

Polar molecules Bonds within a molecule can be polar Molecules themselves can be polar or nonpolar If the center of negative charge overlaps with the center of positive charge within the molecule, the molecule itself is nonpolar, e.g., methane (CH4), CO2. If the centers of charge do not overlap, the molecule contains a dipole, and is considered polar, e.g, H2O, ammonia (NH3).

Polarity Model δ-O-Hδ+ lower case Greek letter delta indicates charge ↓ center of negative charge δ-O Hδ+ ‌ x ← center of positive charge Hδ+ The molecule has a dipole.

Polarity and Solubility “Like Dissolves Like” Polar solvents like water can dissolve ionic and polar compounds The negative parts of the water molecule attract the positive ions. Positive part of water can attract negative ions Ionic solids are pulled apart and dissolved

Polarity and Solubility II Polarity explains why oil and water don’t mix. An intermediate solvent like some alcohols will mix with both water and nonpolar solvents like gasoline Polar solvents have positive and negative parts that do not overlap Nonpolar solvents do