Polarity within a Molecule

Covalently bonded molecules are either polar or non-polar. Draw the Lewis Dot Structures of water and CCl4. Both bonds are polar, but only water is a polar molecule because of the shape of the molecule.

Symmetrical molecules with the same outer atoms (linear, trigonal planar, tetrahedral) = Non-polar molecules Asymmetrical molecules (angular, bent, trigonal pyramidal) or with different outer atoms = Polar molecules

The electric charge on a CCl4 molecule measured at any distance from the center of the molecule is identical to the charge measured at the same distance on the opposite side.

Non-polar molecules are not attracted by an electric field, but polar molecules will. https://www.youtube.com/watch?v=7b-w0oWttN0

Intermolecular Forces

Covalent and ionic bonds (intramolecular bonds) between atoms are strong, but there are other forces between molecules that are weak. These weak forces between molecules are called intermolecular forces.

Intermolecular Forces (cont.)

There are several types of intermolecular forces: Attraction forces (Van der Waals ) Dispersion forces (London) Dipole-Dipole forces Hydrogen bonds

Attraction forces (van der Waals) between molecules cause some materials to be solids, some to be liquids, and some to be gases at the same temperature.

Dispersion forces (London Forces) are weak forces in non-polar molecules that result from temporary shifts in density of electrons in electron clouds.

Dipole-dipole forces are attractions between oppositely charged regions of polar molecules.

Hydrogen bonds are special, stronger dipole-dipole attractions that occur between molecules that contain a hydrogen atom bonded to a small, highly electronegative atom with at least one lone pair of electrons, typically fluorine, oxygen, or nitrogen.

Many physical properties are due to intermolecular forces. Weak forces result in the relatively low melting and boiling points of molecular substances. Many covalent molecules are relatively soft solids.

Non-polar molecules dissolve (soluble) only in non-polar substances. Polar molecules and ionic substances are usually soluble in polar substances. Substances that form Hydrogen bonds tend to dissolve better in substances with hydrogen bonds, and those with dipole-dipole intermolecular forces dissolve better in those with dipole-dipole.