Valence Shell Electron Pair Repulsion Theory Planar triangular Trigonal bipyramidal Octahedral

VSEPR theory (Valence Shell Electron Pair Repulsion) states that repulsion between valence electrons causes these sets to be oriented as far apart as possible.

VSEPR theory This theory predicts the geometry of the molecules. There are 2 types of electron pairs, shared pairs that form bonds and unshared pairs or lone pairs that are nonbonding electrons.

VSEPR theory We will study 5 common shapes of molecules.

VSEPR theory If the molecule has 2 bonds and 0 lone pairs the shape is linear and the bond angles are 180°. Examples:

VSEPR theory If the molecule has 2 bonds and 2 lone pairs the shape is bent or angular and the bond angles are 105°. Example:

VSEPR theory If the molecule has 3 bonds and 1 lone pair the shape is trigonal pyramidal and the bond angles are 107°. Examples:

VSEPR theory If the molecule has 4 bonds and 0 lone pair the shape is tetrahedral and the bond angles are 109.5°. Example:

VSEPR theory If the molecule has 3 bonds and 0 lone pair the shape is trigonal planar and the bond angles are 120°. Example:

Atoms bonded to central atom (B) Molecular Shape Atoms bonded to central atom (B) Lone pairs (E) Linear 2 Bent or angular Trigonal-planar 3

Atoms bonded to central atom Molecular Shape Atoms bonded to central atom Lone pairs Tetrahedral 4 Trigonal- pyramidal 3 1

Polarity of Molecules The shape of the molecule tends to make the molecule stable. It is a polar molecule if there are areas of partial charge. A molecule that has an area of difference has 2 poles and is called polar.

Polar Molecule vs. Nonpolar Molecule One very important property of molecule is whether it is polar or nonpolar. If the electrons in a molecule are not evenly distributed, the molecule can have a negative and positive side. A polar molecule will dissolve in another polar substance (such as water). Nonpolar molecule will dissolve in another nonpolar substance (such as carbon tetrachloride).

Linear molecules can be both polar and nonpolar. Examples:

Bent or angular molecules (example:water) are polar.

Pyramidal molecules are polar. Example:

Tetrahedral molecules can be both nonpolar and polar. Examples:

Polar vs. Nonpolar Rules If the central atom has no lone pairs and has all the same types of atoms attached to it, then the molecule is nonpolar. If the central atom has no lone pairs but different atoms attached to it, the molecule is polar. If the central atom has lone pairs, the molecule is polar.

The Properties of Molecular (Covalent) Compounds

Intermolecular Forces Reviewing what we know Gases Solids Low density Highly compressible Fill container High density Slightly compressible Rigid (keeps its shape)

Intermolecular forces – occur between molecules Intramolecular forces – occur inside the molecules

Dipole – dipole attraction A dipole is created by an uneven charge distribution (electronegativity) Dipole-Dipole is an electrostatic attraction between polar molecules.

Hydrogen Bonding Occurs between H and highly electronegative atom (for example N, O, F)

Hydrogen Bonding Affects physical properties Boiling point

London Dispersion Forces Attraction of instantaneous and induced dipoles; exist between all molecules. Formation of instantaneous dipoles

London Dispersion Forces Nonpolar molecules

London Dispersion Forces Become stronger as the sizes of atoms or molecules increase