Covalent Bonding Sec. 8.4: Molecular shape

Objectives n Discuss the VSEPR bonding theory n Predict the shape of and the bond angles in a molecule n Define hybridization

VSEPR Model n Molecular shape determines many physical and chemical properties of compounds n alence Shell lectron air epulsion model is used to determine molecular geometry (shape). n The Valence Shell Electron Pair Repulsion model is used to determine molecular geometry (shape).

VSEPR Model n Molecular shape is determined by the overlap of orbitals that are sharing electrons n Atoms/orbitals assume a shape that minimizes the repulsion of shared and unshared pairs of electrons around the central atom.

VSEPR Model n Shared pairs (covalent bonds) repel one another n Lone pairs also repel one another n In addition, lone pairs repel shared pairs and push shared pairs closer to each other (because of the relatively large orbitals of the lone pairs) n Consider an analogy...

VSEPR Model n Repulsions of electron pairs for each other results in atoms being at fixed angles to each other. –The angle formed by 2 terminal atoms and the central atom is a bond angle. n Table 6 (pg. 263) summarizes molecular shapes and angles predicted by the VSEPR theory.

Molecular shape: Linear No lone pairs present on central atom. 2 bonding pairs present Maximum separation is attained at a bond angle of Ex. BeCl 2

Molecular shape: Linear The shape for a central atom with double or triple bonds is linear. These types of bonds are rigid and hold the atoms involved in a linear configuration.

Trigonal Planar n No lone pairs present on central atom. n 3 bonding pairs present. –Maximum separation is attained at bond angles of Ex. AlCl 3

Tetrahedral n No lone pairs present on central atom. n 4 bonding pairs present. –Maximum separation is attained at bond angles of Ex. CH 4

Trigonal Pyramidal n 1 lone pair present on central atom n 3 bonding pairs present –Lone pair takes up more space than a bonding pair –Bonding pairs are pushed closer together –Bond angles are

Bent n 2 lone pairs present n 2 bonding pairs present –Lone pairs take up more space than bonding pairs –Bond angles are

Trigonal Bipyramidal n No lone pairs present n 5 bonding pairs present –Bond angles are 90 0 vertical to horizontal –Bond angles are horizontal to horizontal

Octahedral n No Lone pairs present n 6 bonding pairs present –Bond angles are 90 0

Hybridization n When 2 of the same type of object combine, a hybrid results that has characteristics of both objects. n During bonding, orbitals undergo hybridization. –Hybridization is a process in which atomic orbitals are mixed to form new, identical hybrid orbitals.

CH 4 (methane) n Carbon has 4 valence electrons: [He] 2s 2 2p 2 n The electrons in the s orbital unpair: [He] 2s 1 2p 3 n Hybrid orbitals - formed in bonding from the 1 “s” and 3 “p” orbitals - are called sp 3

Hybridization n Like the carbon in methane, hybridization of orbitals occurs in all covalent compounds. n Depending on exactly which orbitals hybridize, the “name” of the hybridization can vary. Recall it is called “sp 3 ” for methane. n The 5 th column of the table on pg. 263 gives the names of the hybridizations associated with each shape.

Practice Problems n Determine the shape, bond angles, and hybridization of –PH 3 –BF 3 –NH 4 + –OCl 2 –KrF 2