Covalent Bonding Sec. 8.4: Molecular shape

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

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

Hybridization For example, think about the carbon atom. Its 4 valence electrons are located in the s and p orbitals. In reality, the s and p orbitals hybridize. All 4 valence electrons, instead of being located in separate s and p orbitals, are found in 4 orbitals that are identical in shape to each other. These are called hybrid orbitals.

Molecular Shape The number and size of the hybrid orbitals (which overlap with those of another atom to form a bond) determines the location of atoms relative to each other in a compound. The location of the atoms in space, called molecular geometry or molecular shape, determines many physical and chemical properties of compounds.

VSEPR Model Many theories have been developed to explain and predict the shape of molecules. The Valence Shell Electron Pair Repulsion model uses Lewis structures to determine molecular geometry (shape).

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

VSEPR Model Orbitals containing shared pairs (covalent bonds) repel one another. Orbitals with lone pairs also repel one another 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) Consider an analogy ...

VSEPR Model 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. Table 6 (pg. 263) summarizes molecular shapes and angles predicted by the VSEPR theory.

Molecular shape: Linear The central atom has no lone pairs present. The central atom has 2 bonding pairs Maximum separation is attained at a bond angle of 1800. Ex. BeCl2

Molecular shape: Linear Double and triple bonds have a sigma and 2 pi bonds. Only the sigma bonds occupy hybrid orbitals. Thus, a linear configuration is seen between atoms with double & triple bonds.

Trigonal Planar There are no lone pairs present on central atom. There are 3 bonding pairs present. Maximum separation is attained at bond angles of 1200 Ex. AlCl3

Tetrahedral No lone pairs are present on the central atom. 4 bonding pairs are present. Maximum separation is attained at bond angles of 109.50 Ex. CH4

Trigonal Pyramidal 1 lone pair is present on the central atom. 3 bonding pairs are present A lone pair takes up more space than a bonding pair. The bonding pairs are pushed closer together as a result. Bond angles are 107.30

Bent 2 lone pairs are present on the central atom. 2 bonding pairs are also present. Lone pairs take up more space than bonding pairs. Bond angles are 104.50

Trigonal Bipyramidal No lone pairs are present 5 bonding pairs are present Bond angles are 900 vertical to horizontal Bond angles are 1200 horizontal to horizontal

Octahedral No lone pairs are present 6 bonding pairs are present Bond angles are 900

More on hybridization . . . Recall that carbon has 4 valence electrons: [He] 2s2 2p2 The electrons in the s orbital unpair: [He] 2s1 2p3 Therefore, the hybrid orbitals that are formed in bonding come from the 1 “s” and 3 “p” orbitals. The 4 identical hybrid orbitals formed are called sp3. CH4 (Methane)

Hybridization Notice that the number of atomic orbitals that hybridize equals the total number of bonding pairs of electrons CH4 has 4 hybrid orbitals for the 4 bonding pairs, formed from a mix of 1 “s” and 3 “p” orbitals

Hybridization Lone pairs also occupy hybrid orbitals. H2O has 4 electron pairs available for bonding - 2 lone pairs and 2 shared pairs 4 hybrid orbitals are formed - all sp3 Review hybridizations given on pg. 263

Practice Problems Determine the shape, bond angles, and hybridization of PH3 BF3 NH4+ OCl2 KrF2