Covalent BondingCovalent Bonding

Sec. 8.4: Molecular shapeSec. 8.4: Molecular shape

ObjectivesObjectives

Define hybridizationDefine hybridization Discuss the VSEPR bonding theoryDiscuss the VSEPR bonding theory Predict the shape of and the bond Predict the shape of and the bond

angles in a moleculeangles in a molecule

HybridizationHybridization

When 2 of the same type of object When 2 of the same type of object combine, a combine, a hybridhybrid results that has results that has characteristics of both objects.characteristics of both objects.

During bonding, orbitals undergo During bonding, orbitals undergo hybridization.hybridization.– HybridizationHybridization is a process in which is a process in which

atomic orbitals are mixed to form atomic orbitals are mixed to form new, identical hybrid orbitals.new, identical hybrid orbitals.

HybridizationHybridization For example, think about the carbon For example, think about the carbon

atom. Its 4 valence electrons are atom. Its 4 valence electrons are located in the s and p orbitals.located in the s and p orbitals.

In reality, the s and p orbitals hybridize. In reality, the s and p orbitals hybridize. All 4 valence electrons, instead of being All 4 valence electrons, instead of being

located in separate s and p orbitals, are located in separate s and p orbitals, are found in 4 orbitals that are identical in found in 4 orbitals that are identical in shape to each other. These are called shape to each other. These are called hybrid orbitals.

Molecular ShapeMolecular Shape The number and size of the hybrid orbitals The number and size of the hybrid orbitals

(which overlap with those of another atom (which overlap with those of another atom to form a bond) determines the location of to form a bond) determines the location of atoms relative to each other in a atoms relative to each other in a compound. compound.

The location of the atoms in space, called The location of the atoms in space, called molecular geometry or molecular shape, molecular geometry or molecular shape, determines many physical and chemical determines many physical and chemical properties of compounds.properties of compounds.

VSEPR Model VSEPR Model

Many theories have been Many theories have been developed to explain and predict developed to explain and predict the shape of molecules.the shape of molecules.

The Valencealence SShell hell Electron lectron Pair air Repulsion model uses Lewis epulsion model uses Lewis structures to determine molecular structures to determine molecular geometry (shape).geometry (shape).

VSEPR ModelVSEPR Model Molecular shape is determined by Molecular shape is determined by

the overlap of hybrid orbitals that the overlap of hybrid orbitals that are sharing electrons.are sharing electrons.

Atoms/orbitals assume a shape Atoms/orbitals assume a shape that that minimizes the repulsionminimizes the repulsion of of shared and unshared pairs of shared and unshared pairs of electrons around the central electrons around the central atom.atom.

VSEPR ModelVSEPR Model Orbitals containing shared pairs Orbitals containing shared pairs

(covalent bonds) repel one another.(covalent bonds) repel one another. Orbitals with lone pairs also repel one Orbitals with lone pairs also repel one

another another In addition, lone pairs repel shared pairs In addition, lone pairs repel shared pairs

and push shared pairs closer to each and push shared pairs closer to each other (because of the relatively large other (because of the relatively large orbitals of the lone pairs) orbitals of the lone pairs)

Consider an analogy ...Consider an analogy ...

VSEPR ModelVSEPR Model Repulsions of electron pairs for Repulsions of electron pairs for

each other results in atoms being each other results in atoms being at fixed angles to each other.at fixed angles to each other.– The angle formed by 2 terminal atoms The angle formed by 2 terminal atoms

and the central atom is a and the central atom is a bond angle.bond angle. Table 6 (pg. 263) summarizes Table 6 (pg. 263) summarizes

molecular shapes and angles molecular shapes and angles predicted by the VSEPR theory.predicted by the VSEPR theory.

Molecular shape: LinearMolecular 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: LinearMolecular 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 PlanarTrigonal Planar

There are no lone There are no lone pairs present on pairs present on central atom.central atom.

There are 3 bonding There are 3 bonding pairs present.pairs present.

– Maximum separation Maximum separation is attained at bond is attained at bond angles of 120angles of 12000

Ex. AlCl3

TetrahedralTetrahedral No lone pairs No lone pairs

are present on are present on the central the central atom.atom.

4 bonding pairs 4 bonding pairs are presentare present..– Maximum Maximum

separation is separation is attained at bond attained at bond angles of 109.5angles of 109.500

Ex. CH4

Trigonal PyramidalTrigonal Pyramidal

1 lone pair is present 1 lone pair is present on the central atom.on the central atom.

3 bonding pairs are 3 bonding pairs are presentpresent– A lone pair takes up A lone pair takes up

more space than a more space than a bonding pair.bonding pair.

– The bonding pairs are The bonding pairs are pushed closer together pushed closer together as a result.as a result.

– Bond angles are 107.3Bond angles are 107.300

BentBent 2 lone pairs are 2 lone pairs are

present on the present on the central atom.central atom.

2 bonding pairs 2 bonding pairs are also present.are also present.– Lone pairs take up Lone pairs take up

more space than more space than bonding pairs.bonding pairs.

– Bond angles are Bond angles are 104.5104.500

Trigonal BipyramidalTrigonal Bipyramidal No lone pairs are No lone pairs are

presentpresent 5 bonding pairs 5 bonding pairs

are presentare present– Bond angles are Bond angles are

909000 vertical to vertical to horizontalhorizontal

– Bond angles are Bond angles are 12012000 horizontal to horizontal to horizontalhorizontal

OctahedralOctahedral No lone pairs are No lone pairs are

presentpresent 6 bonding pairs 6 bonding pairs

are presentare present– Bond angles are Bond angles are

909000

More on hybridization . . .More on hybridization . . .

Recall that carbon has 4 Recall that carbon has 4 valence electrons: [He] 2svalence electrons: [He] 2s22 2p2p22

The electrons in the s orbital The electrons in the s orbital unpair: [He] 2sunpair: [He] 2s11 2p 2p33

Therefore, the hybrid orbitals Therefore, the hybrid orbitals that are formed in bonding that are formed in bonding come from the 1 “s” and 3 come from the 1 “s” and 3 “p” orbitals.“p” orbitals.

The 4 identical hybrid orbitals The 4 identical hybrid orbitals formed are called formed are called spsp33..

CH4 (Methane)

HybridizationHybridization

Notice that the number of atomic Notice that the number of atomic orbitals that hybridize equals the orbitals that hybridize equals the total number of bonding pairs of total number of bonding pairs of electronselectrons– CHCH44 has 4 hybrid orbitals for the 4 has 4 hybrid orbitals for the 4

bonding pairs, formed from a mix of 1 bonding pairs, formed from a mix of 1 “s” and 3 “p” orbitals“s” and 3 “p” orbitals

HybridizationHybridization Lone pairs also occupy hybrid orbitals.Lone pairs also occupy hybrid orbitals. HH22O has 4 electron pairs available for O has 4 electron pairs available for

bonding - 2 lone pairs and 2 shared pairsbonding - 2 lone pairs and 2 shared pairs 4 hybrid orbitals are formed - all sp4 hybrid orbitals are formed - all sp33

Review hybridizations given on pg. 263

Practice ProblemsPractice Problems

Determine the shape, bond angles, Determine the shape, bond angles, and hybridization ofand hybridization of– PHPH33

– BFBF33

– NHNH44++

– OClOCl22– KrFKrF22