Daniel L. RegerScott R. GoodeDavid W. Ball

http://academic.cengage.com/chemistry/reger

Chapter 10 Molecular Structure and

Bonding Theories

Valence-Shell Electron-Pair Repulsion Model (VSEPR) predicts shape from Lewis Structures.

• VSEPR Rule 1: A molecule has a shape that minimizes electrostatic repulsions between valence-shell electron pairs. • Minimum repulsion results when the electron

pairs are as far apart as possible.

VSEPR

• Steric number = (number of lone pairs on central atom) + (number of atoms bonded to central atom)• The steric number is determined from the

Lewis structure.• Steric number determines the bonded-

atom lone-pair arrangement, the shape that maximizes the distances between the valence-shell electron pairs.

Steric Number

Geometric Arrangements

Geometric Arrangements

• In the Lewis structure of BeCl2,

beryllium has two bonded atoms and no lone pairs, steric number = 2.

• A linear geometry places the two pairs of electrons on the central beryllium atom as far apart as possible.

Steric Number = 2

BeCl Cl

• The Lewis structure of HCN (H-CN:) shows that the carbon atom is bonded to two atoms and has no lone pairs, steric number = 2.

• The bonded-atom lone-pair arrangement is linear.

• The number of bonded atoms, not the number of bonds, determines the steric number.

Molecules with Multiple Bonds

• The Lewis structure of BF3

shows the boron atom has a steric number = 3; the bonded-atom lone-pair arrangement is trigonal planar.

Steric Number = 3

B FF

F

• The Lewis structure of CH4

shows the carbon

atom has a steric number = 4; the bonded-atom lone-pair arrangement is tetrahedral.

Steric Number = 4

C H

H

H

H

• The phosphorus atom in PF5 has a steric number = 5; the bonded-atom lone-pair arrangement is trigonal bipyramidal.

Steric Number = 5

• The sulfur atom in SF6 has a steric number = 6; the bonded-atom lone-pair arrangement is octahedral.

Steric Number = 6

• The Lewis structure of H2O is

• Steric number = 4, 2 bonded atoms and 2 lone pairs.

• The bonded-atom lone-pair arrangement is tetrahedral.

Central Atoms with Lone Pairs

OHH

• Molecular shape is the arrangement of the atoms in a species.

• The bonded-atom lone-pair arrangement of H2O is tetrahedral (top); the molecular shape is bent or V-shaped (bottom).

Molecular Shape of H2O

• What is the electron pair geometry and molecular shape of NH3?

Molecular Shape of NH3

• The measured bond angle in H2O (104.5o) is smaller than the predicted angle (109.5o)

• Explanation: repulsions vary lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair

Electron Pair Repulsions

• The favored structure for a trigonal bipyramid minimizes 90o lone pair interactions – the one on the right.

Two structuresare possible:

Location of Lone Pair in SF4

• Lone pairs always occupy the equatorial positions in a trigonal bipyramid so that lone pair-lone pair repulsions are oriented at 120o.

Lone Pairs in Trigonal Bipyramids

• The structure on right has no 90o lone pair-lone pair interactions and is favored.

Location of Lone Pairs in XeF4

• What is the steric number, the bonded-atom lone-pair arrangement, and the molecular shape of ClF3?

Test Your Skill

• The geometry of each central atom is determined separately.

• The CH3 carbon in CH3CN has tetrahedral geometry and the other carbon has linear geometry.

Multiple Central Atoms

• What are the bonded-atom lone-pair arrangements and the shapes about each central atom in NH2SH?

• Draw the Lewis structure.

• The bonded-atom lone-pair arrangements of both are tetrahedral, the nitrogen shape is trigonal pyramidal and sulfur is “V” shaped.

Shapes of Molecules

N SH

HH

• Ethylene, C2H4 , could be planar (left) or nonplanar (right). The VSEPR model does not predict which is preferred.

Overall Shape of C2H4

• The bond dipoles in CO2 cancel because the linear shape orients the equal magnitude bond dipoles in exactly opposite directions.

Polarity of Molecules

• The bond dipoles do not cancel in COSe; they are oriented in the same direction and are of unequal length. They do not cancel in OF2 because the V-shape of the molecule does not orient them in opposite directions.

Polarity of Molecules

• The bond dipoles in BCl3 and CCl4 cancel because of the regular shape and equal magnitude.

Polarity of Molecules

• The bond dipoles in BCl2F and CHCl3 do not cancel because they are not of the same magnitude.

Polarity of Molecules

• Are the following molecules polar or nonpolar: H2S, SiF4, CH2Cl2?

Test Your Skill

• Valence bond theory describes bonds as being formed by overlap of partially filled valence orbitals.

Valence Bond Theory

• Identify the orbitals that form the bond in HCl.

Test Your Skill

• The observed bond angles of 107.5o in NH3 are not consistent with the angles of

90o expected if the bonds formed from N 2p orbitals.

Bonding in NH3

• Hybrid orbitals are orbitals obtained by mixing two or more atomic orbitals on the same central atom.

• Appropriate hybrid orbitals formed by mixing one s and xp atomic orbitals make bonds at either 180o (x = 1), 120o (x = 2), or 109.5o (x = 3).

Hybrid Orbitals

Analogy for Hybrid Orbitals

sp Hybrid Orbitals

Shape of Hybrid Orbitals• For clarity, hybrid orbitals are pictured as

elongated with the small lobe omitted.

• The bonds in BeCl2 arise from the overlap of two sp hybrid orbitals on the beryllium atom with the 3p orbitals on the two chlorine atoms.

Bonding in BeCl2

sp2 Hybrid Orbitals

• The bonds in BF3 arise from the overlap of three sp2 hybrid orbitals on the boron atom with 2p orbitals on the three fluorine atoms.

Bonding in BF3

sp3 Hybrid Orbitals

• The bonds in CH4 arise from the overlap of four sp3 hybrid orbitals on the carbon atom with 1s orbitals on the four hydrogen atoms.

Bonding in CH4

• Hybrid orbitals can hold lone pairs as well as make bonds.

Lone Pairs and Hybrid Orbitals

• Hybrid orbitals of central atoms with steric numbers of 5 or 6 involve d orbitals.

Hybridization with d Orbitals

Steric Number

Electron pair geometry

Hybridization

2 linear sp 3 trigonal planar sp2 4 tetrahedral sp3 5 trigonal

bipyramid sp3d

6 octahedral sp3d2

Hybrid Orbitals

Test Your Skill• Identify the hybrid orbitals on the central

atoms in SiH4 and HCN.

• Sigma bonds (): the shared pair of electrons is symmetric about the line joining the two nuclei of the bonded atoms.

Types of Bonds: Sigma Bonds

• The C-C sigma bond in C2H4 arises from overlap of sp2 hybrid orbitals and the four C-H sigma bonds from overlap sp2 hybrid orbitals on C with 1s orbitals on H.

• The second C-C bond forms from sideways overlap of p orbitals.

Bonding in C2H4

• Pi bonds () places electron density above and below the line joining the bonded atoms – they form by sideways overlap of p orbitals.

Types of Bonds: Pi Bonds

• The double bond in C2H4 is one sigma bond and one pi bond – each bond is of similar strength.

Bonding in C2H4

• C2H4 is planar (A) because pi overlap is at a maximum. Rotation of one end by 90o (B) reduces pi overlap to zero.

Proof of Pi Bonds: Shape of C2H4

• The triple bond in C2H2 is one sigma bond and two pi bonds between the sp hybridized carbon atoms.

Triple Bonds

• Each carbon atom in benzene, C6H6, forms three sigma bonds with sp2 hybrid orbitals.

Sigma Bonds in Benzene

• The remaining p orbital on each carbon atom (top) overlap to form three pi bonds.

Pi Bonds in Benzene

Test Your Skill• Describe the bonds made by the

carbon atom in HCN.

• Molecular orbital theory is a model that combines atomic orbitals to form new molecular orbitals that are shared over the entire molecule.

• A bonding molecular orbital concentrates electron density between atoms in a molecule.

• An antibonding molecular orbital reduces electron density between atoms in a molecule.

Molecular Orbital Theory

• Addition of the 1s orbitals of two H atoms forms a sigma bonding molecular orbital and subtraction forms a sigma antibonding molecular orbital, indicated with a * symbol.

Hydrogen Molecule

• Bonding molecular orbitals are more stable and antibonding molecular orbitals are less stable than the atomic orbital that are combined.

Molecular Orbital Diagram: H2

• Bond order = 1/2 [number of electrons in bonding orbital - number of electrons in antibonding orbitals]

• Bond order in H2 = 1/2 [2 - 0] = 1

Bond OrderBond Order

• Bond order in He2 = 1/2 [2 - 2] = 0; the molecule does not form.

Molecular Orbital Diagram: He2

Sigma Molecular Orbitals from p

Pi Molecular Orbitals from p

MO Diagram Second-Period Diatomics

• The electron configuration is(2s)2(*2s)2(2p)4(2p)2.

• The bond order in N2 is three and there are no unpaired electrons.

• Lewis theory (:NN:) predicts the same result.

Molecular Orbital Diagram: N2

• The electron configuration is (2s)2(*2s)2 .

• Bond order in Be2 is zero and the molecule does not exist.

Molecular Orbital Diagram: Be2

• Draw the molecular orbital diagram of O2. What is the electron configuration, the bond order and how many unpaired electrons are present?

Molecular Orbital Diagram for O2

• Draw the molecular orbital diagram of B2. What is the electron configuration, the bond order and number of unpaired electrons?

Test Your Skill