Ch5 Bonding Theories - Explaining Molecular Geometry

Concepts: molecular geometry, isomers, dipoles, valance bond theory, hybridization, chirality, valence-shell electron-pair repulsion theory (VSEPR)

Valence-shell Electron-Pair Repulsion Theory (VSEPR)

electrons have negative charges & will repel each other; predicts arrangement of valence e- pairs around a central atom

Steric Number (SN) formula

SN = (# of atoms bonded to central atom) + (# of lone e- pairs on central atom)

To be a polar molecule…

asymmetrical molecular geometry, contain polar bonds

If SN = 2+0 = 2, what is the molecular geometry? Bond angle?

linear; 180°

If SN = 3+0 = 3, what is the molecular geometry? Bond angle?

trigonal planar; 120°

If SN = 2+1 = 3, what is the molecular geometry? Bond angle?

bent; <120°

If SN = 4+0 = 4, what is the molecular geometry? Bond angle?

tetrahedral; 109.5°

If SN = 3+1 = 4, what is the molecular geometry? Bond angle?

trigonal pyramidal; <109.5°

If SN = 2+2 = 4, what is the molecular geometry? Bond angle?

bent; <120°

If SN = 5+0 = 5, what is the molecular geometry? Bond angle(s)?

trigonal bipyramidal; 90°, 120°

If SN = 4+1 = 5, what is the molecular geometry? Bond angle(s)?

seesaw; <90°, <120°

If SN = 3+2 = 5, what is the molecular geometry? Bond angle?

t-shaped; <90°

If SN = 2+3 = 5, what is the molecular geometry? Bond angle?

linear; 180°

If SN = 6+0 = 6, what is the molecular geometry? Bond angle?

octahedral; 90°

If SN = 5+1 = 6, what is the molecular geometry? Bond angle?

square pyramidal; <90°

If SN = 4+2 = 4, what is the molecular geometry? Bond angle?

square planar; 90°

If SN = 3+3 = 6, what is the molecular geometry? Bond angle?

t-shaped; <90°

If SN = 2+4 = 6, what is the molecular geometry? Bond angle?

linear; 180°

Identify the molecular geometry & bond angle? (image)

linear; 180° (image)

Identify the molecular geometry & bond angle? (image)

bent; <120° (image)

Identify the molecular geometry & bond angle? (image)

trigonal planar; 120° (image)

Identify the molecular geometry & bond angle? (image)

trigonal pyramidal; <109.5° (image)

Identify the molecular geometry & bond angle? (image)

tetrahedral; 109.5° (image)

Identify the molecular geometry & bond angle? (image)

trigonal bipyramidal; 90°, 120° (image)

Identify the molecular geometry & bond angle? (image)

seesaw; <90°, <120° (image)

Identify the molecular geometry & bond angle? (image)

t-shaped; <90° (image)

Identify the molecular geometry & bond angle? (image)

octahedral; 90° (image)

Identify the molecular geometry & bond angle? (image)

square pyramidal; <90° (image)

Identify the molecular geometry & bond angle? (image)

square planar; 90° (image)

Dipole (Debye unit) equation

µ = q*r (separated charge*distance between center of charges)

Valence Bond Theory

a chemical bond forms when atomic orbitals overlap

sp3 hybrid orbital

4 hybrid orbitals, SN = 4, single bond

sp2 hybrid orbital

3 hybrid orbitals, SN = 3, double bond

sp hybrid orbital

2 hybrid orbitals, SN = 2, triple bond

Sigma Bond (σ)

covalent bond in which the highest e- density lies between the 2 atoms along the bond axis

Pi Bond (π)

covalent bond in which the highest e- density lies above & below the internuclear axis

Bond Conjugation

alternation of single & double bonds in a molecule

In an organic structure, what signals delocalized electrons?

when the electron pair of a double bond cannot be moved anywhere else & still make sense

In an organic structure, what signals localized electrons?

when the electron pair of a double bond can be moved anywhere else & still make sense

Rank NH3, CH4, and H2O in order of decreasing bond angles.

NH3 < CH4< H2O

Do either of these molecules have a permanent dipole?

CH2O (formaldehyde), CH2Cl2 (dichloromethane)

CH2O (formaldehyde): yes

CH2Cl2 (dichloromethane): yes

Which of the following halocarbons are polar & which are nonpolar?

CF2Cl2, CCl4, CH3Cl, CF3-CF3

polar: CF2Cl2, CH3Cl

nonpolar: CCl4, CF3-CF3

Determine the hybridization of carbon & oxygen in CO2.

SN = 2 —> sp-hybridization

Determine the hybridization of carbon in CH2Cl2.

SN = 4 —> sp3-hybridization

Determine the hybridization of carbon in H2C=O.

SN = 3 —> sp2-hybridizaton

Determine the hybridization of carbon in HC=-CH

SN = 2 —> sp-hybridization

Determine the hybridization of carbon in C=-O.

SN = 2 —> sp-hybridization

Identify the chiral center(s).

b

Identify the chiral center(s).

b

Identify the chiral center(s).

b, c

Identify the chiral center(s).

a

Identify the chiral center(s).

none

Identify the chiral center(s).

a, b

Structural Isomer

molecules w/ the same composition, different structure

Identify the type of isomer.

structural isomer

Stereo Isomer

atoms in molecules are joined the same way, arranged differently in space

Identify the type of isomer.

stereo isomer

Optical Isomer

molecules obtained by mirror-image of each other

Identify the type of isomer.

optical isomer

To be a chiral molecule…

contain at least one chiral center, asymmetrical