Ch 11 - Molecular Shape and Bonding Theories

valence shell electron pair repulsion (VSEPR)

predicts the shape of a molecule based on the number of electrons in surrounding the central atom

VSEPR rules

1. regions of higher electron density are either bonding or LP which repel one another to minimize the electron-electron repulsion

2. multiple bonds are treated the same as single bonds

3. central atoms without LP: consider the positions of atoms when reporting molecule shape

4. central atom with LP: LP and atoms impact the final 3D molecular shape

linear shape (no LP)

AX₂, 180 degrees

trigonal planar shape (no LP)

AX₃, 120 degrees

tetrahedral shape (no LP)

AX₄, 109 degrees

trigonal bipyramidal (no LP)

AX₅, 90 degrees between the top and side, 120 degrees between the sides

octahedral

AX₆, 90 degrees all around 

an atom has 2 electron pairs and 1 LP…

bent or angular shape, slightly less than 120 degrees (trigonal planar)

an atom has 4 electron pairs and 1 LP….

trigonal pyramid with, slightly less than 109 degrees 

an atom has 4 electron pairs and 2 LP….

bent or angular with even less than 109 degrees

 an atom has 5 electron pairs and 1 LP….

sawhorse or seesaw, with slightly less than 90 degrees between the top and sides, and slightly less than 120 degrees between the sides

an atom has 5 electron pairs and 2 LP….

t shape, with slightly less than 90 degrees 

an atom has 5 electron pairs and 3 LP….

linear, with 180 degrees

an atom has 6 electron pairs and 1 LP….

square pyramid with slightly less than 90 degress

an atom has 6 electron pairs and 2 LP….

square planar, with 90 degress

an atom has 6 electron pairs and 3 LP….

T shape, with slightly less than 90 degrees

an atom has 6 electron pairs and 4 LP….

linear, with 180 degress

polar molecule

a compound that has a permanent dipole

valence (VB) theory

a covalent bond forms from an overlap in the valence orbitals of the atoms involved

sigma bond

the overlap in single bonds occurs around and along the internuclear axis

According to VB theory, what are double and triple bonds?

• double bonds: sigma bond and one pi-bond

• triple bonds: sigma bond + 2 pi-bonds

hybrid orbitals

mathematical combos of atomic orbitals that result in max overlap of orbitals in bonds, the number of hybrid orbitals = the number of atomic orbitals (AOs) being combones

What type of overlap do sigma bonds refer to?

head-to-head overlap as long as the orbital occurs along the internuclear axis

What happens when the two C atoms in ethene create a sigma bond?

• the unhybridized p orbitals also become close enough to overlap

• forms a second bone with both lobes of the p orbitals overlapping side to side (one above and below internuclear axis, pi bond)

2 electron pairs 

sp hybrid orbitals, linear shape

3 electron pairs

sp² hybrid orbitals, trigonal planar shape

4 electron pairs

sp³ hybrid orbitals, tetrahedral shape

5 electron pairs

sp³d hybrid orbitals, trigonal pyramidal shape

6 electron pairs

sp³d² hybrid orbitals, octahedral shape

steric shape

the number of electrons pairs a central atom has

bond order equation

(the number of bonding electrons - the number antibonding electrons)/2

antibonding has more or less energy?

more

bonding has more or less energy?

less