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