Molecular Geometry and Bonding Theories

Hybrid Orbitals


Hybrid Orbitals

For polyatomic molecules we would like to be able to explain:

sp Hybrid Orbitals

Consider the Lewis structure of gaseous molecules of BeF2:

The fluorine atom electron configuration:

The Be atom electron configuration:

This would actually result in two unpaired electrons, one in a 2s orbital and another in a 2p orbital

However, the structure of BeF2 is linear and the bond lengths are identical

Note:

sp2 and sp3 Hybrid Orbitals

Whenever orbitals are mixed (hybridized):

BF3

Boron electron configuration:

NOTE: sp2 refers to a hybrid orbital being constructed from one s orbital and two p orbitals. Although it looks like an electron configuration notation, the superscript '2' DOES NOT refer to the number of electrons in an orbital.

CH4

NOTE: sp3 refers to a hybrid orbital being constructed from one s orbital and three p orbitals. Although it looks like an electron configuration notation, the superscript '3' DOES NOT refer to the number of electrons in an orbital.

ANOTHER NOTE: the two steps often observed when constructing hybrid orbitals is to 1) promote a valence electron from the ground state configuration to a higher energy orbital, and then 2) hybridize the appropriate valence electron orbitals to achieve the desired valence electron geometry (i.e. the correct number of hybrid orbitals for the appropriate valence electron geometry)

H2O

Oxygen

Hybridization Involving d Orbitals

Atoms in the third period and higher can utilize d orbitals to form hybrid orbitals

PF5

Similarly hybridizing one s, three p and two d orbitals yields six identical hybrid sp3d2 orbitals. These would be oriented in an octahedral geometry.

Steps in predicting the hybrid orbitals used by an atom in bonding:

1. Draw the Lewis structure

2. Determine the electron pair geometry using the VSEPR model

3. Specify the hybrid orbitals needed to accommodate the electron pairs in the geometric arrangement

NH3

1. Lewis structure

2. VSEPR indicates tetrahedral geometry with one non-bonding pair of electrons (structure itself will be trigonal pyramidal)

3. Tetrahedral arrangement indicates four equivalent electron orbitals

Valence Electron Pair Geometry

Number of Orbitals

Hybrid Orbitals

Linear

2

sp

Trigonal Planar

3

sp2

Tetrahedral

4

sp3

Trigonal Bipyramidal

5

sp3d

Octahedral

6

sp3d2