VSEPR Theory – Valence Shell Electron Pair Repulsion Theory
- Pairs of electrons in a compound repel each other so as to be as far apart as
possible
- Unbonded (or lone) pairs repel more than bonded pairs
- Used to predict the geometric shapes of molecules
Let's take a moment now to elaborate on how atoms share electrons when they make a
covalent bond.
Chemists have the idea that a covalent bond occurs when
the electron orbitals of two different atoms overlap one
another so that electrons will be in both orbitals and thus in
both atoms.
Usually each of the overlapping orbitals starts with one electron so that they both
"have" two electrons when the overlap is complete and the bond is formed. This is
believed to be the mechanism by which the sharing of electrons between atoms occurs-orbitals overlap. The overlapping orbitals merge to form a bonding orbital. The
overlapping orbitals can be atomic orbitals or hybrid orbitals or one of each.
Just as you can alter your clothing (e.g. zip up a jacket and raise its hood, etc.) to
respond to different situations, so an atom can alter the arrangement of its electrons to
fit the conditions in which it finds itself. You know that electrons take up space and
that the space they take up is referred to as an orbital. There are many kinds of
orbitals. In the past we talked about s, p, d and f orbitals. These are all atomic orbitals.
They represent the ways that electrons can arrange themselves in isolated, individual
atoms. When atoms bond to one another, the electrons have to change their
arrangement in order to accommodate influence of other atoms. This is called
hybridization.
Let’s look at some specific examples to see how VSEPR theory and hybridization
explain the shapes of molecules:
BeF2: Beryllium is an exception to the octet rule – it only needs 4 electrons to be stable
Lewis Dot diagram: 16 v.e.

Be:
1s22s2
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In the ground state, there are no unpaired electrons (the Be atom is incapable of
forming a covalent bond with a fluorine atom
However, the Be atom could obtain an unpaired electron by promoting an
electron from the 2s orbital to the 2p orbital:
This would actually result in two unpaired electrons, one in a 2s orbital and another
in a 2p orbital


The Be atom can now form two covalent bonds with fluorine atoms
We would not expect these bonds to be identical (one is with a 2s electron
orbital, the other is with a 2p electron orbital)
However, the structure of BeF2 is linear and the bond lengths are identical


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We can combine wavefunctions for the 2s and 2p electrons to produce a "hybrid"
orbital for both electrons
This hybrid orbital is an "sp" hybrid orbital
(http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/hybrv18.swf)
The orbital diagram for this hybridization would be represented as:
Let’s fill in this type of molecular geometry on the grid that I gave you. This shape is
called linear. The pendant atoms are 180 apart. That is as far apart as their electrons
can get.
Now we will look at the other possible shapes and hybridizations. Here’s something to
keep in mind - whenever orbitals are mixed (hybridized):


The number of hybrid orbitals produced is equal to the sum of the orbitals
being hybridized
Each hybrid orbital is identical except that they are oriented in different
directions
BF3
Boron electron configuration: Boron will need 3 unpaired electrons to form3 bonds
with F.

The three sp2 hybrid orbitals have a trigonal planar arrangement to minimize
electron repulsion
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
Carbon electron configuration: Carbon will need 4 unpaired electrons to form4 bonds
with H.

Thus, using valence bond theory, we would describe the bonds in methane as
follows: each of the carbon sp3 hybrid orbitals can overlap with the 1s orbitals of
a hydrogen atom to form a bonding pair of electrons. This shape is called
tetrahedral.
H2O
Oxygen:
This shape is called bent.
NH3
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.