Bonding : Predicting Molecular Shapes - VSEPR
To predict the shapes of molecules, the valence-shell electron-pair repulsion (VSEPR) theory is used.
Valence-shell electron-pair repulsion (VSEPR) theory

Assumes that each atom in a molecule will be positioned so that there is minimal repulsion between the valence
electrons of that atom.
In simple molecules in which there are no nonbonding electrons, there are five basic shapes:
1.
LINEAR - Bond angle = 180
o All diatomic molecules are linear.
o Molecules with two atoms around a central atom such as BF2 are linear because positioning the two attachments
at opposite ends of the central atom minimizes electron repulsion.
o Generic Formula: MX or MX2 (where M is the central atom and X is are the bonding atoms).
2.
TRIGONAL PLANAR - Bond angle = 120
o Molecules with three atoms around a central atom such as BF3 are trigonal planar because electron repulsion is
minimized by positioning the three attachments toward the corners of an equilateral triangle.
o Generic Formula: MX3 (where M is the central atom and X is are the bonding atoms).
3.
TETRAHEDRAL - Bond angle = 109.5
o Molecules with four atoms around a central atom such as CH 4 are tetrahedral because electron repulsion is
minimized by position the four attachments toward the corners of a tetrahedron.
o Generic Formula: MX4 (where M is the central atom and X is are the bonding atoms).
4.
TRIGONAL BIPYRAMIDAL
o Bond angle within the equatorial plane = 120
o Bond angle between equatorial and axial plane = 90
o Molecules with five atoms around a central atom such as PF5 are trigonal bipyramidal. Three of the
attachments are positioned in a trigonal plane with 120 bond angles. The remaining two attachments are
positioned perpendicular (90 ) to the trigonal plane at opposite ends of the central atom. This arrangement of
atoms minimizes electron repulsion.
o Generic Formula: MX5 (where M is the central atom and X is are the bonding atoms).
5.
OCTAHEDRAL - Bond angle = 90
o Molecules with six atoms around a central atom such as SF6 are octahedral. Four of the attachments are
positioned in a square plane with 90 bond angles. The remaining two attachments are positioned perpendicular
(90 ) to the square plane at opposite ends of the central atom. This arrangement of atoms minimizes repulsion.
o Generic Formula: MX6 (where M is the central atom and X is are the bonding atoms).
There are seven shapes for molecules with one or more pairs of nonbonding electrons.
1.
BENT (ANGULAR or V-SHAPED)
o Molecules with two atoms and one or two pairs of nonbonding electrons around a central atom such as H 2O are
bent. It can be imagined that a linear molecule with two atoms attached to a central atom is altered when
electrons are added to the top of the central atom. The repulsion caused by the addition of these extra electrons
causes the molecule to become bent. The angle of bent molecules is less than 120 if there is one pair of
nonbonding electrons and is less than 109.5 if there are two pairs of nonbonding electrons.
o Some molecules, such as NO2 have two atoms and a single unpaired electron around a central atom. These
molecules are also bent due to the repulsion of the single atom added to the central atom.
o Generic Formula: MX2E or MXE2 (where M is the central atom, X is are the bonding atoms, and E are
nonbonding pairs of electrons).
2.
TRIGONAL PYRAMIDAL
o Molecules with three atoms and one pair of nonbonding electrons around a central atom such as NH 3 are
trigonal pyramidal. These molecules are essentially tetrahedral molecules with one of the attached atoms
replaced by a pair of nonbonding electrons. The force of repulsion of these electrons makes the bond angle
between the attached atoms less than 109.5 . For example, in NH3, the H-N-H bond is 107.5 .
o Generic Formula: MX3E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs
of electrons).
3.
SEESAW-SHAPED (DISTORTED TETRAHEDRAL)
o Molecules with four atoms and one pair of nonbonding electrons around a central atom such as SF 4 are seesawshaped. These molecules are essentially trigonal bipyramidal molecules with one of the equatorial-positioned
atoms (in the trigonal plane) replaced by a pair of nonbonding electrons. This leaves the two axial-positioned
atoms and two of the equatorial-positioned atoms in the shape of a seesaw or a teeter-totter.
o Generic Formula: MX4E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs
of electrons).
4.
T-SHAPED
o Molecules with three atoms and two pairs of nonbonding electrons around a central atom such as ClF 3 are Tshaped. These molecules are essentially trigonal bipyramidal molecules with two of the equatorial-positioned
atoms (in the trigonal plane) each replaced by a pair of nonbonding electrons. This leaves the two axialpositioned atoms and one of the equatorial-positioned atoms in a T-shape.
o Generic Formula: MX3E2 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs
of electrons).
5. LINEAR
o Molecules with two atoms and three pairs of nonbonding electrons around a central atom
such as XeF2 are linear. These molecules are essential trigonal bipyramidal molecules with
all three of the equatorial-positioned atoms (in the trigonal plane) each replaced by a pair of
nonbonding electrons. This leaves only the two axial-positioned atoms which are still 180
from each other on opposite ends of the central atom.
o Generic Formula: MX2E3 (where M is the central atom, X is are the bonding atoms, and E
are nonbonding pairs of electrons).
6. SQUARE PYRAMIDAL
o Molecules with five atoms and one pair of nonbonding electrons around a central atom such
as BrF5 are square pyramidal. These molecules are essentially octahedral molecules with one
of the attached atoms replaced by a pair of nonbonding electrons. This leaves four atoms in a
plane as a square base and one atom positioned perpendicular (90 ) to this plane.
o Generic Formula: MX5E (where M is the central atom, X is are the bonding atoms, and E are
nonbonding pairs of electrons).
7. SQUARE PLANAR
o Molecules with four atoms and two pairs of nonbonding electrons around a central atom such
as XeF4 are square planar. These molecules are essentially octahedral molecules with two of
the attached atoms opposite each other around the central atom each replaced by a pair of
nonbonding electrons. This leaves four atoms in a square plane.
o Generic Formula: MX4E2 (where M is the central atom, X is are the bonding atoms, and E
are nonbonding pairs of electrons).
Hybrid Atomic Orbitals
1931 - Linus Pauling

Proposed that the outermost (valence) orbitals of an atom could be combined to form hybrid atomic
orbitals.
o Sigma bond ( ) - The end-to-end overlapping of an s orbital with a p orbital to form a sp
hybrid orbital.
o Pi bond ( ) - The side-to-side overlapping of two p orbitals.



Single bonds are made up of one sigma bond.
Double bonds are made up of one sigma bond and one pi bond.
Triple bonds are made up of one sigma bond and two pi bonds.
Hybridization - A mixture of two or more atomic orbitals.
Generic
Formula
Number of Places
Where Electrons
are Found
Molecular Shape
Hybridization
MX
1
linear
---
MX2
2
linear
sp
MX3
3
trigonal planar
sp2
MX2E
3
bent (angular, v-shaped)
sp2
MX4
4
tetrahedral
sp3
MX3E
4
trigonal pyramidal
sp3
MX2E2
4
bent (angular, v-shaped)
sp3
MX5
5
trigonal bipyramidal
sp3d
MX4E
5
seesaw (distorted tetrahedral)
sp3d
MX3E2
5
T-shaped
sp3d
MX2E3
5
linear
sp3d
MX6
6
octahedral
sp3d2
MX5E
6
square pyramidal
sp3d2
MX4E
6
square planar
sp3d2