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 o o
All diatomic molecules are linear.
Molecules with two atoms around a central atom such as BeH
2
are linear because positioning the two attachments at opposite ends of the central atom minimizes electron repulsion.
Generic Formula: MX or MX
2
(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 BF
3
are trigonal planar because electron repulsion is minimized by positioning the three attachments toward the corners of an equilateral triangle.
o Generic Formula: MX
3
(where M is the central atom and X is are the bonding atoms).
3.
TETRAHEDRAL - Bond angle = 109.5 o 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.
Generic Formula: MX
4
(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 PF
5
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: MX
5
(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 SF
6
are octahedral. Four of the attachments are positioned in a square plane with 90 bond angles. The remaining two attachments are positioned perpendicular) to the square plane at opposite ends of the central atom. This arrangement of atoms minimizes repulsion.
o Generic Formula: MX
6
(where M is the central atom and X is are the bonding atoms).
There are several 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
2
O 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 o o 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.
Some molecules, such as SO
2
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.
Generic Formula: MX
2
E or MXE
2
(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
o attached atoms less than 109.5 For example, in NH
3
, the H-N-H bond is 107.5
Generic Formula: MX
3
E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons).
