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
   • All diatomic molecules are linear.
   • 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.
   • Generic Formula:  MX or MX₂ (where M is the central atom and X is are the bonding atoms).

     

2. TRIGONAL PLANAR - Bond angle = 120
   • 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.
   • Generic Formula:  MX₃ (where M is the central atom and X is are the bonding atoms).

     

3. TETRAHEDRAL - Bond angle = 109.5
   • Molecules with four atoms around a central atom such as CH₄ are tetrahedral because electron repulsion is minimized by position the four attachments toward the corners of a tetrahedron.
   • Generic Formula:  MX₄ (where M is the central atom and X is are the bonding atoms).

     

4. TRIGONAL BIPYRAMIDAL
   • Bond angle within the equatorial plane = 120
   • Bond angle between equatorial and axial plane = 90
   • 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.
   • Generic Formula:  MX₅ (where M is the central atom and X is are the bonding atoms).

     

5. OCTAHEDRAL - Bond angle = 90
   • 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.
   • 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)
   • Molecules with two atoms and one or two pairs of nonbonding electrons around a central atom such as H₂O 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.
   • 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.
   • 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
   • Molecules with three atoms and one pair of nonbonding electrons around a central atom such as NH₃ 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.
   • 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)
   • Molecules with four atoms and one pair of nonbonding electrons around a central atom such as SF₄ are seesaw-shaped.  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.
   • 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
   • Molecules with three atoms and two pairs of nonbonding electrons around a central atom such as ClF₃ are T-shaped.  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 axial-positioned atoms and one of the equatorial-positioned atoms in a T-shape.
   • Generic Formula:  MX₃E₂ (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons).

     

5. LINEAR
   • 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.
   • 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
   • 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.
   • 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
   • 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.
   • Generic Formula:  MX₄E₂ (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons).

     

Check out awesome 3-D graphics and animations of molecular shapes

• Shapes of Molecules - Animated Molecules
  • Has chart of 3-D models of generic molecular shapes.
  • Click on a picture of a molecule to view an animated model.
  • Also, listed are more details about the molecular shape, including text descriptions of the arrangement of orbitals, hybridization, and a downloadable gif image of the molecular shape.
• Valence Shell Electron Pair Repulsion - Shapes of Molecules
  1. Chart One: Shapes of molecules with 2-4 places where electrons can be found
  2. Chart Two:  Shapes of molecules with 5-6 places where electrons can be found
     • Colorful charts of Lewis Structures compared to their respective VSEPR molecular shapes.
     • Downloadable to view rotating models of the molecular shapes with RASMOL.

Hybrid Atomic Orbitals

1931 - Linus Pauling

• Proposed that the outermost (valence) orbitals of an atom could be combined to form hybrid atomic orbitals.
  • Sigma bond () - The end-to-end overlapping of an s orbital with a p orbital to form a sp hybrid orbital.
  • 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.

Learn more about hybrid atomic orbitals and hybridization

• Chem 177:  Orbitals and Bonding
  • Includes chart of molecular shapes.
  • Valence Bond and Hybrid Orbitals notes are toward the bottom of the page.

Next:  "Intermolecular Forces of Attraction"