The Valence Shell Electron-Pair Repulsion (VSEPR) model that is based on the repulsive behavior of electron-pairs. This model
will show the three dimensional structure of simple molecular (covalent) compounds and polyatomic ions.
How to determine molecular geometry (molecular shape) using the table below:
1. Determine the number of valence electrons for all the atoms in a molecule.
2. Add all the electrons for the entire molecule. If the molecule/polyatomic ion has a negative charge, add the number of electrons
added. If it has a positive charge, subtract the electrons lost.
3. Determine the arrangement of the molecule, i.e., identify the central atom and the terminal atoms.
4. Distribute all the electrons (by pairs) around the atoms as follows:
First: between atoms
Second: around terminal atoms (outer atoms)
Third: around central atoms
5. Check on stability (8e- around) of each atom. Move electron when needed.
6. When stable electron dot structure is written, count the bonding pairs and non-bonding pairs around the central atom.
Note: a. bonding pair of electrons - those electrons shared by the central atom and any atom to which it is
bonded.
Double bonds and triple bonds are counted as 1.
b. non-bonding pairs of electrons - those pairs of electrons on an individual atom that are not shared with
another atom.
7. Determine the shape and angle using the table below.
Determine the shape and angle of the following molecules:
1. BCl3
2. HCN
3. CO2
4. BF3
5. SO3
6. NO2
7. NH3
8. CH4
9. PCl5
10. SF6
The table below summarizes the molecular and electron-pair geometries for different combinations of bonding groups and nonbonding
pairs of electrons on the central atom.
# of bonding pair/s of electron on
'central' atom
# of lone pair of electrons on
'central' atom
2
3
2
4
0
0
1
0
Electron-pair
Geometry
linear
trigonal planar
trigonal planar
tetrahedral
3
1
tetrahedral
2
2
tetrahedral
5
0
4
1
3
2
2
3
6
0
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
octahedral
5
1
octahedral
4
2
octahedral
Molecular
Bond
Geometry
Angle
linear
180o
trigonal planar
120 o
bent
less than 120 o
tetrahedral
109.5 o
trigonal
less than
pyramidal
109.5 o
less than
bent
109.5 o
trigonal
90 o, 120 o and
bipyramidal
180 o
90 o, 120 o and
seesaw
180 o
T-shaped
90 o and 180 o
linear
180 o
octrahedral
square
pyramidal
square planar
90 o and 180 o
90 o and 180 o
90 o and 180 o
Note: for bent molecular geometry when the electron-pair geometry is trigonal planar the bond angle is slightly less than 120
degrees, around 118 degrees. For trigonal pyramidal geometry the bond angle is slightly less than 109.5 degrees, around 107
degrees. For bent molecular geometry when the electron-pair geometry is tetrahedral the bond angle is around 105 degrees.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
http://intro.chem.okstate.edu/1314F00/Lecture/Chapter10/VSEPR.html
http://web.gccaz.edu/~ksmith8/VSEPR%20handout.pdf
http://www.google.com/search?hl=en&sugexp=les%3B&gs_rn=1&gs_ri=hp&cp=7&gs_id=u&xhr=t&q=vsepr+chart&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.41642
243,d.b2U&biw=1441&bih=654&wrapid=tljp1359506588469012&um=1&ie=UTF8&tbm=isch&source=og&sa=N&tab=wi&ei=iW0IUcfiLJD82gWUroDQBA#imgrc=3VmAuBlIqxrusM%3A%3B6UEnGnaST7D2eM%3Bhttp%253A%252F%252Fwww.
molecularmodelscompany.com%252FPortals%252F0%252Fimages%252FVSEPR%252520diagram%252FVSEPR-theorydiagram.jpg%3Bhttp%253A%252F%252Fwww.molecularmodelscompany.com%252FProducts%252FVSEPR%252FVSEPRtheorychart.aspx%3B688%3B491