In the last two posts we discussed the VBT at length. We shall discuss its drawbacks and the hybridization theory in the next post.However, before proceeding to the hybridization theory , let us study the VSPER model first , which will help us understand the shape of the molecules explicitly.
Valence shell electron pair repulsion model (VSEPR)
VSEPR model ((pronounced “vesper“) is a way proposed to explain shape of the molecules.According to this model, the valence electron pairs surrounding the atoms will orient themselves in such a way as to minimize electron-electron repulsion between them. Thus, it is these valence electron pairs that determines the geometry of the molecule.VSEPR is not a theory as such , it is just a way of predicting the shape of molecules.
Before we proceed any further , we must know the difference between bonding pair of electrons and non- bonding / Lone pair of electrons.
Lone pair of electrons ⇒ electrons that are present on a single atom , which are not shared with any other atom.
Bonding pair of electrons ⇒ electrons that are shared between the two atoms forming the bond.
e.g.- In methane (CH4) molecule, all electrons are bonding pair of electrons as they are shared between the two atoms , namely – Carbon (C) and Hydrogen (H).Whereas, in ammonia (NH3) molecule, there are two electrons which are not shared and they reside on nitrogen atom only.(We shall discuss why these lone pair of electrons are not shared, in a later post, when we study hybridization of orbitals).
Electron domain – The region the electron pairs occupy in space. In VSEPR model, we talk about the electron domains and their effect on the geometry of molecules.
The AXE method.
The AXE method is used to count the # of electron pairs on the central atom, in a molecule.
A ⇒ Central atom.
X ⇒ Bonding atom.
E ⇒ # of lone pairs.
STERIC NUMBER (SN) = no.of atoms bonded to the central atom + no. of lone pair of electrons.
∴SN = X+ E
NOTE –In VSEPR, while calculating SN , double and triple bonds are also treated as single bonds, which means that for a double/triple bond , the no.of bonding pair of electrons is just one.
e.g. – In, CO2 molecule , SN = 2 + 0 = 2.
How is the geometry determined using this model ?
- We begin by drawing Lewis structures for molecules( Refer to post no 53 to know more about Lewis structures).
- We draw all the electron pairs and lone pairs around the central atom (this is usually the least electronegative atom).Each lone pair and each electron pair is one single group. Each bond counts as a single group even if it is a double or triple bond.
e.g. –Methane, CH4 Lewis structure: 
Central atom: Carbon Valence electrons on central atom: 4 4H contribute 1 electron each: 4 Total: 8 Divide by 2 to give electron pairs 4 VSEPR geometry(AX4E0) Tetrahedral. Carbon di oxide, CO2 Lewis structure: 
Central atom: Carbon Valence electrons on central atom: 4 2O contribute 6 electron each: 12 Total: 16 Divide by 2 to give electron pairs 8 (4 lone pairs which are NOT on central atom, so they wont be considered and two double bonda are considered as a single bonds.)
8 – 4 – 2 = 2VSEPR geometry(AX2E0) Linear - According to this model , electron pairs occupy positions in space such that there is minimal electrostatic repulsion between them. The geometries are given in the following table –
|
Electron groups (# Bond pair) |
Geometry |
Bond angle |
Figure |
Examples |
Structure |
|
2 (AX2E0)
|
Linear |
1800 |
 |
CO2,BeCl2,BeH2 |
|
|
3 (AX3E0)
|
Triagonal planar |
1200 |
 |
BCl3 |
 |
|
4 (AX4E0) |
Tetrahedral |
109.50 |
 |
CH4 |
 |
|
5 (AX5E0) |
Trigonal bipyramidal |
900,1200 |
 |
PCl5 |
 |
|
6 (AX6E0) |
Octahedral |
900 |
 |
SF6 |
 |
All above examples have no lone pair of electrons and so its E0.
Why do these shapes get the respective names?
- Linear means in a line – when molecules have all atoms in a straight line, the geometry is termed linear.
- Trigonal planar – when three atoms occupy three vertices of a triangle(trigonal) and they all lie in one plane, the geometry is trigonal planar.
3.Tetrahedral – When atoms occupy the four corners of a tetrahedron , the geometry is tetrahedral.
- Trigonal bipyramidal – In this geometry, the atoms form two pyramidal structures(on top and bottom to the plane containing three atoms) and the three atoms (all in one plane) lie at the vertices of a triangle.
5.Octahedral – In this geometry, atoms occupy six vertices of an octahedron.
These were the geometries of molecules with no lone pair electrons.In our next post we shall study what happens with introduction of one or more lone pairs of electrons in a molecule. Till then,
Be a perpetual student of life and keep learning…
Good day !
Image source –
2)https://chem.libretexts.org/Textbook_Maps/General_Chemistry_Textbook_Maps/Map%3A_Chemistry%3A_The_Central_Science_(Brown_et_al.)/09._Molecular_Geometry_and_Bonding_Theories/9.2%3A_The_VSEPR_Model
3)https://www.google.co.in/search?biw=1280&bih=648&tbm=isch&sa=1&ei=Tmp4WuzlNYnxvgSUjp24Cg&q=tetrahedron+&oq=tetrahedron+&gs_l=psy-ab.3..0l10.154534484.154535008.0.154535800.3.2.0.1.1.0.240.430.0j1j1.2.0….0…1c.1.64.psy-ab..0.3.447…0i67k1.0.5wb5twmvRm4#imgrc=A0mqO1cd5ncZGM:
4)https://www.google.co.in/search?biw=1280&bih=648&tbm=isch&sa=1&ei=J1l9WoObL8X6vATww6bABg&q=octahedron+pink+color&oq=octahedron+pink+color&gs_l=psy-ab.3…0.0.1.5467.0.0.0.0.0.0.0.0..0.0….0…1c..64.psy-ab..0.0.0….0.0wlGGSTHU_0#imgdii=qlSQEyXBQhlcnM:&imgrc=lcTFD9aP3gNZRM:
5)https://www.google.co.in/search?biw=1280&bih=648&tbm=isch&sa=1&ei=J1l9WoObL8X6vATww6bABg&q=octahedron+pink+color&oq=octahedron+pink+color&gs_l=psy-ab.3…0.0.1.5467.0.0.0.0.0.0.0.0..0.0….0…1c..64.psy-ab..0.0.0….0.0wlGGSTHU_0#imgrc=lcTFD9aP3gNZRM:
