57.CHEMICAL BONDING (4)- Covalent Bonding(3)- VESPR model(1).

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 (CH4molecule, 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).
583

 

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, COmolecule , SN = 2 + 0 = 2.
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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: Methane-2D-square
    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: 7a6b1fdf5d0a049d80d56c6f3e16a15a.jpg
    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 = 2
    VSEPR 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
(Note that the lone pairs are NOT on the central atom C)

1800

 800px-Linear-3D-balls.png

CO2,BeCl2,BeH2

 7a6b1fdf5d0a049d80d56c6f3e16a15a.jpg

3 (AX3E0)

Triagonal  planar

1200

 682px-Trigonal-3D-balls.png

BCl3

 f82a056091bdc902c2702bd274e4a8d4.jpg

4 (AX4E0)

Tetrahedral

109.50

 571px-Tetrahedral-3D-balls.png

CH4

 tetrahedral_carbon01.png

5 (AX5E0)

Trigonal bipyramidal

900,1200

 528px-Trigonal-bipyramidal-3D-balls.png

PCl5

3514e1ef95435477535a0695203c4770

6 (AX6E0)

Octahedral

900

574px-Octahedral-3D-balls.png

SF6

 968436a310a2bdcdea1f30097677fa14.jpg

All above examples have no lone pair of electrons and so its E0.

Why do these shapes get the respective names?

  1. Linear means in a line – when molecules have all atoms in a straight line, the geometry is termed linear.

22

  1. Trigonal planar – when three atoms occupy three vertices of a triangle(trigonal) and they all lie in one plane, the geometry is trigonal planar.
23.jpg
Atoms are at vertices of the triangle.

3.Tetrahedral – When atoms occupy the four corners of a tetrahedron , the geometry is tetrahedral.

  1. 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.
25.jpg
Trigonal bipyramidal geometry.

 

5.Octahedral  – In this geometry, atoms occupy six vertices of an octahedron.

 

26 copy

593px-Octahedron-4-3D-balls
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 –

1) https://www.google.co.in/search?q=tetrahedral+shape&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjemOmf_o7ZAhULpY8KHb-0B0gQ_AUICigB&biw=1280&bih=604#imgrc=Gu_GAORZqKORFM:

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:

 

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