Vsepr theory chart

Introduction :

The two scientists R.S Nyholmm and R.J Gillespie proposed the VSPER theory in 1957. VSEPR theory helps us in explaining the repulsion caused between the atoms, bonds and lone electron pairs in a molecule. VSEPR theory was developed to predict the shapes of the molecule in which atoms are bonded, including the repulsion facts.

VSEPR Theory Definition:

Valence Shell Electron Pair Repulsion (VSEPR) theory is a phenomenon used in chemistry to predict the shapes of the individual molecules based on the repulsion acting between the electrons pairs in a molecule. The other name if VSPER theory is Gillespie-Nyholm theory, named after its two main developers. In this theory, it is explained that, valance electron pairs surrounding an atom mutually repel each other and hence they will arrange themselves in such a geometry, which minimizes their repulsion between electron pairs. The number of electron pairs around an atom, that is bonding and non-bonding are called steric number. The number of electron pairs in the valance shell of a central atom is concluded by drawing the Lewis structure of the molecule in which all lone electron pairs will be shown with bonds.
Diagram of VSEPR theory Chart

                                   

Postulates of VSEPR Theory:

The main postulates of VSEPR theory are:

• The total number of electron pairs (bonding and non-bonding) determines the shape of the molecule and on the orientation of the electron pairs around the central atom.
• To minimize the repulsion between the atoms, the electron pairs arrange themselves fare away from the central atom.
• The electron pairs around the central atom can be shared electron pairs or lone pairs. The shared electron pairs are known as Bond pairs.
• The strength of repulsions between different electron pairs is in the order:

Lone pair - Lone pair > Lone pair - Shared pair > Shared pair - Shared pair.

Prediction of shapes using VSEPR theory

 VSEPR theory could predict the shapes of a molecules correctly . The following examples prove this :
a) BeCl₂ has two single Be-Cl bonds and these two bond pairs of electrons on Be atom are oriented farthest in the opposite directions to have minimum repulsions between them . BeCl₂ is a linear molecule . Similarly CO₂ molecule has two carbon - oxygen double bonds . These bond pairs of electrons are oriented in opposite directions to have minimum repulsion . Hence , CO₂ is called linear molecule O = C = O .
b) In BCl₃ there are three B-Cl single bonds or there  are three bonded electron pairs around 'B' in the valence shell and they are oriented farthest apart to have minimum repulsion among them . Hence BCl₃  molecule has trigonal planar structure with `|__ClBCl` bond angle 120^o .
c) All the four electron pairs in methane (CH₄) are bond pairs only. Therefore , the molecule is tetrahedral with a bond angle of 109^o29' .
d) Nitrogen in NH₃ has four electron pairs .
e) In water molecule, the central atom , oxygen has two lone pairs and two bond pairs of electrons . The number of lone pairs on oxygen is grater than those on nitrogen in ammonia . Due to stronger lone pair - lone lone pair repulsions , the bond angle decreases to 104^o30'. Thus , the water molecule is 'V' shaped and not linear.

Conclusion on Vsepr Theory:

VSEPR theory proved very useful in predicting the shapes of all molecule sand it also helps in explaining the distortion of angles between the atoms. VSEPR theory also explains the idea why the shape of some molecule is different with the others.