VSEPR theory is defined as the electron pairs surrounding the central atom must be arranged in space as for apart as possible to minimize the electrostatic repulsion between them.
A Central Atom……
A central atom is is any atom that is bonded to two or more than two other atoms. The first and the most important rule of the VSEPR theory is that the bond angles about a central atom are those that minimize the total repulsion between the Electron pairs in the atom’s valence shell.
While out the shapes of molecules from this theory, the following rules must b remembered;
01: A lone pair of electrons occupy more space than a bonding pair.there is because lone pair of electron is under the influence of only one nucleous of the central atom, they are expected to occupy more space with a greater electron density than the bond pair electrons which are under the influence of two nuclei.The decreasing order of repulsion is
Lone pair-Lone pair>Lone pair-Bond pair>Bond pair- bond pair
02: Repulsion forces decreases sharply with increasing interpair angle.They are strong at 90 degree much weak at 120 degree and very weak at 180 degree.
03: The influence of a bonding electron pair decreases with the increasing value of electronegativity of an atom forming a molecule.
04: Multiple bonds behave as a single electron pair for the purpose of VSEPR bond theory.
05: The two electron pair of a double bond occupy more space than one electron pair of a single bond.
06: The lone pair repels bond pair giving rise to some distortions in the molecular shape.
Application of VSEPR Theory:
Let us now apply the valance shell electron pair repulsion theory to predict the shapes of molecules.The first step is the VSEPR method for determining the geometry of the molecule to determine the number of electron pair around the central atom.
The second step is to determine the total number of electrons around the central atom.The third step is to determine the number and location of lone pairs.
The molecules can be classified to the number of electron pair around the central atom.
Gillespie proposed the following rules to explain the shapes of electron pair of inorganic molecules:
If the central atom of a molecule is surrounded only by bonding electron pairs and not by non bonding electron pair called lone pairs the geometry of the molecule will be the regular I.e.,
it will be the linear,triangular,planer,tetrahedral,trigonal,bipyramid and regular octahedral for 2,3,4,5 and 6 bonding electron pair.
When central atom in a molecule is surrounded by both lps the molecules does not have a regular shape.The alternation or distortion in shape due to the alternation in bond angles which arises due to the presence of lps on the central atom.
B-A-B bond angle decreases with the increase in electronegativity of atom B in AB molecule wherein A is the central atom. This is due to the fact that with the increase in electronegativity of atom B the average position of bonding electron-pair moves further from the central atom A. Hence the repulsion exerted by the bonding electron pair on the electron pair on atom A decreases.
Bonding angles involving multiple bond are generally longer than those involving only single bonds. However the multiple bonds do not effect the geometry of molecules.
Repulsion between electrons pairs in filled shells are longer than the repulsion between electron pairs In incompletely filled shells. E.g compare H-O-H and H-S-H bond angles in H2O and H2S Molecules H2O>>H2S.
The Valence shell electron pair repulsion (vsepr) model predicts the shapes of molecules and ions in which valence shell electron pair are arranged about atom. So that electron pair kept as for away from one another as possible thus minimizing electron pair repulsion.
Molecular Geometry is the general shape of a molecule, as determine by the relative positions of the atomic nuclei. For the general molecular formula A refers to atoms attached to central atom and E refers to unbounded electron pairs.
AB2; Molecules with two electron pairs around a central atom:
There are several molecules and and ions that consist of a central atom plus atom of another element. There are no lone pair on the central atoms. This kind of molecule is abbreviated as AB2. Typical compounds include BeCl2,BeBr2 and CO2, as well as CdX2 and HgX2 where X=Cl,Br or I.A polyatomic ion in NO2. All of these are known to be linear.
The VSEPR Theory predicts a linear structure for BeCl2,and for all other molecules of this type.
BeCl (Lewis structure) or Cl- Be -Cl
The Lewis structure for BeCl2 shows that the central atom Be has two bonding electron pairs.
AB3; Molecules With Three Electron Pairs Around A Central Atom:
When the central atom of a molecule is surrounded only by three bonding electron pairs. The geometry of the molecule will be triangular planer.
Boron forms many covalent compounds bonding to three other atoms. Typical examples include BF3,BCl3,BBr3Bl3.
All are trigonal planer.
Shows that the central atom B has three bonding electron pairs and has no lone pair of electrons.
VSEPR theory predicts a trigonal planer structure for BF3 molecules. This is because of the structure gives maximum sap-ration among the three bonding electron pairs.
The structure of BCl3,BBr3 and BI3 are similar.
Angle between them is 120 degree.
AB4; Molecules With Four Electron Pairs Around A Central Atoms:
When the central atom of a molecule is bonded to the four other atoms and has no lone pair of electron on it. The molecular geometry will be tetrahedral,e.g
The group IVA elements from numerous covalent compound by shearing their four,electrons with four other atoms. These are obbreviated as AB4 molecules.
The lewis structure for methone,CH4 are..
The central carbon atoms in both molecules have four bonding electron pairs. So, VSEPR theory predicts tetrahedral geometry Since this gives maximum separation. The four H atoms in CH4 and four F atom in CF4,lie at the corners of a regular tetrahedron. The H-C-H bond angles are all 109 degree and 28`.