# Vsepr Theory

Topics: VSEPR theory, Atom, Molecule Pages: 10 (2501 words) Published: October 8, 2011
UNIT I

Geometry:
In VSEPRT, you will see two kinds of geometries Electron pair geometry: This geometry includes all the electron pairs (bp + lp) around the central atom. Molecular geometry: This includes only placement of atoms (bp) in the molecule. When there is no lone pair in molecules the two geometries are same.

PART C

VSEPR Theory
(Valence Shared Electron Pair Repulsion Theory)

Syllabus: Various rules under VSEPR theory to explain molecular geometry (following examples may be taken to explain various rules- BeCl2, BF3, CH4 , NH4+ , PCl5 , SF6, IF7, SnCl2 , NH3, H2O, SF4, ClF3, ICl2, ICl4, BrF5, XeF6, SOF4, COF2, PCl3, PBr3, PI3, F2O, H2S). Limitations of VSEPR theory.

How to draw VSEPR geometry?
To predict the shape of a covalent molecule, follow these steps: Step 1: Draw the Lewis structure of molecule. Make sure that you have drawn all the electron pairs around the central atom. Step 2: Count the number of electron pairs around the central atom. And determine the electron pair geometry using the following table. No. of electron pairs 2 3 4 5 Electron Pair Geometry Linear Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral Pentagonal bipyramidal Bond angles 180° 120° 109.28° 120° & 90° 90° 72° & 90°

Introduction:
Shape of a covalent molecule can be predicted using the Valence Shell Electron Pair Repulsion theory. VSEPR theory says that the electron pairs in a molecule will arrange themselves around the central atom of a molecule so that the repulsion between them is as small as possible. In other words, the electron pairs arrange themselves so that they are as far apart as they can be. Depending on the number of electron pairs in the molecule, it will have a different shape.

Postulates of VSEPR theory:
1. The shape of a molecule depends upon the number of electron pairs (bonded or nonbonded) in valence shell of the central atom. 2. Pairs of electrons in the valence shell repel one another. 3. These pairs of electrons tend to occupy such positions in space that minimize repulsion and thus maximize distance between them. 4. If the central atom has only bond pairs of electrons then the molecule has regular geometry. 5. If the central atom has both bond pair and lone pair of electrons then the molecule has distorted geometry. 6. More the number of lone pairs, greater is the distortion in geometry. 7. Bond angle decreases with increase in electronegativity of attached atoms. 8. Multiple bonds are counted as single bonds. 9. Repulsion between filled shells is greater than the repulsion between incompletely filled shells.

6 7

E.g. A molecule with two electron pairs has a linear shape. Step 3: Finally, identify the molecular geometry i.e. the arrangement of atoms in the molecule. For this consider only bond pairs of electrons.

Examples:
1. BeCl2
In BeCl2 molecule, central atom is Be, which is having the configuration 1s2, 2s1, 2px1 in excited state. It is able to form two bonds with the two chlorine atoms because of the presence of two unpaired electrons in valence shell. Thus we can write its Lewis structure as follows.

The repulsive interaction of electron pairs decrease in the order: Lone pair (lp) – Lone pair (lp) pair (bp)

In BeCl2, the central atom is surrounded by two atoms thus the molecule is having linear geometry. The angle between two Be-Cl bonds is 180°.

> Lone pair (lp) – Bond
> bp–bp

> Bond pair (bp) – Bond pair (bp)
> lp–bp

lp–lp

2. BF3
In BF3 molecule, central atom is B, which is having the configuration 1s2, 2s1, 2px1, 2py1 in excited state. It is able to form three bonds with the three fluorine atoms because of the presence of three unpaired electrons in valence shell. Thus we can write the Lewis structure as follows.

It is able to form four bonds because of the presence of four unpaired electrons in valence shell. Thus we can write the Lewis structure as follows.

In CH4, the central atom is surrounded by four electron pairs thus the...