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Chemical Bonding Part 6

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Chemical Bonding Part 6

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  • June 23, 2024
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  • 2023/2024
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Chemical Bonding Part 6
THE VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY
• Covalent molecules (except those which form macromolecules such as diamond and
silica) exhibit variety of shapes owing to the directional nature of the covalent bond.
• This property arises because pairs of electrons occupy orbitals which repel each other so
that they are oriented in space as far away as possible.
• The definite relative arrangement of the bonded atoms in a molecule is known as
geometry or shape of the molecule.
• Sidgwick and Powell (1904) proposed a simple theory based on the repulsive
interactions of the electron pairs on the valence shell of-the atoms.
• They suggested that the shape of molecules can be determined by the number of
us
electron pairs in the valence shell of the central atom and proposed å theory known as
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Valence Shell Electron Pair Repulsion (VSEPR) theory.
e
0) ac


The important postulates of VSEPR theory are:
45 i R




I. The shapes of molecules depend upon the number of electron pairs (bond as well as
non-bonded) in the valence shell of the central atom.
31 ar




II. The electron pairs Surrounding the central atom repel each other. As a result, the
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electron pairs try to stay as far apart as possible to acquire a state of minimum
06 h




repulsion and hence maximum stability.
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III. The repulsive interactions between two lone pairs (Ip) are different from those
between two bond pairs (bp) or between one lone pair and one bond pair. The
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repulsive interactions between various electron pairs arc in the order lp – lp > lp – bp
a




> bp – bp.
im




IV. After repulsion, the electron pairs achieve definite positions around the central atom
ah




giving a permanent geometry to the molecule.
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Molecules in which the central atom has only bond pairs in the valence shell will have
regular geometry. The presence of Ione pairs in addition to bond pairs in the valence shell of
the central atom causes distortion in the geometry of the molecule.

Shapes of Molecules Containing Bond Pairs Only
VSEPR theory helps to predict the geometry of molecules. It can be explained by considering
the following examples.

, i. BeF2
• In BeF2, the central atom Be (Z = 4 and E.C = 2, 2) has two electrons in the valence
shell.
• In the formation of BeF2, each of these valence electrons is shared by each of the
two fluorine atoms.
• As a result, Be atom is surrounded by two bond pairs of electrons.
• To minimise repulsion and to have maximum stability, the fluorine atoms are placed
on the opposite sides of Be with a bond angle of 1800 and the molecule is linear.




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Pl
ii. BF3
e

• In BF3, the central atom B (Z = 5 and E.C = 2, 3) has three electrons in the valence
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shell.
45 i R




• In the formation of BF3, each of these valence electrons is shared by each of the
31 ar




three fluorine atoms.
13 ak




• As a result, B atom is surrounded by three bond pairs of electrons.
06 h
Sa




• To minimise repulsion and to have stability, the bonded electron pairs are directed
towards the comers of an equilateral triangle.

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Therefore BFS molecule has a trigonal planar structure with bond angle 120 0.
a
im
ah
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iii. CH4
• The central atom carbon has four valence electrons.

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