VSEPR Theory (by chemisTREEpk)

VSEPR Theory

INTRODUCED IN 1940 BY SIDGWICK & POWELL AND MODIFIED BY RONALD GILLESPIE & RONALD NYHOLM IN 1957.

Principle & Main Assumption:

• Shape  of the molecule  will be decided  by the total  number  of electron  pairs (Bond  Pairs  & Lone 

• Pairs) around the central atom.

• And these electron pairs remain at maximum distance from each other so that the repulsion between them is maximum.

• A lone pair occupies more space on an atom in a molecule as compared to a bond pair.

• The repulsion order between electron pair is: L.P — L.P > L.P — B.P > B.P — B.P

• The electron density of three bond pairs of a triple bond or two-electron pairs of a double bond lies between two atoms in a molecule just like that of one molecule of a single bond hence three electron pairs of a  triple bond and two-electron pairs of a  double bond must be considered as one electron pair.

• Three electron pairs of a triple bond occupies more space than two electron pairs of a double bond which occupies more space than one electron pair of a single bond.

• The order of repulsion between electron pairs is: e , e > = , = > — ,

𝐴𝐵2

BeCl2

• Central atom is Beryllium.
• Atomic number of Be = 4
• Number of electrons in valence shell of Be = 2
• Cl-Be-Cl
• Total number of electron pairs around Be in BeCl2 = 2
• Type of molecule = AB₂ BeCl₂
• Number of bond pairs = 2
• Number of lone pairs = 0
• Geometry = Linear
• Angle = 180 degree

HCN

• Central atom is Carbon.
• Atomic number of C = 6
• Number of electrons in valence shell of C = 4
• H−C≡N
• Total number of electron pairs around C in HCN = 2
• Type of molecule = AB2
• Number of bond pairs = 2
• Number of lone pairs = 0
• Geometry = Linear
• Angle = 180 degree

CO2

• Central atom is Carbon.
• Atomic number of C = 6
• Number of electrons in valence shell of C = 4
• O=C=O
• Total number of electron pairs around C in CO2 = 2
• Type of molecule = AB2
• Number of bond pairs = 2
• Number of lone pairs = 0
• Geometry = Linear
• Angle = 180 degree

𝐴𝐵3

BF3

• Central atom is Boron(B).
• Atomic number of boron = 5
• Numbers of electron in valence shell of Boron = 3
• Total number of electron pairs around B in BF3 = 3
• Number of Bond Pairs = 3
• Number of Lone Pairs = 0
• Type of molecule = AB3
• Geometry = Triangular Planner
• Angle = 120 degree

SO3

• Central atom is Sulphur (S).
• Atomic number of Sulphur = 16
• Numbers of electrons in the valence shell of Sulphur = 6
• Total number of electron pairs around S in 𝑆𝑂3 = 3
• Number of Bond Pairs = 3
• Number of Lone Pairs = 0
• Type of molecule = AB3
• Geometry = Triangular Planner
• Angle = 120 degree

[CO3]-2

• Central atom is Carbon (C).
• Atomic number of Carbon = 6
• Numbers of electron in valence shell of Carron = 4
• Total number of electron pairs around C in 𝐶𝑂3 −2 = 3
• Number of Bond Pairs = 3
• Number of Lone Pairs = 0
• Type of molecule = AB3
• Geometry = Triangular Planner
• Angle = 120 degree

𝐴𝐵2E

SnCl2

• Central atom in SnCl2 is Sn
• Atomic number of Sn = 50
• number of electrons in the Valence shell of Sn = 4
• Number of electron pairs around Sn in SnCl2 = 3
• Number of Bond Pairs = 2
• Number of Lone Pairs = 1
• Type of molecule = AB2E
• Geometry = Angular
• Angle = Less than 180 degree

SO2

• Central atom in SO2 is Sulphur.
• Atomic number of S = 16
• number of electrons in the Valence shell of S = 6
• Number of electron pairs around S in SO2 = 3
• Number of Bond Pairs = 2
• Number of Lone Pairs = 1
• Type of molecule = AB2E
• Geometry = Angular
• Angle = Less than 180 degree

 

𝐴𝐵4

CH4

• Central atom is Carbon in CH4.
• Atomic number of C = 6
• number of electrons in the valence shell of C = 4
• Total number of electron pairs around C in CH4 = 4
• Type of molecule = AB4
• Number of bond pairs = 4
• Number of lone pairs = 0
• Geometry = Tetrahedral
• Angle = 109.5 degree

N(+)H4

• Central atom is 𝑁+ in 𝑁+𝐻4.
• Atomic number of N = 7
• Atomic number of 𝑁+ = 6
• Number of electrons in valence shell of 𝑁+ = 4
• Total number of electron pairs around 𝑁+ in 𝑁+𝐻4 = 4
• Type of molecule = AB4
• Number of bond pairs = 4
• Number of lone pairs = 0
• Geometry = Tetrahedral
• Angle = 109.5 degree

B(-)F4

• Central atom is 𝐵− in 𝐵−𝐹4.
• Atomic number of B = 5
• Atomic number of 𝐵− = 6
• Number of electrons in valence shell of 𝑁+ = 4
• Total number of electron pairs around 𝐵− in 𝐵−𝐹4 = 4
• Type of molecule = AB4
• Number of bond pairs = 4
• Number of lone pairs = 0
• Geometry = Tetrahedral
• Angle = 109.5 degree

[SO4]-2

• Central atom is S in 𝑆𝑂4 −2.
• Atomic number of Sulphur = 16
• Number of electrons in valence shell of S = 6
• Total number of electron pairs around S in [𝑆𝑂4] −2 = 4
• Type of molecule = AB4
• Number of bond pairs = 4
• Number of lone pairs = 0
• Geometry = Tetrahedral
• Angle = 109.5 degree

𝐴𝐵3E

NH3

• Central atom in NH3 is N.
• Atomic number of Nitrogen = 7
• number of electrons in the Valence shell of Nitrogen = 5
• Number of electron pairs around Nitrogen in NH3 = 4
• Number of Bond Pairs = 3
• Number of Lone Pairs = 1
• Type of molecule = AB3E
• Geometry = Trigonal Pyramidal
• Angle = 107.5 degree

PCl3

• Central atom in PCl3 is P.
• Atomic number of P = 15
• number of electrons in the Valence shell of P = 5
• Number of electron pairs around Nitrogen in NH3 = 4
• Number of Bond Pairs = 3
• Number of Lone Pairs = 1
• Type of molecule = AB3E
• Geometry = Trigonal Pyramidal
• Angle = Less than 109.5 degrees

𝐴𝐵2E2

H2O

• Central atom in H2O is Oxygen.
• Atomic number of O = 8
• Number of an electron in the Valence shell of O = 6
• Number of electron pairs around Oxygen in H20 = 4
• Number of Bond Pairs = 2
• Number of Lone Pairs = 2
• Type of molecule = AB2E2
• Geometry = Angular
• Angle = 104.5 degree

𝐴𝐵5

PCl5

• Central atom in PCl5 is P.
• Atomic number of P = 15
• Number of electron in Valence shell of O = 5
• Number of electron pairs around P in PCl5 = 5
• Number of Bond Pairs = 5
• Number of Lone Pairs = 0
• Type of molecule = AB5
• Geometry = Trigonal Bipyramidal
• Angle = 120, 90, 180 degree

𝐴𝐵4E

SF4

• Central atom in SF4 is S.
• Atomic number of S = 16
• number of electrons in the Valence shell of S = 6
• Number of electron pairs around S in SF4 = 5
• Number of Bond Pairs = 4
• Number of Lone Pairs = 1
• Type of molecule = AB4E
• Geometry = Distorted Tetrahedral

𝐴𝐵6

SF6

• Central atom in SF6 is S.
• Atomic number of S = 16
• number of electrons in the Valence shell of S = 6
• Number of electron pairs around S in SF6 = 6
• Number of Bond Pairs = 6
• Number of Lone Pairs = 0
• Type of molecule = AB6
• Geometry = Octahedral
• Angle = Square Planner Bipyramidal

𝐴𝐵7

IF7

• Central atom in IF7 is I.
• Atomic number of I = 53
• number of electrons in the Valence shell of I = 7
• Number of electron pairs around me in IF7 = 7
• Number of Bond Pairs = 7
• Number of Lone Pairs = 0
• Type of molecule = AB7
• Geometry = Pentagonal Bipyramidal

𝐴𝐵2E3

XeF2

• Central atom in XeF2 is Xe.
• Atomic number of Xe = 54
• Number of electron in Valence shell of Xe = 8
• Number of electron pairs around Xe in XeF2 = 5
• Number of Bond Pairs = 2
• Number of Lone Pairs = 3
• Type of molecule = AB2E3
• Geometry = Linear
• Angle = Less than 180 degree

𝐴𝐵4E2

XeF4

• Central atom in XeF4 is Xe.
• Atomic number of Xe = 54
• Number of electron in Valence shell of Xe = 8
• Number of electron pairs around Xe in XeF4 = 6
• Number of Bond Pairs = 4
• Number of Lone Pairs = 2
• Type of molecule = AB4E2
• Geometry = Planer
• Angle = 90 degree

 

Limitations of VSEPR Theory:

• It does not explain how are bonds formed.
• It doesn't tell about the bond length and bond strength.
• It doesn't explain the magnetic behavior of the molecules or ions.
• It does not explain the shapes of certain transition complexes as well as certain molecules.

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