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VSEPR Model: Predicting Molecular Geometry and Dipole Moments, Lecture notes of Geometry

Geometry

The Valence-Shell Electron Pair Repulsion (VSEPR) model is a fundamental concept in chemistry used to predict the molecular geometry of various molecules and ions. rules and figures to help discern electron pair arrangements and predict molecular geometries and dipole moments. It covers various molecular geometries such as linear, trigonal planar, bent or V-shaped, tetrahedral, trigonal pyramidal, and octahedral.

Typology: Lecture notes

2021/2022

Uploaded on 09/12/2022

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VSEPR Model
•The structure around a given
atom is determined principally
by minimizing electron pair
repulsions.
The Valence-Shell Electron
Pair Repulsion Model
•The valence-shell electron pair repulsion
(VSEPR) model predicts the shapes of
molecules and ions by assuming that the
valence shell electron pairs are arranged as
far from one another as possible.
• To predict the relative positions of atoms around
a given atom using the VSEPR model, you first
note the arrangement of the electron pairs
around that central atom.
Figure 8.14: The
molecular
structure of
methane. The
tetrahedral
arrangement of
electron pairs
produces a
tetrahedral
arrangement of
hydrogen atoms.
Predicting Molecular
Geometry
• The following rules and figures will help
discern electron pair arrangements.
1. Draw the Lewis structure
2. Determine how many electrons pairs are around the
central atom. Count a multiple bond as one pair.
3. Arrange the electrons pairs are shown in Table 8.8.
The direction in space of the bonding electron pairs gives
the molecular geometry
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pf4
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pf8
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VSEPR Model

• The structure around a given

atom is determined principally

by minimizing electron pair

repulsions.

The Valence-Shell Electron

Pair Repulsion Model

  • The valence-shell electron pair repulsion (VSEPR) model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible.
  • To predict the relative positions of atoms around a given atom using the VSEPR model, you first note the arrangement of the electron pairs around that central atom.

Figure 8.14: The molecular structure of methane. The tetrahedral arrangement of electron pairs produces a tetrahedral arrangement of hydrogen atoms.

Predicting Molecular

Geometry

  • The following rules and figures will help discern electron pair arrangements.
  1. Draw the Lewis structure
  2. Determine how many electrons pairs are around the central atom. Count a multiple bond as one pair.
  3. Arrange the electrons pairs are shown in Table 8.8. The direction in space of the bonding electron pairs gives the molecular geometry

Predicting Molecular

Geometry

  • The following rules and figures will help discern electron pair arrangements.
  1. Obtain the molecular geometry from the directions of bonding pairs, as shown in Figures 10.3 and 10.8.

Figure 10.3: Molecular geometries.

Figure 8.18: (a) In a bonding pair of electrons, the electrons are shared by two nuclei. (b) In a lone pair, both electrons must be close to a single nucleus and tend to take up more of the space around that atom.

Predicting Molecular

Geometry

  • Two electron pairs (linear arrangement).
    • You have two double bonds, or two electron groups about the carbon atom.
    • Thus, according to the VSEPR model, the bonds are arranged linearly, and the molecular shape of carbon dioxide is linear. This molecule has an AX 2 general formula with “2 bonding pairs” & no lone pairs. The bond angle is 180 o.

O C O:

Predicting Molecular

Geometry

  • Three electron pairs - (trigonal planar arrangement - AX 3 with “3 bonding pairs” & no lone pairs on the central atom).
  • The three groups of electron pairs are arranged in a trigonal plane. Thus, the molecular shape of COCl 2 is trigonal planar. Bond angle is 120 o.

Cl

C

O

Cl :

: :

Predicting Molecular

Geometry

  • Three electron pairs - (trigonal planar arrangement - AX 2 with “2 bonding pairs” & 1 lone pair on central atom).
  • Ozone has three electron groups about the central oxygen. One group is a lone pair.
  • These groups have a trigonal planar arrangement.

O O

: O

Predicting Molecular

Geometry

  • Three electron pairs (trigonal planar arrangement).
  • Since one of the groups is a lone pair, the molecular geometry is described as bent or v- shaped. When lone pairs are present in a bent molecule with bond angle $ 120 o^ very little distortion occurs.

O O

: O

Predicting Molecular

Geometry

  • Three electron pairs (trigonal planar arrangement).
  • Note that the electron pair arrangement includes the lone pairs, but the molecular geometry refers to the spatial arrangement of just the atoms.

O O

: O

Predicting Molecular

Geometry

  • Four electron pairs (tetrahedral arrangement).
    • Four electron pairs about the central atom lead to three different molecular geometries.

: Cl :

:

:Cl::

:Cl

: : C^ Cl:

: :

H

N

H

: H O:

H

: H

Predicting Molecular Geometry

  • Four electron pairs (tetrahedral arrangement).

: Cl :

:

: Cl: :

: Cl

: :

C

H

N

H

: H O:

H

: H

tetrahedral

Cl :

: :

Solid line - in plane of screen, dotted lines projecting back behind screen, dark wedge projecting toward you.

Predicting Molecular

Geometry

  • Four electron pairs (tetrahedral arrangement).

: Cl :

:

: Cl: :

: Cl

: :

C O:

H

: H

tetrahedral

Cl :

: : H

N H H

:

trigonal pyramid

Models of

CH 4 , NH 3 ,

H 2 O.

tetrahedral

trigonal planar

Bent or V-shaped

Figure

Molecular

geometries

Predicting Molecular

Geometry

  • Five electron pairs (trigonal bipyramidal arrangement). - This structure results in both 90 o^ and 120 o^ bond angles.

: F :

:

: F ::

F :

: : : F

: : P^ F :

: :

Predicting Molecular

Geometry

  • Other molecular geometries are possible when one or more of the electron pairs is a lone pair.

S ClF^3 XeF^2

F

F

F

F

see-saw

Predicting Molecular

Geometry

  • Other molecular geometries are possible when one or more of the electron pairs is a lone pair.

XeF (^2)

see-saw

S

F

F

F

F

: Cl

F

F

F

T-shape

Predicting Molecular

Geometry

  • Other molecular geometries are possible when one or more of the electron pairs is a lone pair.

see-saw

S

F

F

F

F

: Cl

F

F

F

T-shape

Xe

F

F

linear

Figure 8.20: Three possible

arrangements of the electron

pairs in the I 3

ion.

I 3 -^ (3 x 7 e) + 1 e = 22 e or 11 pairs I-I-I Place 1 pair between each peripheral I and central I, 3 prs on each peripheral I, and 3 pairs on central I.

Predicting Molecular

Geometry

  • Other molecular geometries are possible when one or more of the electron pairs is a lone pair.

SF 4 ClF 3 XeF (^2)

  • Let’s try their Lewis structures.

Octahedral

electron

arrangement

for Xe

Figure 8.19: Possible electron-

pair arrangements for XeF 4.

Figure 8.21: The molecular structure of methanol. (a) The arrangement of electron pairs and atoms around the carbon atom. (b) The arrangement of bonding and lone pairs around the oxygen atom. (c) The molecular structure.

Dipole Moment and Molecular

Geometry

  • The dipole moment is a measure of the degree of charge separation in a molecule. - We can view the polarity of individual bonds within a molecule as vector quantities.

O C O

  • Thus, molecules that are perfectly symmetric have a zero dipole moment. These molecules are considered nonpolar. (see Table 10.1)

δ −^ δ^ −

δ +

Return to Slide 27

Dipole Moment and Molecular

Geometry

  • However, molecules that exhibit any asymmetry in the arrangement of electron pairs would have a nonzero dipole moment. These molecules are considered polar. δ −

δ +

H

N H H

:

δ +

δ −