Lecture : Molecular Geometry

YEASMIN AKTHER Assistant Professor Dept: Chemistry

Cumilla Victoria Govt College.

Lecture : Molecular Geometry

V ^alence S ^hell

E ^lectron P ^air

R ^{epulsion Theory}

Planar triangular

Tetrahedral

Trigonal pyramidal

Bent

VSEPR Theory

• Based on Electron Dot (Lewis structures)

• Theory predicts shapes of compounds

• abbreviated VSEPR

• VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion

• VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves)

• Electrons around central nucleus

repel each other. So, structures

have atoms maximally spread out

VSEPR overview

• Each shape has a name (you will have to know these

)

• Names of Shapes:

• tetrahedral

• trigonal pyramidal

• Bent

• Linear

• trigonal planar

Triangular Planar Tetrahedral

Trigonal pyramidal

Linear

Bent or V Models

methane, CH ₄

Bonds are all evenly spaced electrons

109.5°

Tetrahedral

Less repulsion between the bonding pairs of electrons

.. ammonia NH ₃

.. .. ..

Trigonal Pyramidal

.. ..

..

109.5° (109.5°) 109.5° (107°) 109.5° (104.5°)

water, H₂O

.. ..

..

Bent or V

2 unshared pairs of e’s at top of O repel bonds and force them to bend

Molecular Geometry

Molecules of different substances have diverse shapes. Atoms attach to one another in various geometric arrangements. The overall molecular shape of a molecule is determined by its bond angles in three dimensions. The shape of a molecule is very important for its physical and chemical properties.

VSEPR Valence Bond

Theory

Molecular Orbital Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory

It is a method for predicting the shape of a molecule from the knowledge of the groups of electrons around a central atom.

Electron pairs (bonding and nonbonding electrons) repel one another, as a result , the electron pairs remain as far apart as possible from another as possible to minimize the repulsion.

• Two electron pairs in the valence orbital are arranged linearly

• Three electron pairs are organized in a trigonal planar arrangement

• Four electron pairs are organized in a tetrahedral arrangement

• Five electron pairs are arranged in a trigonal bipyramid

• Six electron pairs are organized in an octahedral arrangement

The repulsion of lone pair electrons is grater than the repulsion of bond pair electrons

Electron pairs assume orientations about an atom to minimize repulsions.

Applying the VSEPR theory.

• draw a plausible Lewis structure of the molecule or polyatomic ion.

• Determine the number of unshared electron pairs and numbers of bonds around the central atom ( multiple bonds count as a single bond)

• B + No

• Establish the geometrical orientation of the electron pairs

around the central atom as linear, trigonal planar, tetrahedral,, trigonal bipyramid or octahedral

• Describe the molecular geometry

question: predict the shape of CO² HCN CH⁴, NH³ SO² PCl⁵, SF⁶ and H²O by using VSEPR theory.

question: predict the shape of CO² HCN CH⁴, NH³ SO² PCl⁵, SF⁶ and H²O by using VSEPR theory.

The nonbonding electron pairs are as important as bonding electron pairs in determining the structure.

Nonbonding electrons take up more space in the valence shell than the bonding electrons.

If one or more of the electron pairs are lone pairs, the distribution of electron pair and the geometrical shape of the molecule must be different.

The bond angles decrease as the number of nonbonding electron pairs increases

Repulsion strengths

lone pair -lone pair  lone pair e-bond pair  bond pair-bond pair

Valence bond theory

The covalent bonds are formed by overlap of atomic orbitals each of which contains one electron of opposite spin.

The valence bond method predicts molecule shapes from the shapes and orientation of the atomic orbitals and their overlap regions when two atoms approach.

In most cases the orbitals that overlap are reconfigured orbitals, called hybrid orbitals, having different shapes and orientations than pure orbitals.

The process of hybridization corresponds to a mathematical mixing of the valence- shell atomic orbitals.

Bonding in Methane

Valence Electron Pair Geometry

Number of Orbitals

Hybrid Orbitals

Linear 2 sp

Trigonal Planar 3 sp²

Tetrahedral 4 sp³

Trigonal Bipyramidal 5 sp³d

Octahedral 6 sp³d²

question : find the hybridazation type and geometry of CH⁴, PCl⁵, SF⁶. NH3 BeF2

Multiple Covalent Bonds