Valence shell electron p air repulsion
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Valence Shell Electron P air Repulsion. VSPER. Molecular Shapes. The shape of a molecule plays an important role in its reactivity. By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule. What Determines the Shape of a Molecule?.

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Valence Shell Electron P air Repulsion

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Valence shell electron p air repulsion

Valence Shell Electron Pair Repulsion

VSPER


Molecular shapes

Molecular Shapes

  • The shape of a molecule plays an important role in its reactivity.

  • By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.


What determines the shape of a molecule

What Determines the Shape of a Molecule?

  • Simply put, electron pairs, whether they are bonding or nonbonding, repel each other.

  • By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.


Vsper

VSPER

  • The basis of this model is that the valence electrons repel one another. As a result the molecule forms predictable 3-D shapes.

    • Distance between electron areas is maximized

    • VSPER accounts for the 3-D arrangement of electrons AROUND A CENTER ATOM


Electron domains

Electron Domains

  • We can refer to the electron pairs as electronareas.

  • In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain.

  • This molecule has four electron domains.


Why do we need to know this

Why do we need to know this?

  • 3-D shapes determine the polarity or electron distribution of the molecule

  • This determines the strength of the intermolecular forces

    • Strong IMF have higher melting and boiling points and are more likely to be a solid at room temp.


How to determine the shape two factors

How to determine the shape?Two Factors

  • Electron geometry: The THEORETICAL shape of the molecule based on all the electron areas (bonds and lone pairs)

  • Molecular geometry: the ACTUAL shape of the molecule based on the electron repulsion. Only bonds are counted– lone pairs are invisible. (They exist and take up space but are not visible as an electron area.)


To determine electron geometries

To determine Electron Geometries

  • Count the number of electron areas in the Lewis structure.

  • The geometry will be that which corresponds to that number of electron domains.


Eg linear

EG: Linear


Molecular geometry

Molecular Geometry

  • Describes the actual shape of the molecule

  • When there are no lone pairs on the center atom  the molecular geometry is the SAME as the electron geometry. The shapes are the same.


2 electron areas electron geometry linear

2 Electron Areas:Electron geometry = linear

  • Two electron areas are always linear electron geometry

  • No lone pairs – so shape of molecule is also linear.


Three electron areas

Three Electron Areas

  • Electron geometry is trigonal planar

  • There are two molecular geometries (shapes):

    • Trigonal planar, if all the electron domains are bonding

    • Bent, if one of the domains is a nonbonding pair.


4 electron areas tetrahedral electron geometry

4 Electron Areas: Tetrahedral Electron geometry

  • All 4 atoms: the shape is same: tetrahedral shape

  • 3 atoms and one lone pair: Trigonal Pyramidal shape

  • 2 atoms and 2 lone pairs: Bent shape


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