2.2.4 The valence shell electron pair repulsion model( VSEPR )

• The two dimensional representations of the Lewis Formulas do not reflect the three-dimensional arrangement of atoms in the molecule. The study of these 3 dimensional shape of a molecule is called Molecular geometry, its important as it determines its properties.

  • Molecular geometry is explored using the VSEPR model

    • Valence

    • Shell

    • Electron

    • Pair

    • Repulsion

  This model is based on a few premises:

1. Electron pairs repel each other and $\therefore$arrange themselves as far as possible

2. Lone electrons occupy more space than bonding pairs ( single bond )

3. Double and triple bonds occupy more space than single bonds

A prequisite definition is the term

Electron domain --> Region of high electron density due to electron pairs being present

It can be:

1. a non-bonding pair ( lone pairs )

2. bonding pair of electrons ( single pair )

3. Double or triple bond ( which involves multiple electrons )

• Exploring the geometries of molecules involving 2,3,4 electron domains using VSEPR. Predicting the VSEPR shape involves two steps:

1. Count the # of electron domains around central atom to deduce electron domain geometry

2. Determine how many are bonding domains and are non-bonding domains

Two domains: Linear Geometry

180degrees away from each other as the electron pairs in the domains repel each other

• Called Linear to showcase that the bond along with its central atom can be displayed on a straight line

• Molecules with this electron domain geometry also have a linear molecular geometry

Three domains: Trigonal Planar Geometry

• 3 bonding domains calls for the pairs to adopt positions 120 degrees away from each other

• This electron domain geometry is called Trigonal planar.

  • Trigonal --> since form a triangle

  • planar --> Since atoms lay flat on a plane ( 2D )

• 2 possible molecular geometry

  • When all 3 are bonding domains --> trigonal planar molecular geometry

  • when $\frac{2}{3}$of domains are bonding domains, molecule has bent ( V-shaped ) geometry

Four domains: Tetrahedral Geometry

• pairs adopts 109.5 degrees away from each other

• Called tetrahedral because ends of domain form corners of a tetrahedron

  • Domains arrange themselves to maximize bond angles and thus distances between them.

3 Molecular geometries

• All bonding --> Tetrahedral

• $\frac{3}{4}$bonding --> trigonal pyramidal

• $\frac{2}{4}$bonding --> bent ( V -shaped )

Non-bonding pairs occupy more space which leads to decreased bond angles along with the greater repulsion.

• Wedges --> bonds that are coming out of the plane of the page at an angle

• Dashes --> bonds that are going into the plane of the page

Multiple Bonds

--> Triple bonds exert more repulsion than a single bond because of the extra electrons, causing values to deviate