# 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
