# Chemical Bonding revisedPage 2

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So far we have seen molecules represented in 2-D

However, molecules are actually 3 dimensional

To predict 3 dimensional molecular shapes we use VSEPR theory (Valence-Shell Electron-Pair Repulsion)

Based on the electrostatic repulsion of electron pairs

Slide 11

Note that the repulsion force is strongest between two lone pairs and the weakest between two bonded pairs, and the repulsion between a lone pair and a bonded pair is intermediate

We apply the VSEPR theory to a central atom that has an octet of electrons in its valence shell, and there are three categories of shapes; linear, trigonal planar, and tetrahedral

In VSEPR, an electron group is a bond (single or multiple) or a lone pair

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## Linear

Forms when a central atom has two electron groups

The shape is linear because the electron groups try to arrange themselves as far apart as possible

The bond angle between the electron groups is 1800

The central atom is bonded to two other atoms by two double bonds or a combination of a single bond and a triple bond

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## Trigonal Planar

A central atom with three electron groups has a trigonal planar shape

The bonding angle between the electron groups is 120o

The central atom is either bonded to three atoms(trigonal planar), or two atoms and a lone pair(bent or V-shaped)

Slide 14

## Tetrahedral

A central atom with four electron groups has a tetrahedral shape

The bonding angles between the electron groups is 109.5o

The central atom can be bonded to four atoms (tetrahedral), three atoms and a lone pair (trigonal pyramidal), or two atoms and two lone pairs (bent or V-shaped)

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Information Summary

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## Steps to Predicting Molecular Shapes

Draw a primary LDD of the molecule

Determine the total number of electron groups around the central atom

Determine the types of electron groups (bonding pairs or lone pairs)

Determine which shape will accommodate the combination of electron groups

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## Electronegativity

Electronegativity is a measure of the relative ability of an element’s atoms to attract the shared electrons in a chemical bond.

Higher electronegativities mean a greater attraction for the electrons.

Fluorine is the highest with a value of 4.0

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