Bonding, Molecular Shape & Structure

Page
6

The ideal tetrahedron has a bond angle = 109.5°

The lone electron pair exerts a little extra repulsion on the three bonding hydrogen atoms to create a slight compression to a 107° bond angle.

VSEPR model explains distortions of molecules

Less repulsion is exerted by a bonding pair of electrons because they feel attraction from two nuclei, while a non-bonding pair feels attraction from only one nucleus.

Non-bonding pairs spread out more!

Slide 35

AB4 is Tetrahedral

The carbon has 4 valence electrons and thus needs 4 more electrons from four hydrogen atoms to complete its octet. The hydrogen atoms are as far apart as possible at 109° bond angle. This is tetrahedral geometry. The molecule is three dimensional.

Slide 36

Valence-Shell Electron-Pair Repulsion Theory (VSEPR)

In molecules there are 2 types of electron

1. Bonding Pairs

2. Non-bonding or lone pairs

The combinations of these determine the shape of the molecule

Single bonds have a big impact on shape, double bonds have little effect

The outer pairs of electrons around a covalently bonded atom minimize repulsions between them by moving as far apart as possible

Slide 37

Water is a bent molecule with bond angles of 104.5°

Notice – the bond angle decreases as the number of non-bonding pairs increases

AB2 - classification

H2O

Slide 38

Ozone

O3 ; number of valence electrons = 18 electrons

Resonance structures

AB3 - classification

Slide 39

Valence Shell Electron-Pair Repulsion Theory (VSEPR)

Procedure

Sum the total Number of Valence Electrons

Drawing the Lewis Structure

2. The atom usually written first in the chemical formula is the Central atom in the Lewis structure

Complete the octet bonded to the Central atom. However, elements in the third row have empty d-orbitals which can be used for bonding.

If there are not enough electrons to give the central atom an octet try multiple bonds.

Predicting the Shape of the Molecule

Sum the Number of Electron Domains around the Central Atom in the Lewis Structure; Single = Double = Triple Bonds = Non-Bonding Lone Pair of Electrons = One Electron Domain

From the Total Number of Electron Domains, Predict the Geometry and Bond Angle(s); 2 (Linear = 180º); 3 (Trigonal Planar = 120º); 4 (Tetrahedral = 109.5º); 5 (Trigonal Bipyramidal = 120º and 90º); 6 (Octahedral = 90º)