Bonding Theories Part 2: VSEPR Theory Objectives Describe how VSEPR theory helps predict the shapes of molecules Identify ways in which orbital
hybridization is useful in describing molecules Important Vocabulary Tetrahedral angle VSEPR theory Hybridization
VSEPR Theory States that the repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible For example, methane molecules are threedimensional The hydrogen in the molecule are at the four corners of a geometric solid called a regular
tetrahedron In this arrangement, all of the H-C-H angles are 109.5, the tetrahedral angle Methane The four shared pairs are NOT at the maximum distance apart when on a flat plane
Instead, they position themselves at the corners of a tetrahedron Its shape, then is tetrahedral Electron Pairs & Molecular Shape Unshared pairs of electrons are also important in predicting the
shapes of molecules For example: CO2 The two shared pairs that form each double bond repel each other and remain as far apart as possible Thus, this molecule is linear Carbon Dioxide
What about BF3? Remember boron does not always obey the octet rule The three unshared pairs of electrons on each F atoms will repel each other to the maximum distance apart.
Its molecular shape is called trigonal planar BF3 What about Unshared Electron Pairs? When the central atom has an unshared
pair of electrons, they influence the shape of the molecule In VSEPR theory, unshared pairs occupy more space around the central atom than shared pairs Thus, the shared pairs and the unshared pair cause the shape to be bent
Examples of Bent Molecules 9 Possible Molecular Shapes Predicting Molecular Shapes
1. Draw the Lewis structure for the molecule 2. Count the shared and unshared pairs of electrons around the central atom 3. Use VSEPR theory to find the shape that allows the shared and unshared pairs of electrons to be spaced as far apart as possible
4. Verify the structure by making sure that all the atoms, except hydrogen, obey the octet rule Examples 1. BeCl2 3. SO3
2. SO42- 4. PF5 Hybrid Orbitals The VSEPR theory works well when accounting for molecular shapes, but it
does not help much in describing the types of bonds formed Orbital hybridization provides information about both molecular bonding and molecular shape In hybridization, several atomic orbitals mix to form the same total number of equivalent hybrid orbitals
Hybridization Involving Single Bonds Considering methane CH4 The carbon atoms outer electron configuration is 2s22p2 But one of the 2s electrons is promoted to a 2p orbital This gives one 2s electron and three 2p
electrons, allowing carbon to bond to 4 hydrogen atoms All of these bonds are identical and can be explained by orbital hybridization Hybridization Involving Single Bonds The one 2s orbital and three 2p orbitals of a carbon atom mix to form four sp3 hybrid
orbitals These are at the tetrahedral angle of 109.5 The sp3 orbitals extend further into space than either s or p orbitals, allowing a great deal of overlap with the hydrogen 1s orbitals The 8 available valence electrons fill the molecular orbitals to form four CH sigma bonds
The overlap results in unusually strong covalent bonds Hybridization of Methane Hybridization Involving Double Bonds Ethene is a relatively simple molecule that
has one carbon-carbon double bond and 4 carbon-hydrogen single bonds The bond angles in ethene are 120 Two sp2 hybrid orbitals form from the combination of one 2s and two 2p atomic
orbitals Five sigma bonds and one pi bond hold the molecule together Hybridization of Ethene Hybridization Involving Triple Bonds
Ethyne (C2H2) also called acetylene, forms a carbon-carbon triple bond It is a linear molecular It creates two sp hybrid orbitals for each carbon In total 3 sigma bonds and two pi bonds hold the molecule together
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