Bonding Theories Part 2: VSEPR Theory

Objectives Describe how VSEPR theory helps predict the shapes of molecules Describe how VSEPR theory helps predict the shapes of molecules Identify ways in which orbital hybridization is useful in describing 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 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 three- dimensional For example, methane molecules are three- dimensional The hydrogen in the molecule are at the four corners of a geometric solid called a regular tetrahedron 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 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 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 Instead, they position themselves at the corners of a tetrahedron Its shape, then is tetrahedral Its shape, then is tetrahedral

Electron Pairs & Molecular Shape Unshared pairs of electrons are also important in predicting the shapes of molecules Unshared pairs of electrons are also important in predicting the shapes of molecules For example: CO 2 For example: CO 2 The two shared pairs that form each double bond repel each other and remain as far apart as possible The two shared pairs that form each double bond repel each other and remain as far apart as possible Thus, this molecule is linear Thus, this molecule is linear

Carbon Dioxide

What about BF 3 ? Remember boron does not always obey the octet rule 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. 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 Its molecular shape is called trigonal planar

BF 3

What about Unshared Electron Pairs? When the central atom has an unshared pair of electrons, they influence the shape of the molecule 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 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 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. BeCl 2 2.SO SO 3 4. PF 5

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 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 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 In hybridization, several atomic orbitals mix to form the same total number of equivalent hybrid orbitals

Hybridization Involving Single Bonds Considering methane CH 4 Considering methane CH 4 The carbon atom’s outer electron configuration is 2s 2 2p 2 The carbon atom’s outer electron configuration is 2s 2 2p 2 But one of the 2s electrons is promoted to a 2p orbital 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 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 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 sp 3 hybrid orbitals The one 2s orbital and three 2p orbitals of a carbon atom mix to form four sp 3 hybrid orbitals These are at the tetrahedral angle of 109.5° These are at the tetrahedral angle of 109.5° The sp 3 orbitals extend further into space than either s or p orbitals, allowing a great deal of overlap with the hydrogen 1s orbitals The sp 3 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 C ‒ H sigma bonds The 8 available valence electrons fill the molecular orbitals to form four C ‒ H sigma bonds The overlap results in unusually strong covalent 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 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° The bond angles in ethene are 120° Two sp 2 hybrid orbitals form from the combination of one 2s and two 2p atomic orbitals Two sp 2 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 Five sigma bonds and one pi bond hold the molecule together

Hybridization of Ethene

Hybridization Involving Triple Bonds Ethyne (C 2 H 2 ) also called acetylene, forms a carbon-carbon triple bond Ethyne (C 2 H 2 ) also called acetylene, forms a carbon-carbon triple bond It is a linear molecular It is a linear molecular It creates two sp hybrid orbitals for each carbon It creates two sp hybrid orbitals for each carbon In total 3 sigma bonds and two pi bonds hold the molecule together In total 3 sigma bonds and two pi bonds hold the molecule together

Hybridization of Ethyne