Chemistry, Julia Burdge, 2st Ed McGraw Hill Chapter 10 Chemical Bonding II Mr Truong Minh Chien ; losedtales@yahoo.com http://tailieu.vn/losedtales http://mba-programming.blogspot.com 2011, NKMB Co., Ltd Structure Determines Properties! • properties of molecular substances depend on the structure of the molecule • the structure includes many factors, including: the skeletal arrangement of the atoms the kind of bonding between the atoms ionic, polar covalent, or covalent the shape of the molecule • bonding theory should allow you to predict the shapes of molecules Chemistry, Julia Burdge, 2nd e., McGraw Hill Molecular Geometry • Molecules are 3-dimensional objects • We often describe the shape of a molecule with terms that relate to geometric figures • These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure • The geometric figures also have characteristic angles that we call bond angles Chemistry, Julia Burdge, 2nd e., McGraw Hill Using Lewis Theory to Predict Molecular Shapes • Lewis theory predicts there are regions of electrons in an atom based on placing shared pairs of valence electrons between bonding nuclei and unshared valence electrons located on single nuclei • this idea can then be extended to predict the shapes of molecules by realizing these regions are all negatively charged and should repel Chemistry, Julia Burdge, 2nd e., McGraw Hill VSEPR Theory • electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory since electrons are negatively charged, they should be most stable when they are separated as much as possible • the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule Chemistry, Julia Burdge, 2nd e., McGraw Hill Chemistry, Julia Burdge, 2nd e., McGraw Hill VSEPR electron domain animation Chemistry, Julia Burdge, 2nd e., McGraw Hill Electron Groups • the Lewis structure predicts the arrangement of valence • • electrons around the central atom(s) each lone pair of electrons constitutes one electron group on a central atom each bond constitutes one electron group on a central atom  regardless of whether it is single, double, or triple •• •O • •• N •• O• • •• Chemistry, Julia Burdge, 2nd e., McGraw Hill there are electron groups on N lone pair single bond double bond Molecular Geometries • there are basic arrangements of electron groups around a central atom  based on a maximum of bonding electron groups  though there may be more than on very large atoms, it is very rare • each of these basic arrangements results in different basic molecular shapes  in order for the molecular shape and bond angles to be a “perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent • for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same Chemistry, Julia Burdge, 2nd e., McGraw Hill Linear Geometry • when there are electron groups around the central • • atom, they will occupy positions opposite each other around the central atom this results in the molecule taking a linear geometry the bond angle is 180° •• • • Cl •• •• Be Cl • • •• • • O •• C O • • •• Chemistry, Julia Burdge, 2nd e., McGraw Hill 10 Li2 σ bonding MO HOMO Tro, Chemistry: A Molecular Approach σ∗ Antibonding MO LUMO 140 Interaction of p Orbitals Tro, Chemistry: A Molecular Approach 141 Interaction of p Orbitals Tro, Chemistry: A Molecular Approach 142 O2 • dioxygen is paramagnetic • paramagnetic material have unpaired electrons • neither Lewis Theory nor Valence Bond Theory predict this result Tro, Chemistry: A Molecular Approach 144 O2 as described by Lewis and VB theory Tro, Chemistry: A Molecular Approach 145 Oxygen Atomic Orbitals 2p Since more electrons are in bonding orbitals than are in antibonding orbitals, net bonding interaction BO = ½( be – abe) BO = σ∗ π∗ 2p O2 MO’s π σ σ∗ 2s Tro, Chemistry: A Molecular Approach Oxygen Atomic Orbitals Since there are unpaired electrons in the antibonding orbitals, O2 is paramagnetic 2s σ 146 O2 π∗ Antibonding MO HOMO π Bonding MO Tro, Chemistry: A Molecular Approach HOMO-1 π∗ Antibonding MO LUMO σ∗ Antibonding MO LUMO+1 147 Draw a molecular orbital diagram of N and predict its bond order and magnetic properties Tro, Chemistry: A Molecular Approach 148 Nitrogen Atomic Orbitals 2p BO = ½( be – abe) BO = σ∗ π∗ 2p N2 MO’s σ π σ∗ 2s Tro, Chemistry: A Molecular Approach Nitrogen Atomic Orbitals Since there are no unpaired electrons, N2 is diamagnetic 2s σ 149 Heteronuclear Diatomic Molecules • the more electronegative atom has lower energy orbitals • when the combining atomic orbitals are identical and equal • energy, the weight of each atomic orbital in the molecular orbital are equal when the combining atomic orbitals are different kinds and energies, the atomic orbital closest in energy to the molecular orbital contributes more to the molecular orbital  lower energy atomic orbitals contribute more to the bonding MO  higher energy atomic orbitals contribute more to the antibonding MO • nonbonding MOs remain localized on the atom donating its atomic orbitals Tro, Chemistry: A Molecular Approach 150 NO Free-Radical σ2s Bonding MO mainly O’s 2s atomic orbital Tro, Chemistry: A Molecular Approach 151 HF Tro, Chemistry: A Molecular Approach 152 Polyatomic Molecules • when many atoms are combined together, the atomic orbitals of all the atoms are combined to make a set of molecular orbitals which are delocalized over the entire molecule • gives results that better match real molecule properties than either Lewis or Valence Bond theories Tro, Chemistry: A Molecular Approach 153 Ozone, O3 Delocalized π bonding orbital of O3 Tro, Chemistry: A Molecular Approach 154 ... rather than shared like bonding electron groups • relative sizes of repulsive force interactions is: Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair • this effects... the skeletal arrangement of the atoms the kind of bonding between the atoms ionic, polar covalent, or covalent the shape of the molecule • bonding theory should allow you to predict the shapes... Chemistry, Julia Burdge, 2nd e., McGraw Hill 27 Effect of Lone Pairs The bonding electrons are shared by two atoms, nonbonding electrons are localized on the central atom, so area of charge is