Atomic Orbitals
1s atomic orbital 2s atomic orbital 2s atomic orbital (with node) 2px orbital 2py orbital 2pz orbital
Valence Bond Theory and ybridized Atomic Orbitals
Bonding in 2 1s 1s
Atomic Orbital Overlap and Bonding in 2S S S isolated atoms 2S S: [Ne] 3s 2 3px 2 3py 1 3pz 1 :1s 1 expected bond angle: 90º measured bond angle: 92º
Simple Atomic Orbital Overlap Does Not Explain the Structure of Methane
Valence Bond Theory formation of a molecule, 1. There is a hybridization of atomic orbitals to form molecular orbitals. 2. The valence atomic orbitals in a molecule are different from those in isolated atoms. 3. A set of overlapping atomic orbitals has a maximum of two electrons with opposite spins. 4. The greater the orbital overlap, the stronger and more stable is the bond.
ybrid Orbitals 1. The number of atomic orbitals mixed. 2. The atomic orbitals mixed. 3. Types of ybrid Orbitals
The sp 3 ybridization Scheme for Carbon Energy 2p 2s 1s excitation 2p 2s 1s } hybridization 2sp 3 1s Carbon: ground-state electron configuration Carbon: excited-state electron configuration Carbon: sp 3 -hybridized electron configuration
The sp 3 ybridization Scheme
The Orbital Picture of Methane C4 4 hydrogen 1s atomic orbitals C C Methane 4 carbon sp 3 hybrid orbitals
Ethane, C3C3 The sp 3 hybrid orbitals of two carbons overlap The remaining sp 3 hybrid orbitals overlap with hydrogen s orbitals 7 sigma bonds
Ethane, C3C3 The sp 3 hybrid orbitals of two carbons overlap The remaining sp 3 hybrid orbitals overlap with hydrogen s orbitals 7 sigma bonds
The sp 2 ybridization Scheme B BF3
The sp 2 ybridization Scheme for Carbon Energy 2p 2s 1s excitation 2p 2s 1s } hybridization 2p 2sp 2 1s Carbon: ground-state electron configuration Carbon: excited-state electron configuration Carbon: sp 2 -hybridized electron configuration
The sp 2 hybrid orbitals of two carbons overlap Ethene, C2C2 The remaining sp 2 hybrid orbitals overlap with hydrogen s orbitals 5 sigma bonds
Ethene, C2C2 bond contains 2 electrons. The remaining unhybridized p orbitals overlap to form a bond. 5 sigma bonds and 1 bond
The sp ybridization Scheme Be BeCl2
The sp ybridization Scheme for Carbon Energy 2p 2s 1s excitation 2p 2s 1s } hybridization 2p 2sp 2 1s Carbon: ground-state electron configuration Carbon: excited-state electron configuration Carbon: sp 2 -hybridized electron configuration
The sp hybrid orbitals of two carbons overlap Ethyne, CC The remaining sp hybrid orbitals overlap with hydrogen s orbitals 3 sigma bonds
Each bond contains 2 electrons. The remaining unhybridized p orbitals overlap to form 2 bonds. 3 sigma bonds and 2 bonds
Bonding in CC C C C C
Bonding in CN C N C N
sp 3 hybridized sp 2 hybridized Summary of ydrocarbon Bonding 1.09 Å 1.08 Å C C C C 1.54 Å 1.33 Å 1.20 Å C C sp hybridized 1.06 Å
Molecular Orbital Theory
Problems with Valence Bond Theory VB theory predicts properties better than Lewis theory bonding schemes, bond strengths, lengths, rigidity There are still properties it doesn t predict perfectly magnetic behavior of certain molecules strength of bonds VB theory presumes the electrons are localized in orbitals doesn t account for delocalization
Molecular Orbital Theory In MO theory, we apply Schrödinger s wave equation to the molecule to calculate a set of molecular orbitals. The equation solution is estimated. We start with good guesses as to what the orbitals should look like, then test the estimate until the energy is minimized The electrons belong to the whole molecule orbitals are delocalized
LCAO The simple guess starts with atomic orbitals of the atoms adding together to make molecular orbitals, the Linear Combination of Atomic Orbitals. The waves can combine either constructively or destructively.
Molecular Orbitals When wave functions combine constructively, the resulting molecular orbital has less energy than the original atomic orbitals it is called a Bonding Molecular Orbital σ, π most of the electron density between the nuclei Amplitudes of wave functions added
Molecular Orbitals When wave functions combine destructively, the resulting molecular orbital has more energy than the original atomic orbitals it is called an Antibonding Molecular Orbital σ*, π* most of the electron density outside the nuclei nodes between nuclei Amplitudes of wave functions subtracted.
σ* antibonding molecular orbital 1s atomic orbital 1s atomic orbital σ bonding molecular orbital
σ* antibonding molecular orbital 2p atomic orbital 2p atomic orbital σ bonding molecular orbital
π* antibonding molecular orbital 2p atomic orbital 2p atomic orbital π bonding molecular orbital
σ* antibonding molecular orbital sp 3 atomic orbital sp 3 atomic orbital σ bonding molecular orbital
VSEPR Theory
Lewis Theory Predicts Electron Groups Electron groups - regions of electrons around an atom Regions result from placing shared pairs of valence electrons between bonding nuclei Regions result from placing unshared valence electrons on a single nuclei
Lewis Theory of Molecular Shapes Electron groups repel each other. Predicting the shapes of molecules 1) The arrangement of the electron groups will be determined by trying to minimize repulsions between them. 2) The arrangement of atoms ( molecular shape ) surrounding a central atom will be determined by where the bonding electron groups are. 3) 1 and 2 are not necessarily the same
VSEPR Theory Electrons should be most stable when they are separated as much as possible. Valence shell electron pair repulsion (VSEPR) - Electron groups around the central atom will be most stable when they are as far apart as possible. The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule.
180º 120º
Tetrahedral Electron Geometry When there are four electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom. This results in the electron groups taking a tetrahedral geometry. The bond angle is 109.5
Tetrahedral Molecular Geometry
Pyramidal Molecular Geometry: a Derivative of Tetrahedral Electron Geometry When there are four electron groups around the central atom, and one is a lone pair, the result is called a pyramidal shape. The bond angle is less than 109.5 N3
Bent Molecular Geometry a Derivative of Tetrahedral Electron Geometry 2 O Tetrahedral Electron Geometry Bent Molecular Geometry
Molecules with Multiple Central Atoms Methanol O N C C O Glycine
Dipole Moments and Molecular Polarity
Polarity of Molecules For a molecule to be polar, it must have polar bonds, and have an unsymmetrical shape Polarity affects the intermolecular forces of attraction and therefore affects boiling points and solubilities Nonbonding pairs affect molecular polarity.
C4 N3 2O
Predicting Polarity of Molecules 1. Draw the Lewis structure and determine the molecular geometry. 2. Determine whether the bonds in the molecule are polar. 3. Determine whether the polar bonds add together to give a net dipole moment.
: Molecular Polarity F N F F μ = 0.234 D