1s atomic orbital 2s atomic orbital 2s atomic orbital (with node) 2px orbital 2py orbital 2pz orbital

Transcription

1 Atomic Orbitals

2 1s atomic orbital 2s atomic orbital 2s atomic orbital (with node) 2px orbital 2py orbital 2pz orbital

3 Valence Bond Theory and ybridized Atomic Orbitals

4 Bonding in 2 1s 1s

5 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

6 Simple Atomic Orbital Overlap Does Not Explain the Structure of Methane

7 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.

8 ybrid Orbitals 1. The number of atomic orbitals mixed. 2. The atomic orbitals mixed. 3. Types of ybrid Orbitals

9 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

10 The sp 3 ybridization Scheme

11 The Orbital Picture of Methane C4 4 hydrogen 1s atomic orbitals C C Methane 4 carbon sp 3 hybrid orbitals

12 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

13 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

14 The sp 2 ybridization Scheme B BF3

15 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

16 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

17 Ethene, C2C2 bond contains 2 electrons. The remaining unhybridized p orbitals overlap to form a bond. 5 sigma bonds and 1 bond

18 The sp ybridization Scheme Be BeCl2

19 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

20 The sp hybrid orbitals of two carbons overlap Ethyne, CC The remaining sp hybrid orbitals overlap with hydrogen s orbitals 3 sigma bonds

21 Each bond contains 2 electrons. The remaining unhybridized p orbitals overlap to form 2 bonds. 3 sigma bonds and 2 bonds

22 Bonding in CC C C C C

23 Bonding in CN C N C N

24 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

25 Molecular Orbital Theory

26 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

27 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

28 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.

29 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

30 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.

31 σ* antibonding molecular orbital 1s atomic orbital 1s atomic orbital σ bonding molecular orbital

32 σ* antibonding molecular orbital 2p atomic orbital 2p atomic orbital σ bonding molecular orbital

33 π* antibonding molecular orbital 2p atomic orbital 2p atomic orbital π bonding molecular orbital

34 σ* antibonding molecular orbital sp 3 atomic orbital sp 3 atomic orbital σ bonding molecular orbital

35 VSEPR Theory

36 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

37 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

38 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.

39 180º 120º

40 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

41 Tetrahedral Molecular Geometry

42 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 N3

43 Bent Molecular Geometry a Derivative of Tetrahedral Electron Geometry 2 O Tetrahedral Electron Geometry Bent Molecular Geometry

44 Molecules with Multiple Central Atoms Methanol O N C C O Glycine

45 Dipole Moments and Molecular Polarity

46 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.

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48 C4 N3 2O

49 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.

50 : Molecular Polarity F N F F μ = D