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Molecular Geometries and Bonding Unit 10: Molecular Geometries and Bonding Theories Dr. Jorge L. Alonso Miami-Dade College – Kendall Campus Miami, FL CHM.

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Presentation on theme: "Molecular Geometries and Bonding Unit 10: Molecular Geometries and Bonding Theories Dr. Jorge L. Alonso Miami-Dade College – Kendall Campus Miami, FL CHM."— Presentation transcript:

1 Molecular Geometries and Bonding Unit 10: Molecular Geometries and Bonding Theories Dr. Jorge L. Alonso Miami-Dade College – Kendall Campus Miami, FL CHM 1045: General Chemistry and Qualitative Analysis Textbook Reference: Chapter # 8 (skip 9) Module #12 (skip 13)

2 Molecular Geometries and Bonding Molecular Shape and Reactivity: Lock & Key Theory The shape of a molecule plays an important role in its reactivity t-RNA Receptor, active site {MolecularShape&Reactivity} Molecular Shape determines function!

3 Molecular Geometries and Bonding Molecular Shape: Effect of Cocaine on CNS Dopamine binding to receptors and uptake pumps. Cocaine concentrates in areas of the brain that are rich in dopamine synapses. Cocaine binding to uptake pumps; inhibition of dopamine uptake when cocaine is present in the synapse. Cocaine (turquoise) binds to the uptake pumps and prevents them from removing dopamine from the synapse. This results in more dopamine in the synapse, and more dopamine receptors are activated. Presynaptic Neuron Postsynaptic Neuron synapse

4 Molecular Geometries and Bonding Molecular Shape determines function! DopamineCocaine

5 Molecular Geometries and Bonding Molecular Shapes 2. electron pairs, whether they be bonding or nonbonding, repel each other. 1.By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule. 3. By assuming the electron pairs are placed as far as possible from each other.

6 Molecular Geometries and Bonding Bonding Theories (2) Valence Bond (VB) Theory: considers the valence electrons and the quantum mechanical orbitals and how they hybridize to form bonds.(VB) Theory (1) Valence Shell Electron Pair Repulsion (VSEPR) Theory: considers only the valence electrons and their geometry when they repel each other when they form bonds. (VSEPR) Theory (3) Molecular Orbital (MO) Theory: uses bonding and antibonding molecular orbitals which best explain the energy characteristics of molecules.(MO) Theory

7 Molecular Geometries and Bonding Electron Domains We can refer to the electron pairs as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. This molecule has four electron domains.

8 Molecular Geometries and Bonding “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.” Valence Shell Electron Pair Repulsion Theory (VSEPR)

9 Molecular Geometries and Bonding These are the electron-domain geometries for two through six electron domains around a central atom. Electron- Domain Geometries {VSEPRTheory ElectrDomain}

10 Molecular Geometries and Bonding Electron-Domain Geometries How would the structure of CH 4 and NH 3 differ? Count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to that number of electron domains.

11 Molecular Geometries and Bonding Molecular Geometries The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs. The electron-domain geometry is often not the shape of the molecule, however.

12 Molecular Geometries and Bonding Linear Electron Domain In this domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

13 Molecular Geometries and Bonding Trigonal Planar Electron Domain There are two molecular geometries:  Trigonal planar, if all the electron domains are bonding  Bent, if one of the domains is a nonbonding pair.

14 Molecular Geometries and Bonding Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds. Therefore, they also affect bond angles.

15 Molecular Geometries and Bonding Tetrahedral Electron Domain There are three molecular geometries:  Tetrahedral, if all are bonding pairs  Trigonal pyramidal if one is a nonbonding pair  Bent if there are two nonbonding pairs

16 Molecular Geometries and Bonding

17 Molecular Geometries and Bonding Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

18 Molecular Geometries and Bonding Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry:  Axial  Equatorial SF 4 Where will the nonbonding electrons be for SF 4 ?

19 Molecular Geometries and Bonding Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial (where there is more space), rather than axial, positions in this geometry.

20 Molecular Geometries and Bonding Trigonal Bipyramidal Electron- Domain

21 Molecular Geometries and Bonding Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries:  Octahedral  Square pyramidal  Square planar

22 Molecular Geometries and Bonding Electron Domains determined from Molecular Geometries What is the electron domain geometries that led to the formation of these molecular structures? CO 2 : two double bonds SO 2 : trigonal planar SO 3 : trigonal planar (1 double bond) NF 3 : tetrahedral ClF 3 : trigonal bipyramidal (5 e- pairs on Cl)

23 Molecular Geometries and Bonding Larger Molecules: HC 2 H 3 O 2 In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole

24 Molecular Geometries and Bonding Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule. {OtherLargeMolecules}

25 Molecular Geometries and Bonding Molecular Structure

26 Molecular Geometries and Bonding Polarity Just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

27 Molecular Geometries and Bonding Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

28 Molecular Geometries and Bonding Polarity

29 Molecular Geometries and Bonding Valence Bond Theory : Orbital (s,p,d,f) Overlap and Bonding Covalent bonds are formed by the sharing of electrons from adjacent atoms whose orbitals overlap. Which orbitals are involved in the bonding of the molecules shown below? H2H2 HCl Cl 2

30 Molecular Geometries and Bonding Atomic Orbitals & Molecular Geometries? ……..could arise the molecular geometries we know to exist: tetrahedral, trigonal planar, etc. Atomic Orbitals Molecular Geometries It is hard to imagine that from the atomic orbitals we know (s, p x, p y & p z ) p orbitals have Angles that are 90 0 from each other sp sp 2 sp 3

31 Molecular Geometries and Bonding Hybrids: Mixing Orbitals But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds. Consider beryllium (Be): In its ground electronic state, it would not be able to form bonds because it has no singly- occupied orbitals. sp hybrid With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1s orbital but lower than the 2p. BeF 2 = : F : Be : F : (Be & B, accept less than octet) ¨¨ ¨¨

32 Molecular Geometries and Bonding Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals.  These sp hybrid orbitals have two lobes like a p orbital.  One of the lobes is larger and more rounded as is the s orbital. sp hybrid orbitals for Beryllium

33 Molecular Geometries and Bonding Hybrid Orbitals These two degenerate orbitals would align themselves 180  from each other. This is consistent with the observed geometry of beryllium compounds: linear. BeF 2 = : F : Be : F : (Be & B, accept less than octet) ¨¨ ¨¨ ¨ : ¨ : ¨ ¨

34 Molecular Geometries and Bonding Hybrid Orbitals Using a similar model for boron (B) leads to… BF 3

35 Molecular Geometries and Bonding Hybrid Orbitals …three degenerate sp 2 orbitals. BF 3

36 Molecular Geometries and Bonding Hybrid Orbitals With carbon we get… CH 4

37 Molecular Geometries and Bonding Hybrid Orbitals …four degenerate sp 3 orbitals. CH 4

38 Molecular Geometries and Bonding Hybrid Orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids. PF 5

39 Molecular Geometries and Bonding Hybrid Orbitals This leads to five degenerate sp 3 d orbitals… …or six degenerate sp 3 d 2 orbitals. PF 5 SF 6

40 Molecular Geometries and Bonding Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom. {HybridOrbitals} (1) (2) (3) (4) (5)

41 Molecular Geometries and Bonding Valence Bond Theory: Bond Types Hybridization is a major player in this approach to bonding. There are two ways orbitals can overlap to form bonds between atoms.

42 Molecular Geometries and Bonding Sigma (  ) Bonds Sigma bonds are characterized by  Head-to-head overlap.  Cylindrical symmetry of electron density about the internuclear axis. H 2 s-s bond Cl 2 p x -p x bond HCl s-p x bond

43 Molecular Geometries and Bonding Pi (  ) Bonds Pi bonds are characterized by  Side-to-side overlap.  Electron density above and below the internuclear axis.

44 Molecular Geometries and Bonding Single Bonds Single bonds are always  bonds, because  overlap is greater, resulting in a stronger bond and more energy lowering.

45 Molecular Geometries and Bonding Multiple Bonds In a multiple bond one of the bonds is a  bond and the rest are  bonds.

46 Molecular Geometries and Bonding Multiple Bonds In a molecule like formaldehyde (shown at left) an sp 2 orbital on carbon overlaps in  fashion with the corresponding orbital on the oxygen. The unhybridized p orbitals overlap in  fashion.

47 Molecular Geometries and Bonding Multiple Bonds In triple bonds, as in acetylene, two sp orbitals form a  bond between the carbons, and two pairs of p orbitals overlap in  fashion to form the two  bonds. H C C H

48 Molecular Geometries and Bonding Delocalized Electrons: Resonance When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.

49 Molecular Geometries and Bonding Delocalized Electrons: Resonance In reality, each of the four atoms in the nitrate ion has a p orbital. The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.

50 Molecular Geometries and Bonding Delocalized Electrons: Resonance This means the  electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.

51 Molecular Geometries and Bonding Resonance The organic molecule benzene has six  bonds and a p orbital on each carbon atom.

52 Molecular Geometries and Bonding Resonance In reality the  electrons in benzene are not localized, but delocalized. The even distribution of the  electrons in benzene makes the molecule unusually stable. or

53 Molecular Geometries and Bonding Molecular Orbital (MO) Theory Considers the wave nature of electrons. If waves interact constructively, the resulting orbital is lower in energy: a bonding molecular orbital. If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.

54 Molecular Geometries and Bonding MO Theory In H 2 the two electrons go into the bonding molecular orbital. The bond order is one half the difference between the number of bonding and antibonding electrons (2 - 0) = 1 BOND ORDER:

55 Molecular Geometries and Bonding MO Theory In the case of He 2, the bond order would be 1212 (2 - 2) = 0 Therefore, He 2 does not exist. {MO Theory}MO Theory BOND ORDER:

56 Molecular Geometries and Bonding MO Theory For atoms with both s and p orbitals, there are two types of interactions:  The s and the p orbitals that face each other overlap in  fashion.  The other two sets of p orbitals overlap in  fashion.

57 Molecular Geometries and Bonding MO Theory The resulting MO diagram looks like this. There are both  and  bonding molecular orbitals and  * and  * antibonding molecular orbitals.

58 Molecular Geometries and Bonding MO Theory The smaller p-block elements in the second period have a sizeable interaction between the s and p orbitals. This flips the order of the s and p molecular orbitals in these elements.

59 Molecular Geometries and Bonding Magnetism: Ferro-, Dia- & Para- All substances are magnetic. The science of physics has divided substances into 3 magnetic categories, they are: 1.Ferromagnetic, e.g., Fe, Ni, Permalloy (alloy 20% Fe and 80% Ni) Exert strong lines of force when exposed to a magnetic field, for example, the familiar magnetic compass. 2.Diamagnetic (weakly repelled) e.g., Cu, Ag, H 2 O, Liquid N 2 3.Paramagnetic (weakly attracted) e.g., Air, Liquid Oxygen, Platinum. Magnetic effects are so minute, that they need instrumentation for detection.

60 Molecular Geometries and Bonding

61 Molecular Geometries and Bonding Paramagnetism Unpaired electrons have their spins aligned  or   This increases the magnetic field of the atom. Atoms with unpaired electrons are called paramagnetic.  Paramagnetic atoms are attracted to a magnet.

62 Molecular Geometries and Bonding Diamagnetism Paired electrons have their spins unaligned .  Paired electrons have no net magnetic field. diamagneticAtoms with unpaired electrons are called diamagnetic.  Diamagnetic atoms are repelled by a magnet. {para Vs dia}Vs

63 Molecular Geometries and Bonding Second-Row MO Diagrams


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