Molecular Shapes, Valence Bond Theory, and Molecular

Trick to learn shapes of molecules | Geometry of molecules | VSEPR Theory
Trick to learn shapes of molecules | Geometry of molecules | VSEPR Theory

Molecular Shapes, Valence Bond Theory, and Molecular Lecture Presentation Chapter 10 Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory Catherine MacGowan Armstrong Atlantic State University © 2013 Pearson Education, Inc.

MOLECULAR GEOMETRY AND VSEPR Valence Shell Electron Pair Repulsion VSEPR is used to predict the molecular geometries of compounds based on the number of electron groups attached to the center atom of the molecule. Most important factor in determining the geometry is minimizing the relative repulsion between electron pairs. Molecules’ or ions’ geometry reflects arrangement that favors minimal electron repulsion.

Electron Pair Geometries: Family Groups

Structure Determines Properties The chemical and physical properties of molecular substances depend on the structure of the species. Factors to be considered: Skeletal arrangement of the atoms Who is bonded to who Type of intramolecular bonding between the atoms ionic, polar covalent, or covalent Shape of the molecule Bonding theory should allow you to predict the shapes of molecules.

Molecular Geometry Molecules are three-dimensional objects, so their shape can be described in terms related to geometric figures. Example: Linear Tetrahedral Octahedral Geometric shapes have characteristic: “corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure, and angles (e.g., bond angles)

Lewis Structures and VSEPR Theory A Lewis structure Predicts the number of valence electron pairs 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 a single, double, or triple bond

Lewis Structures and VSEPR Theory Based on the number of electron groups attached to the center atom of the molecule The arrangement of the atoms attached to the center atom minimizing the relative repulsion between electron pairs Note: It is the arrangement of atoms attached to the center atom that dictates the compound’s structure (electron pair geometry) and shape (molecular geometry of atoms).

Geometric Arrangement of Electron Groups Around Center Atom: Electron Group Geometries Five types of arrangements (families) of electron groups around a central atom Types of electron groups Atoms Lone pairs The five geometric arrangements (families) Linear Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral

Geometric Arrangement of Electron Groups Around Center Atom: Electron Group Geometries Linear Two electron groups attached to the center atom of a two-atom molecule Bond angle between electron groups and center atom: 180o

Geometric Arrangement of Electron Groups Around Center Atom: Electron Group Geometries Trigonal planar Three electron groups attached to the center atom with each electron group occupying a corner position on a triangle Bond angle between electron groups: 120o

Geometric Arrangement of Electron Groups Around Center Atom: Electron Group Geometries Tetrahedral Four electron groups surrounding the center atom with electron groups occupying corner positions of a tetrahedron Bond angle between electron groups: 109.5o

Geometric Arrangement of Electron Groups Around Center Atom: Electron Group Geometries Trigonal bipyramidal Five electron groups attached to the center atom The electron groups will occupy positions at the axial position (above and below the center atom) and at the equatorial positions in the base plane of the center atom. Bond angle between equatorial positions is 120°. Bond angle between axial and equatorial positions is 90°.

Geometric Arrangement of Electron Groups Around Center Atom: Electron Group Geometries Octahedral Six electron groups attached or surrounding the center atom Electron groups in the shape of two square base pyramids that are base to base, with the central atom in the center of the shared bases It is called octahedral because the geometric figure has eight sides. All positions are equivalent. The bond angle is 90°.

Geometric Derivations or Why Distortions Occur Why do distortions in a molecule’s three-dimensional structure occur? Lone pairs Require more space Need for space causes a distortion in the bond angle between the atoms. Types of atoms bonding to the center atom Size of atom Types of bonds between the bonding groups. Single vs. multiple What results? The molecular geometry (shape) will vary Shape will be a derivative of one of the basic shapes.

Chart of Electron and Molecular Geometries

Chart of Electron and Molecular Geometries

Geometric Derivations and Placement of Lone Pairs Placement of lone pairs around a center atom: Look at the geometry of the molecule. Want to minimize the electron-electron repulsion Want to occupy a position within the structure that has space to allow for “spreading out” Geometries that DO NOT have ‘”minimizing” positions around the center atom or where all bond angles are equally spaced: Linear, trigonal planar, tetrahedral, and octahedral

The Effect of Lone Pairs on a Compound’s Molecular Geometry (Shape) Lone pair/nonbonding electron groups “occupy more space” on the central atom. Why? Electron density is exclusively on the central atom rather than shared like bonding electron groups. This affects the bond angles, making them smaller than expected. The relative sizes of repulsive force interactions are: (Lone Pair – Lone Pair) > (Lone Pair – Bonding Pair )> (Bonding Pair – Bonding Pair )

Distortion of Trigonal Planar Geometric Group Happens when one of the electron groups is a lone pair One lone pair + 2 atoms attached to center atom Electron geometry is trigonal planar but the shape of molecule (molecular geometry) is BENT. The bond angle between groups is <120°. :O = S – O: :O – S = O: :O = S = O: . . . . . . . . . . . . . . . . . . . . . .

H2O: Tetrahedral Bent Shape Bonding pair H : O : H Lone pair . . . .

Distortion of Tetrahedral Geometric Group It happens when ONE or TWO lone pairs surround the center atom. All belong to the tetrahedral electron geometric group but will have different molecular geometries (shape). Pyramidal: 1 lone pair surrounding center atom + 3 atom groups attached to center atom Bent or V shape: 2 lone pairs surrounding center atom + 2 atom groups attached to the center atom When there are 4 electron groups around the central atom Bond angle for both distorted tetrahedral shapes is <109.5°.

Trigonal Bipyramidal Geometry and the Placement of Lone Pairs Positions of trigonal bipyramidal structure: Equatorial BOND angle between equatorial positions is <120°. Axial bond angle between axial and equatorial positions is <90°. Lone pairs will occupy the equatorial positions because there is more room. .

Distortion of Trigonal Bipyramidal Geometric Group It happens when ONE, TWO, or THREE lone pairs surround the center atom. All belong to the trigonal bipyramidal electron geometric group but will have different molecular geometries (shapes). See Saw or Sawhorse: 1 lone pair at one of equatorial positions surrounding center atom + 4 atom groups attached to center atom

Distortion of Trigonal Bipyramidal Geometric Group It happens when ONE, TWO, or THREE lone pairs surround the center atom. All belong to the trigonal bipyramidal electron geometric group but will have different molecular geometries (shapes). T shape: 2 lone pairs at the equatorial positions surrounding center atom + 3 atom groups attached to the center atom

Distortion of Trigonal Bipyramidal Geometric Group It happens when ONE, TWO, or THREE lone pairs surround the center atom. All belong to the trigonal bipyramidal electron geometric group but will have different molecular geometries (shapes). Linear shape: 3 lone pairs at the equatorial positions surrounding center atom + 2 atom groups attached to the center atom

Distortion of the Octahedral Geometric Group Six electron groups (atoms and/or lone pairs) surrounding the central atom indicates an octahedral electron geometry (family). Distortions happen when one or more lone pairs occupy the six positions. All positions in this geometry have the same bond angle (90o) when atom groups occupy the sites. Therefore, no site is more favorable when substituting lone pair groups for atoms. In distorted geometric structures the bond angle between axial and equatorial positions will be <90°.

Distortion of the Octahedral Geometric Group It happens when ONE or TWO lone pairs surround the center atom. All belong to the octahedral electron geometric group but will have different molecular geometries (shapes). Square pyramidal: 1 lone pair at either the equatorial or axial position surrounding center atom + 5 atom groups attached to the center atom

Distortion of the Octahedral Geometric Group It happens when ONE or TWO lone pairs surround the center atom. All belong to the octahedral electron geometric group but will have different molecular geometries (shapes). Square planar: 2 lone pairs at either the equatorial or axial positions surrounding center atom + 4 atom groups attached to the center atom

Geometric Arrangements and Resonance Structures To achieve the best structure (energy-wise), a compound’s electron groups must be bonds and all the bonds must be equivalent. For molecules that exhibit resonance, the molecular geometry will be the same for all possible arrangements. Example: NO3- Electron geometry (family) is trigonal planar. Molecular geometry (shape) is also trigonal planar.

Predicting the Electron (Family Group) and Molecular (Shape) Geometries of Compounds Draw the Lewis structure of the compound. Determine the number of bonding groups (atoms bonded to center atom) and lone pairs surrounding the central atom. Bonding groups (atoms bonded or attached to center atom): You are counting the number of atoms bonded to the center atom and NOT the number of bonds between the atoms. An atom double bonded to the center atom counts as ONE attachment. Determine the geometries. Electron (family group): TOTAL the number of atoms (attachments) to the center atom + the number of lone pairs surrounding the center atom. Example: If there are 3 atoms bonded (attached) to the center atom plus 2 lone pairs surrounding the center atom, the TOTAL number is 5. 5 = trigonal bipyramidal geometry Molecular (shape): The electron geometry dictates the molecular geometry associated with that geometric grouping. Example: If the compound belongs to the trigonal bipyramidal (total of 5) and has 3 bonded atoms (attachments) and 2 lone pairs surrounding the center atom, then the molecular shape of the molecule is T-shape.

Problem: Determine the electron and molecular geometry for the compound ammonia, NH3. Draw electron dot structure. Count # of attachments and lone pairs on center atom. Three attachments (hydrogen atoms) to the center atom (nitrogen) and one lone pair = 4 3. Electron pair geometry (family) for combination of 4 is tetrahedral . 4. The molecular geometry (shape) is trigonal pyramidal.

Problem: Determine the electron geometry (family) and molecular geometry (shape) for the molecule ClO2F.

Problem: Determine the electron geometry (family) and molecular geometry (shape) for the molecule BF3. Answer: B is the least electronegative of atoms in the molecule so it will be the center atom. All atoms will be attached to the B atom. Draw the Lewis structure: B = 3 e─ F = 3 x 7 e- 21 e─ Total = 24 e─ 3 electron groups on the center atom B Electron geometry group (family) is trigonal planar. All groups surrounding the center atom are atom groups (no lone pairs); therefore ,the molecular geometry (shape) of BF3 is trigonal planar.

Not-Quite-Perfect Geometry NOTE: Because the bonds are not identical, the observed angles are slightly different from the ideal.

Multiple Central Atoms and Their Geometries Large structured molecules have more than ONE center atom implying that they have multiple central atoms Molecules with more than one center atom, therefore, have more than one type of geometry. Geometries (electron and molecular) are assigned for each center atom in the molecule.

VESPR and Multi-Center Molecules Methanol, CH3OH Methanol has two center atoms. Center one: H-C-H = 109o Electron geometry: Tetrahedron Molecular geometry: Tetrahedral Center two: – C-O-H <109o Molecular geometry: Bent

Representing Three-Dimensional Shapes on a Two-Dimensional Surface It is difficult to draw molecules showing their dimensionality. Convention dictates that the central atom is put in the plane of the paper. To indicate on paper the spatial relationship of the atom groups and/or lone pairs to the center atom, the following guidelines and symbols are used: Put as many other atoms as possible in the same plane and indicate with a straight line. For atoms in front of the plane, use a solid wedge. For atoms behind the plane, use a hashed wedge.

Molecular Polarity Question: Why are some molecules polar in nature and others nonpolar? Answer: Look at the bonding and their three-dimensional structure (geometry). Molecules to be polar need to have: Polar covalent bonds within the molecule Difference in bond atoms’ electronegativity A net dipole moment because the molecule’s electron geometry is NOT “symmetric” Symmetric molecules DO NOT have a dipole moment. The molecule is NONPOLAR. Unsymmetric molecules DO have a dipole moment. The molecule is POLAR. Polarity affects the intermolecular forces of attraction. Boiling points and solubilities Like dissolves like Nonbonding pairs affect molecular polarity; they pull the electron density strongly.

Dipole Moment and Polarity

Polar or Nonpolar? Which one is polar: CO2 or H2O? H2O CO2

Molecular Polarity Explains solubility of substances Likes dissolve in likes. Provides information on a substance’s chemical and physical properties Melting points and boiling points H2O vs. CH4 Bp: 100 oC Bp: -161 oC

Molecular shape in food science:the production of artificial sweeteners(人工甘味劑), e.g., aspartame (200 times more sweet than sugar) and saccharin. Artificial sweeteners taste sweet but have few or no calories, because taste and caloric value are independent properties of food. The surface of a taste cell contains specialized protein molecules called taste receptors called Tlr3. When sucrose enters the active site of Tlr3, different subunits of the receptor Split apart and this split causes ion channels in the cell membrane to open, resulting in nerve transmission that registers a sweet taste in the brain. Artificial sweeteners taste sweet because they fit into the receptor pocket that normally binds sucrose.

Chapter 10, Opener

Theories of Chemical Bonding

Problems with Lewis Theory as a Bonding Theory generally predicts trends in properties does not give good numerical predictions for bond strength and bond length A good first approximation of the bond angles in molecules, but usually cannot be used to get the actual angle Resonance: Cannot write a “single” correct structure for many molecules Does not predict the correct magnetic behavior of molecules O2 is paramagnetic, yet Lewis structure predicts it to be diamagnetic.

VALENCE BOND THEORY (VB) MOLECULAR ORBITAL THEORY (MO) Theories of Bonding VALENCE BOND THEORY (VB) Valence electrons are localized between atoms (or are lone pairs). Half-filled atomic orbitals overlap to form bonds. MOLECULAR ORBITAL THEORY (MO) Delocalization of valence electrons within molecule Valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

Pi bonding occurs when: electron orbitals of the atoms overlap sideways, resulting in electron density ABOVE and BELOW the plane of the bond’s two atomic nuclei centers. See pi bonding with p and d orbitals, NOT with s Sigma bonding occurs when: electron orbitals of the atoms overlap within the plane of the bond’s two atomic nuclei centers. Have sigma bonding with: s to s orbitals p to p orbitals ONLY if head to head

Sigma Bond (s) and Pi (P) The electron density between two atoms The electron density within the plane of the bond Can be formed from hybridized or unhybridized orbitals Every bond contains a sigma component. Single bonds have only a sigma component. Pi Bond The electron density above and below the plane of the bond Formed from unhybridized orbitals. Pi bonds are observed in multiple bonds. Double bonds have ONE pi component. Triple bonds have TWO pi components.

Sigma Bond Formation by Orbital Overlap

Valence Bond Theory

Valence Bond Theory It is the theory involving the overlap of atomic orbitals to form bonds. Uses VSEPR. The valence electrons in an atom reside in the quantum-mechanical atomic orbitals or hybrid orbitals. The electrons must be spin paired. A chemical bond results when these atomic orbitals interact (overlap) and there is a total of two electrons in the new molecular orbital. When direct overlap of atomic orbital cannot or does not explain experimental finding, then hybridization of atomic orbitals of the CENTER atom occurs. The shape of the molecule is determined by the geometry of the interacting orbitals. The electron geometry of the molecule or ion dictates the hybridization of the atomic orbitals on the CENTER atom. Problem: Can NOT explain bonding with molecules or ions bringing in an odd number of valence electrons

Bonding in H2S Electron configuration of each of the atom’s valence electrons The Lewis dot structure and distribution of hydrogen’s electrons in sulfur’s atomic orbitals coincide with the atomic orbitals of the hydrogen atoms. The atomic orbitals on the S atom (center atom) can accommodate the 1s atomic orbitals from the hydrogen atoms without hybridization. .. .. S H H

Valence Bond Theory and Hybridization is the “mixing” of the center atom’s valence atomic orbitals arises when the number of partially filled or empty atomic orbitals of the center atom did not predict the number of bonds or orientation of bonds between the center atom and the other bonding atom that actually occurred Example: C = 2s22px12py12pz0 would predict two or three bonds that are 90° apart, rather than four bonds that are 109.5° apart.

Valence Bond Theory and Hybridization To explain inconsistencies, it was postulated that the valence atomic orbitals of the center atom could hybridize before bonding took place. One hybridization of C is to mix all the 2s and 2p orbitals to get four orbitals that point to the corners of a tetrahedron.

Bonding in a Tetrahedron Formation of Hybrid Atomic Orbitals

Electron Geometry (Family) Hybridization Chart

Predict the Bonding in Glycine Molecule

Predict the Bonding in Glycine Molecule

Predict the Bonding in Glycine Molecule

Predict the Bonding in Glycine Molecule

Predict the Bonding in Glycine Molecule

Hybridization and Electron Pair Geometry Relationship Hybridization is about the center atom’s atomic orbitals undergoing a “transformation” to accommodate electrons from other atoms to form bonds. Hybridization type corresponds to the electron geometry (family) of the compound. *Remember: the electron geometry was determined by the number of attachments and nonbonding electrons around the center atom. Hybridization types Electron Geometry Hybridization Comments linear sp triple bond trigonal planar sp2 double bond tetrahedron sp3 trigonal bipyramidal dsp3 octahedron d2sp3

sp2 Hybridization and Double Bonds sp2 hybridization is seen with double bond formation. Double bonds: 4 electrons or 2 electron pairs Remember: not all elements can double bond Elements that can double bond with themselves and each other are C, S, N, O, and P Have sigma and pi components Sigma bonds: electron density within the plane of the bond Pi bonds: electron density above and below the plane of the bond

Formation of sp2 Hybrid Orbitals

Pi (p) Bonding in CH2O (Formaldehyde) Formaldehyde (Formalin): Valence electron configuration for elements in formaldehyde Center atom in the molecule is C. Its atomic orbital needs to undergo hybridization: The unused p orbital on the C atom contains an electron, and this p orbital overlaps the p orbital on the O atom to form a π bond.

Pi (p) Bonding in CH2O (Formaldehyde) Formaldehyde (Formalin):

Movement Around a Double Bond Degrees of rotation (movement) of atoms The more electrons between two atoms in a bond, the more restrictive the rotation. Single > Double > Triple Bonding orbitals that form the s bond point along the internuclear axis; rotation around that bond does not require breaking the interaction between the orbitals. Bonding orbitals that form the p bond interact above and below the internuclear axis, so rotation around the axis requires the breaking of the interaction between the orbitals.

sp in Bonding and Triple Bonds sp hybridization is seen with triple bond formation. Triple bonds: 6 electrons or 3 electron pairs Remember: not all elements can triple bond Elements that can triple bond with themselves and each other are C, S, N, and O Have sigma and pi components Sigma bonds: electron density within the plane of the bond Two pi bonds: electron density above and below the plane of the bond Center atom: Uses hybrid orbitals for s bonds or lone pairs Forms p bonds from two sets nonhybridized p orbitals Electron and molecular geometry linear shape 180° bond angle

sp in Bonding and Triple Bonds

sp in Bonding C2H2: Triple Bonds

sp3d: Hybridization for Trigonal Bipyramidal Geometries Center atom has five electron groups (lone pairs and/or atom groups) around it. Molecular geometry (shape) Trigonal bipyramidal Seesaw T-shape Linear Center atom uses empty d orbitals from valence shell for hybridization. d orbitals can be used to make p bonds.

sp3d: Hybridization for Trigonal Bipyramidal Geometries

sp3d: Hybridization AsF5

sp3d2: Hybridization for Octahedral Geometries Center atom has six electron groups (lone pairs and/or atom groups) around it. Molecular geometry (shape) Octahedral Square pyramidal Square planar Center atom uses empty d orbitals from valence shell for hybridization. d orbitals can be used to make p bonds.

sp3d2: Hybridization for Octahedral Geometries

sp3d2 Hybridization for SF6

Problems with Valence Bond Theory VB theory predicts many properties better than Lewis theory. Bonding schemes Bond strengths Bond lengths Bond rigidity VB theory does not predict perfectly a compound’s magnetic behavior. VB theory presumes that the electrons are localized in orbitals on the atoms in the molecule. It doesn’t account for delocalization.

Molecular Orbital Theory (MO)

VB Theory vs. MO Theory Valance Bond Uses VESPR and hybridization of atomic orbitals to explain bond formation Predicts only the type of hybridization, NOT which molecules will hybridized Can only explain bonding with compounds bringing in an even number of valence electrons Molecular Orbital When elements bond, a bonding orbital forms. Bonding Antibonding(*) Can explain even and odd valence electron compounds Correctly predicts bond strengths and magnetic properties of compounds Unpaired: paramagnetic Paired: diamagnetic Correctly predicts whether a molecule/ion exists Bond order

Molecular Orbital Theory (MO) MO theory makes the assumption that when two elements form a bond, a new molecular orbital bonding entity is formed. NOT the mixing of atomic orbitals, as in VB hybridization Electrons entering MO follow Pauli’s exclusion principle (lowest to highest energy) and Hund’s rule for MO of same energy (electrons will parallel spin) Valence electrons are delocalized. These electrons are NOT located specifically above or below the plane of the bond. Valence electrons are in orbitals (called molecular orbitals) spread over entire molecule. The basis for MO theory comes from quantum mechanics.

Molecular Orbital Theory Electrons in bonding MOs are stabilizing. Lower energy than the atomic orbitals Electrons in anti-bonding MOs are destabilizing. Higher energy than atomic orbitals Electron density located outside the internuclear axis Electrons in anti-bonding orbitals cancel stability gained by electrons in bonding orbitals

Molecular Orbitals When the wave functions combine constructively, the resulting molecular orbital has less energy than the original atomic orbitals and is called a bonding molecular orbital. s, p Most of the electron density between the nuclei When the wave functions combine destructively, the resulting molecular orbital has more energy than the original atomic orbitals and is called an anti-bonding molecular orbital. s*, p* Most of the electron density outside the nuclei Nodes between nuclei

Molecular Orbital Theory 1. The number of MOs formed = number of atomic orbitals used Bonding MOs have lower energies than atomic orbitals. Anti-bonding (*) MO is higher-level energy than atomic orbitals and bonding MOs. Two electrons per molecular orbital Electrons enter into to the lowest energy molecular orbital and upward. MO Diagram

Illustration of Molecular Orbital: ss

Illustration of Molecular Orbital: sp

Illustration of Molecular Orbital: pp

Molecular Orbitals and the Property of Bond Order Bond order is: about strength – the greater the value, the stronger and shorter the bond between the atoms determined by taking the difference between number of electrons in bonding and anti-bonding orbitals Bond order = (# bonding electrons – # anti-bonding electrons) 2 Only need to consider valence electrons May be a fraction A bond order with a value of ZERO indicates an unstable species Less stable than individual atoms

Does He2 Exist as a Molecule? Bond order predicts whether or not a molecule or ion exists. Bond order according to MO theory 1/2 [# e- in bonding MOs – # e- in anti- bonding MOs] He2 bond order: ½ (2 – 2) = 0 A zero bond order indicates that He2 does not exist as a molecule.

Molecular Orbital Diagram of Valence Electrons for B2 B2 brings in 3 valence electrons for each boron atom. Valence electrons enter MO orbitals from lowest energy upward. Two electrons/orbital Bond order 1/2 (4 – 2) = 1 Molecule is paramagnetic Has unpaired electrons

Molecular Orbitals and the Property of Magnetism Bonding theories and magnetism: Lewis theory as well as valance bond theory do not predict a molecule’s magnetic properties. Molecular orbital (MO) theory can predict magnetic properties. Magnetism: results when unpaired electron occur Materials that exhibit magnetic properties are referred to as PARAMAGNETIC. Paramagnetic: unpaired electrons Materials that DO NOT exhibit magnetic properties are known as DIAMAGNETIC. Diamagnetic: all paired electrons Example: O2 is paramagnetic.

Molecular Orbitals and the Property of Magnetism in O2 O2 is paramagnetic. MO diagram for oxygen

Predict the more stable species: ___ * 2p *2p ___ ___ *2p 2p ___ ___ 2p ___ 2p ___ * 2s ___ 2s ___ * 2p *2p ___ ___ *2p 2p ___ ___ 2p ___ 2p ___ * 2s ___ 2s ___ * 2p *2p ___ ___ *2p 2p ___ ___ 2p ___ 2p ___ * 2s ___ 2s Bond order = 2 Bond order = 2 1/2 Bond order = 1 1/2

MO Diagrams of 2nd Row Nonmetals

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