Lecture 13 - VSEPR, Molecular Polarity, Intermolecular Forces

"The basic postulate of the VSEPR model is that the arrangement of the electron pairs in a valence shell is that which places them as far apart as possible" - Ronald Gillespie Chem. Soc. Rev., 1992, 21, 59-69 Class notes for Lecture 1-12: https://bricejurban.github.io/CHEM111/ Assignments this week: ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28699/assignments/998317"HW 6 - Periodic Trends and Ionic Bonding (Fri 2/23) ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28699/assignments/998318"HW 7 - Lewis Structures (Mon 2/26) Reading for this week: Chapter 7.5-7.10, Chapter 8 ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28699/modules/items/3002842"Midterm 2 Practice Exam Assignments next week: ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28699/assignments/945547"Midterm 2 (Tuesday (2/27) in-class) ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28699/assignments/993257"Emerging Tech in Chemistry (Friday (3/1) w/ replies by Sunday (3/3) Office Hours: Friday 11-1 CIC ﷟HYPERLINK "https://calendly.com/bricejurban/office-hours"By appointment Midterm 2 Topics Nomenclature Ionic Compounds Periodic Trends Radius Ionization Energy Quantum Calculations Quantum numbers Electron Configurations Ions, transition metals Lewis structures Formal Charges Resonance Octet Exceptions Today's Schedule: Tuesday (2/22) VSEPR Molecular Polarity Intermolecular Forces Looking Ahead Tuesday (2/27) Schedule Midterm 2 on chapters 4-7 Paper and Pencil Exam (no multiple choice) Bring: ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28698/files/13892147?wrap=1"Periodic Table ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28698/files/14221849?wrap=1"Periodic Chart of the ions, ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28699/modules/items/2977345"Quantum Equations ﷟HYPERLINK "https://boisestatecanvas.instructure.com/courses/28699/modules/items/3004107"Atomic Orbital Handout One single-sided 8.5" x 11" sheet of notes Calculator Molecules exist in three-dimensions dictated by electronic repulsion Valence-Shell Electron Pair Repulsion (VSEPR) (Gillespie-Nyholm) Theory 220px-Ronald_Gillespie.gif Sir-Ronald-Sydney-Nyholm.jpg Sir Ronald Sidney Nyholm (1917–1971) | Art UK
220px-Ronald_Gillespie.gif Sir-Ronald-Sydney-Nyholm.jpg Sir Ronald Sidney Nyholm (1917–1971) | Art UK VSEPR theory is used to predict the shape of molecules based on the idea that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. Consider both bonding (shared between atoms) and lone (non-bonding) pairs of electrons in the valence shell of the central atom. The geometric arrangement of the electron pairs determines the shape of the molecule. Lone pair-lone pair > Lone pair-bonding pair > Bonding pair-bonding pair. This means lone pairs repel more strongly than bonding pairs. Presence of lone pairs can alter the ideal bond angle and shape due to their greater repulsion. For example, NH₃ is trigonal pyramidal, not planar. To predict a molecule's shape, draw its Lewis structure, determine the number of bonded atoms and number of nonbonding electron pairs. Use this table to help you arrange them to minimize repulsion in order to deduce the molecular geometry. https://cms.gutow.uwosh.edu/gutow/VSEPR_TUTORIAL/shtml%20version/VSEPR_model.shtml (Visualizations of each shape) ﷟HYPERLINK "https://en.wikipedia.org/wiki/VSEPR_theory#AXE_method"VSEPR theory - Wikipedia (Helpful information about the theory) Electron Groups (Steric Number SN) (Bonded Atoms + Number of Lone Pairs) Electron Geometry Bonded Atoms (Coordination Number) Number of Nonbonding Lone Pairs Molecular Geometry (Molecular Shape) Bond Angle(s)* Example Molecules 3D Shape (Lone Pairs in Orange or Yellow) 2 Linear 2 0 Linear 180° CO₂, BeCl₂ Untitled picture.png 3 Trigonal Planar 3 0 Trigonal Planar 120° BF₃, AlCl₃ Untitled picture.png 3 Trigonal Planar 2 1 Bent <120° SO2, O3 Untitled picture.png 4 Tetrahedral 4 0 Tetrahedral 109.5° CH₄, SiCl₄ Untitled picture.png 4 Tetrahedral 3 1 Trigonal Pyramidal <109.5° NH₃, PCl₃ Untitled picture.png 4 Tetrahedral 2 2 Bent <109.5° H₂O, H₂S Untitled picture.png lone pair electrons repel more strongly causing a distortion in bond angle AX2 AX3 AX2E AX4 AX3E AX2E2
4 Tetrahedral 2 2 Bent <109.5° H₂O, H₂S Untitled picture.png 5 Trigonal Bipyramidal 5 0 Trigonal Bipyramidal 120°, 90° PCl₅, PF₅ Untitled picture.png 5 Trigonal Bipyramidal 4 1 See-Saw <120°, <90° SF₄, SeCl₄ Untitled picture.png 5 Trigonal Bipyramidal 3 2 T-shaped <90° ClF₃, BrF₃ Untitled picture.png 5 Trigonal Bipyramidal 2 3 Linear 180° XeF₂, I₃⁻ AX2E3-3D-balls.png undefined 6 Octahedral 6 0 Octahedral 90° SF₆, SeF₆ Untitled picture.png 6 Octahedral 5 1 Square Pyramidal <90° BrF₅, IF₅ Untitled picture.png 6 Octahedral 4 2 Square Planar 90° XeF₄, PtCl₄²⁻ 1024px-AX4E2-3D-balls.png undefined Electrons are removed from the equatorial sites first! This is to reduce the number of electron repulsions Two axial atoms and three equatorial atoms results in two different bond angles Water is specifically 104.5° but other atoms will be different AX2E2 AX5 AX4E AX3E2 AX2E3 AX6 AX5E AX4E2
1024px-AX4E2-3D-balls.png undefined 7 (Rare) Pentagonal Bipyramidal 7 0 Pentagonal Bipyramidal 90°, 72°, 180° IF7, ReF7 800px-AX9E0-3D-balls.png undefined 8 (Rare) Square Antiprismatic 8 0 Square Antiprismatic 109.5°, 99.6°, 70.5° TaF83– 800px-AX9E0-3D-balls.png undefined 9 (Rare) Tricapped Trigonal Prismatic 9 0 Tricapped Trigonal Prismatic 60° ReH9– 800px-AX9E0-3D-balls.png undefined *The bond angles in actual molecules may vary slightly Rules for Finding the Molecular Geometry Determine the correct Lewis structure for the molecule. If it is a diatomic (has only two atoms) it is linear. If it has 3 or more atoms continue with step 2. Count the number of electron groups around the central atom. A group of electrons is a bond or a nonbonding electron pair. Each triple or double bond counts as only one group for the purposes of this model. Based on this number of groups around the central atom the molecule falls into one of six basic categories. Within each category there are a number of different names for the shapes depending upon the number of atoms and nonbonding groups around the central atom. Class number (AXE notation): A = central atom X = atom attached to central atom E = nonbonding electron group on central atom TeF6 IF4+ XeOF4 Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings AX7 AX8 AX9
O3 Other examples for you to try! Make sure to complete the questions on the worksheet and submit on Gradescope SiH4 PCl5 BrF3 I3– SeF4 Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Ink Drawings Note the electron removed from the equatorial site <120°, <90° Ink Drawings Ink Drawings
IF5 PH3 Bond lengths and strengths When more electrons are shared between atoms, there is increased orbital overlap between the atoms. Therefore, double bonds have more overlap than single covalent bonds and thus are shorter and stronger. Furthermore, triple bonds have even more overlap than double bonds and are the shortest and strongest bonds. Here's a simple comparison of common bond lengths and their corresponding energies Pick up here next week after the midterm! Bond Bond Length (Å) Bond Energy (kJ/mol) C–C 1.54 345 C=C 1.34 611 C≡C 1.20 837 C–N 1.43 290 C=N 1.38 615 C≡N 1.16 891 C–O 1.43 350 C=O 1.23 741 C≡O 1.13 1080 Molecular Polarity Covalent bonds in a molecule can be polar or nonpolar. Polar bonds are referred to as dipoles because they exhibit a separation of electric charge that gives rise to two poles, a partial positive δ+ and a partial negative δ− creating a dipole moment μ that characterizes the bond's polarity.
Polar bonds are referred to as dipoles because they exhibit a separation of electric charge that gives rise to two poles, a partial positive δ+ and a partial negative δ− creating a dipole moment μ that characterizes the bond's polarity. The bonds in a molecule and its shape determine whether that molecule is classified as polar or nonpolar Nonpolar Molecules: All the bonds are nonpolar or the polar bonds (dipoles) cancel each other out, resulting in no dipole moment. Polar Molecules: One end of the molecule is more negatively charged than the other end, resulting in a net dipole CCl4 HCl Cl2 NH3 CH3F H2O OF2 Intermolecular Forces between Ions, Polar Covalent Molecules, and Nonpolar Covalent Molecules Type of Force Particle Arrangement Example Strength Explanation Between Atoms or Ions Ionic bonds The strong intermolecular forces between positive and negative charges creates solids with very high melting points.
Ionic bonds Na+Cl– The strong intermolecular forces between positive and negative charges creates solids with very high melting points. Covalent bonds (X = nonmetal) Cl−Cl The sharing of valence electrons results in an overlap between atoms that require energy to break. Between Molecules Hydrogen bonds (X = N, O, or F) Hδ+−Fδ− ••• Hδ+−Fδ− Polar molecules can form an attraction between the partially positive hydrogen atom in one molecule and the partially negative atom (F, O, and N) in another. Dipole-Dipole Attractions (X and Y = nonmetals) Hδ+−Clδ− ••• Hδ+−Clδ− For polar molecules, these attractions occur between the positive end of one molecule and the negative end of another. Dispersion Forces (temporary shift of electrons in nonpolar bonds) Fδ+−Fδ− ••• Fδ+−Fδ− Very weak attractions that result from a temporary dipole that align the molecules so the positive end of one is attracted to another. Although, weak they allow for nonpolar molecules to form liquids and solids and are important for protein folding Intermolecular Forces exist between the Base Pairs of DNA How to recognize Hydrogen bond donors and Acceptors In DNA Adenine (A) base pairs with Thymine (T) with 2 hydrogen bonds Guanine (G) base pairs with Cytosine with 3 hydrogen bonds The GC base pair takes more energy to break than an AT base pair, therefore sequences of DNA with lots of guanine and cytosine with have a higher melting point temperature and are less accessible by enzymes. Untitled picture.png Machine generated alternative text: NH -N C CH Adenine (Ade) Untitled picture.png Machine generated alternative text: CH3 Thymine (Thy)l
Untitled picture.png Machine generated alternative text: NH -N C CH Adenine (Ade) Untitled picture.png Machine generated alternative text: CH3 Thymine (Thy)l Untitled picture.png Machine generated alternative text: CH 142N uanine (Gua) Untitled picture.png Machine generated alternative text: NH NCH CH Cytosine (Cyt) There are lots of hydrogen bond donors and acceptors in the base pairs of DNA The left side of Adenine in the above image will base pair with the left side of Thymine because they can form two hydrogen bonds Hydrogen bond donor-acceptor pair 1 above Hydrogen bond acceptor-donor pair 2 above The left side of Guanine in the above image will base pair with the left side of Cytosine because they can form three hydrogen bonds
The left side of Guanine in the above image will base pair with the left side of Cytosine because they can form three hydrogen bonds Hydrogen bond acceptor-donor pair 3 above Hydrogen bond donor-acceptor pair 4 above Hydrogen bond donor-acceptor pair 5 above This base pairing is shown in the image below from the Color Atlas of Biochemistry This base pairing is only possible when the polarity of two strands of DNA are running in opposite directions. This forms a double helix structure! Untitled picture.png Machine generated alternative text: — A. DNA: structure 1. Formula CH3 0 o CH20H CH2 CH2 Minor groove 2. Double strand Major groove OH 2'-deoxy- D-ribose
Untitled picture.png Machine generated alternative text: — A. DNA: structure 1. Formula CH3 0 o CH20H CH2 CH2 Minor groove 2. Double strand Major groove OH 2'-deoxy- D-ribose

 

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