Unit 4 Chemical Bonding Slides PDF

Chemical Bonding Unit 4 How could you act out two different types of bonding? Ionic Molecular (Covalent) Metal + nonmetal (or polyatomic) Nonmetals only 2 elements with very different EN 2 elements with similar EN Transfer of electrons Shari...

Chemical Bonding Unit 4 How could you act out two different types of bonding? Ionic Molecular (Covalent) Metal + nonmetal (or polyatomic) Nonmetals only 2 elements with very different EN 2 elements with similar EN Transfer of electrons Sharing of electrons Lewis Dot Symbols Gilbert Lewis American Chemist (1875 - 1946) Atoms combine to achieve a more stable electron configuration Maximum stability results when an atom is isoelectronic with a noble gas Lewis Dot Symbols Method of keeping track of valence electrons → main actors in bonding One dot for each valence electron Notice Pattern with Periodic Table Columns Ionic Compounds form Lattices Chapter 2.6 p43 Ionic Substance Properties Solids at room temperature (high melting points) Conduct electricity when dissolved in water (aq), or when melted (l) ○ Animation1 and Animation 2 of of ionic substance (salt) breaking up in water Ability to form a solution (solubility) varies with strength of the overall bonding of the lattice (Lattice Energy) Lattice Energy = Energy required to completely separate one mole of solid ionic compound into gaseous ions = a measure of the “stability” of the lattice = - k (q1q2)/r k = constant, q1&q2 = charges on ions, r = distance between ions → as q1 and q2 increase, lattice energy increases For LiF, q1 = +1, q2 = -1 For MgO, q1 = +2, q2 = -2 (multiplicative effect) (Concepts in 9.3, first → as r increases, lattice energy decreases two paragraphs + last paragraph)  Lattice Energy Values Ionic Compound Lattice Energy (kJ/mol) Melting Point (deg C) LiF 1020 845 LiCl 828 610 NaCl 799 801 NaBr 736 750 KCl 699 772 MgCl2 2527 714 MgO 3890 2800 CaO 3414 2580 Table 9.1 p293 Higher lattice E → more stable solid → more tightly held ions → higher mp Graphs & Good Summary Linked Here  Bond Type Bond Length (pm) Average Bond C–H 107 Lengths of C–O 143 Covalent Bonds C=O 121 C–C 154 For same pair of C=C 133 atoms, C☰C 120 Single bonds > double bonds > triple C–N 143 bonds C=N 138 (multiple bonds more C☰N 116 stable) N–O 136 O–H 96  Figure 9.5 p296 Has rounded EN values This chart from: https://en.wiki pedia.org/wiki /Electronegati vity Another resource on EN: https://www.nagwa.com/en/explainers/896120791760/ Tips on Drawing Lewis Dot Structures/Diagrams 1. Place elements symmetrically → Lowest electronegativity in the center → H only forms one single bond (put around the outside) → F only forms one single bond (put around the outside) → Bond # TENDENCIES (not always): HONC-1234 → avoid rings - usually this crowds electrons too much 2. Add up total valence electrons and divide by 2 to get # of pairs → if negative ion, add electrons to total to match charge → if positive ion, remove electrons from total to match charge 3. Place pairs as single bonds and nonbonding pairs first 4. Check octet rule - move pairs as needed to complete → if atom A has enough and B has too few, make A share more → double, triple bond only if needed → don’t erase or add pairs Drawing Lewis Dot Structures - Videos Check the portal on our class page under Topics → Unit 4 for VIDEOS on: How to draw Lewis Structures: part 1, part 2 Formal charge Resonance Octet Exceptions But Molecules are 3D, not just 2D drawings! How do they spread out in space? Molecular Geometries (AKA Shapes of Molecules) Based on experimentally determined bond lengths and angles Geometry impacts properties (melting point, boiling point, density, types of reactions) Predictive Model of Shape: VSEPR VSEPR = Valence Shell Electron Pair Repulsion ○ Outer valence electrons are the participants ○ Negative electrons repel one another Count number of electron domains off the central atom ○ Domain = “branch of electrons” (i.e. nonbonding pair, single bonded pair, double bond) ○ Spread them out in space as equally and with as much space as possible 3D Activity with gumdrops, toothpicks, and marshmallows Molecule Shapes Tables Table 10.1: Arrangements of atoms with no nonbonded (lone) pairs Table 10.2: Arrangements of atoms with nonbonded (lone) pairs Mrs. Robinson’s combo table Now that you know 3D shape of molecules, you can determine if molecules are polar or nonpolar as a whole….. Polar/Nonpolar Bond Reminder…. And New Vocab When atoms of equal electronegativity are bonded → nonpolar bond ○ I.e. H-H or Cl-Cl → ΔEN = 0 = nonpolar ○ Electrons are shared equally between the atoms in a bond When atoms of unequal electronegativity are bonded → polar bond ○ I.e. H-Cl → ΔEN = 3.16 - 2.20 = 0.96 = polar ○ Electrons are shared unequally between atoms in a bond ○ Element with higher EN has a higher electron density (electrons spend more time there) New vocabulary: Polar bonds have a DIPOLE Dipole = unequal distribution of charge “Two poles” are marked with δ+ (smaller EN) and δ- (higher EN) Direction of electron “pull” can be shown with an arrow pointing towards higher EN side of bond with a “dipole moment” arrow Polar/Nonpolar Molecules Nonpolar Molecules One nonpolar bond: H2 or N2 Symmetric arrangement of polar bonds: CH4 Polar Molecules One polar bond: HCl Unsymmetric arrangement of polar bonds: NH3, CH3F, H2O Practice & Review Try to identify shape and polarity of lewis dot structures on past homework problems and worksheets Video (first 5 minutes is most relevant for this unit, last 5 is a sneak peak into understanding our Lab 7 more fully and Unit 6 material)