CHEM 261 1st Edition Lecture 4Outline of Last Lecture:I. Recap of Hybrid OrbitalsII. Electronegativity and Polar Covalent BondsOutline of Current Lecture:I. Polar Covalent Bonds and Electronegativity RecapII. Dipole Moment RecapIII. Formal ChargeIV. ResonanceV. Brønsted-Lowry Acids and BasesPolar Covalent Bonds and Electronegativity Recap:Covalent bond- complete sharing of electronsPolar covalent bond: non-equal sharing of electrons (partial +, partial -)Ionic Bond: transfer of electrons, electrostatic interactionsDipole Moment Recap:-Directed in the direction of the negative charge (where lone electron pairs are). Arrows show where electrons “go”-Lone pair contributes to direction of electrons**absence of dipole moment when μ= 0 (charges cancel each other out)Formal Charge:-Calculate formal charge on individual atoms. Formal charge is related to how many electrons that atom is “seeing”How to calculate: (# of valence electrons) - # of bonds formed by atom - # of nonbonding electrons on atomResonance: -A tool to help draw structures in 2 dimensions-In reality, resonance structures exist somewhere in between the states depictedClass Example:Multiple Resonances:To know about resonance: 1. Resonance bonds only change for π (pi bonds) or non-bonding electrons2. Octet rule still applies3. Remember electronegativity! (negative charges go on electronegative atoms)** you’re never changing the connections!Brønsted-Lowry Acids and Bases:Acid: Donates an H+Base: Accepts an H+XH + SX- + HS (XH= acid, S=base, X-=conjugate base, HS= conjugate acid)Acid strength and pKa:pKa = -log(Ka), where Ka is the acid dissociation constant**big Ka value = strong acid**big pKa value = weak acid** -pKa = very strong
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