Lecture+18_Polyatomic+nomenclature_Begin+VSEPR+and+Hybridization

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University of Pennsylvania *

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Chemistry

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Oct 30, 2023

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Announcements for October 11 th , 2023 The plan: • Polyatomic ions and nomenclature • VSEPR and Hybridization Theory Monday reading : Chapter 8, Sections 6 – 8; Chapter 2, Section 8 (polyatomic ions) Wednesday reading: Chapter 9, Sections 1, 2, 4, 5 ( VSEPR Simulator ) • Homework 6 due Monday, October 16 th by 11:59 PM • Fall Break is Thursday and Friday this week – no recitation, no Friday lecture

Some commonly occurring ions are polyatomic Ammonium: [NH 4 ] + Acetate: [CH 3 CO 2 ] - Hydroxide: [OH] - Carbonate: [CO 3 ] -2 Nitrate: [NO 3 ] - Phosphate: [PO 4 ] -3 Sulfate: [SO 4 ] -2 Cyanide: [CN] - • Molecular ions held together internally by covalent bonds • Have net negative charge (usually) due to octet (closed valence shell) electron configuration Perchlorate: [ClO 4 ] -

Periodic approach to remembering polyatomic anions Group 14 Group 15 Group 16 Group 17 Carbon ate : 𝐶? 3 −2 Nitr ate : ?? 3 − Nitr ite : ?? 2 − Phosph ate : ?? 4 −3 Phosph ite : ?? 3 −3 Sulf ate : 𝑆? 4 −2 Sulf ite : 𝑆? 3 −2 Per chlor ate : 𝐶𝑙? 4 − Chlor ate : 𝐶𝑙? 3 − Chlor ite : 𝐶𝑙? 2 − Hypo chlor ite : 𝐶𝑙? − Max of 3 O’s in Period 2 Max of 4 O’s in Period 3 Net charge becomes less negative (+1 proton to central atom!) Per-[ ]-ate [ ]-ate [ ]-ite Hypo-[ ]-ite -O Oxidation number (charge state) for a set of homologous oxoanions never changes Group 17 oxoanions (Cl, Br, I): Group 14 – 16 oxoanions: [ ]-ate [ ]-ite -O -O -O -O -O -O Cl = Chlor-, Br = Brom-, I = Iod-

Ca CO 3 Nomenclature: Polyatomic ions form ionic bonds with metals Fe 2 (SO 4 ) 3 NH 4 ClO 4 Na 2 SO 3 • Main group metal: Named as Type I • Transition metal: Named as Type II • Polyatomic ion: No change to polyatomic cation or anion name

To neglect or not neglect resonance in hypervalent molecules Sulfate (SO 4 2- ): S — O Bond order = ___ Do bond lengths follow a trend that supports the need of resonance structures? Sulfuric acid (H 2 SO 4 ): S — O Bond order = ____ S — O length = 149 pm S — O length = 142 pm S — OH length = 157 pm S — O Bond order = ___ S — O Bond order = ____ S — OH Bond order = ____ Theory does not support double-bonding properties. What else could be the reason? Octet-abiding FC minimized Scientists still debate this topic! There is still much to be learned about bonding!

Summary of Lewis structures (1) Octet-abiding Lewis structures – model for predicting molecular connectivity (2) Formal charges – electron counting mechanism used to predict molecular electronic distribution ( lower formal charges or (-) on more EN atom = better structure! ) (3) Resonance structures – delocalization of electron density between covalent bonds and/or lone pairs; consequence of multiple acceptable Lewis structure. (4) Hypervalent compounds – Lewis structures which “violate” the octet rule C H H H H I I I −1 0 0 1) Identify central atom and arrange bonding atoms around as far apart as possible 2) Count valence electrons 3) Connect central atom to surrounding atoms 4) Distribute remaining electrons as lone pairs; then multibonds CH 4 • Can occur for 3 rd period and below elements • Larger atomic radius permits this! • Last resort for accommodating bonding FC = 0 0 0 -1 0 +1 • Stabilizing effect for spreading e - density over multiple atoms

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