3.4Covalent Bonds and LewisStructures• In 1916 G. N. Lewis proposed that atomscombine in order to achieve a more stableelectron configuration.• Maximum stability results when an atomis isoelectronic with a noble gas.• An electron pair that is shared between two atoms constitutes a covalent bond.The Lewis Model of ChemicalBondingCovalent Bondingin H2HH..HH..Two hydrogen atoms, each with 1 electron,Two hydrogen atoms, each with 1 electron,can share those electrons in a covalent bond.can share those electrons in a covalent bond.HH::HH• Sharing the electron pair gives each hydrogen anelectron configuration analogous to helium.Covalent Bondingin F2Two fluorine atoms, each with 7 valence electrons,Two fluorine atoms, each with 7 valence electrons,can share those electrons in a covalent bond.can share those electrons in a covalent bond.• Sharing the electron pair gives each fluorinean electron configuration analogous to neon.........FF..FF..::::........FF::FF::::................The Octet Rule• The octet rule is the most useful in casesinvolving covalent bonds to C, N, O, and F.FF::FF::::................In forming compounds, atoms gain, lose, orIn forming compounds, atoms gain, lose, orshare electrons to give a stable electronshare electrons to give a stable electronconfiguration characterized by 8 valenceconfiguration characterized by 8 valenceelectrons.electrons.ExampleExampleCC........FF::..........Combine carbon (4 valence electrons) andCombine carbon (4 valence electrons) andfourfour fluorines fluorines (7 valence electrons each)(7 valence electrons each)to write a Lewis structure for CFto write a Lewis structure for CF44..::FF::........CC::FF::........::FF::........::FF::........The octet rule is satisfied for carbon and The octet rule is satisfied for carbon and each fluorine.each fluorine.ExampleExampleIt is common practice to represent a covalentIt is common practice to represent a covalentbond by a line. We can rewritebond by a line. We can rewrite::FF::........CC::FF::........::FF::........::FF::............CCFFFFFFFF................::::::::::::....asas3.4Double Bonds and Triple BondsInorganic examplesInorganic examplesCC::::::OO....::OO....::::CC::OO....OO....::::::::NN::CC::HH::NNCCHHCarbon dioxideCarbon dioxideHydrogen cyanideHydrogen cyanideOrganic examplesOrganic examplesEthyleneEthyleneAcetyleneAcetylene::::::CC::CC::HHHHCCCCHHHHCC::::CC....HH::::....HHHHHHCCCCHHHHHHHH3.4Formal Charges• Formal charge is the charge calculatedfor an atom in a Lewis structure on thebasis of an equal sharing of bondedelectron pairs.Nitric acid....::....HHOOOOOONN::::........• We will calculate the formal charge foreach atom in this Lewis structure.Formal charge of HFormal charge of HNitric acid....::....HHOOOOOONN::::........• Hydrogen shares 2 electrons with oxygen.• Assign 1 electron to H and 1 to O.• A neutral hydrogen atom has 1 electron.• Therefore, the formal charge of H in nitric acid is0.Formal charge of HFormal charge of HNitric acid....::....HHOOOOOONN::::........• Oxygen has 4 electrons in covalent bonds.• Assign 2 of these 4 electrons to O.• Oxygen has 2 unshared pairs. Assign all 4 ofthese electrons to O.• Therefore, the total number of electrons assignedto O is 2 + 4 = 6.Formal charge of OFormal charge of ONitric acid....::....HHOOOOOONN::::........• Electron count of O is 6.• A neutral oxygen has 6 electrons.• Therefore, the formal charge of O is 0.Formal charge of OFormal charge of ONitric acid....::....HHOOOOOONN::::........• Electron count of O is 6 (4 electrons fromunshared pairs + half of 4 bonded electrons).• A neutral oxygen has 6 electrons.• Therefore, the formal charge of O is 0.Formal charge of OFormal charge of ONitric acid....::....HHOOOOOONN::::........• Electron count of O is 7 (6 electrons fromunshared pairs + half of 2 bonded electrons).• A neutral oxygen has 6 electrons.• Therefore, the formal charge of O is -1.Formal charge of OFormal charge of ONitric acid....::....HHOOOOOONN::::........• Electron count of N is 4 (half of 8 electronsin covalent bonds).• A neutral nitrogen has 5 electrons.• Therefore, the formal charge of N is +1.Formal charge of NFormal charge of N––Nitric acid....::....HHOOOOOONN::::........• A Lewis structure is not complete unlessformal charges (if any) are shown.Formal chargesFormal charges––++Formal ChargeFormal ChargeFormal charge = Formal charge = group numbergroup numberin periodic tablein periodic tablenumber ofnumber ofbondsbondsnumber ofnumber ofunshared electronsunshared electrons––––An arithmetic formula for calculating formal charge.An arithmetic formula for calculating formal charge."Electron counts""Electron counts" and formal and formal charges in NHcharges in NH44+ + and BFand BF44-- 1144NNHHHHHHHH++7744....BBFFFFFFFF................::::::::::::....––3.5Drawing Lewis StructuresConstitution• The order in which the atoms of amolecule are connected is called itsconstitution or connectivity.• The constitution of a molecule mustbe determined in order to write aLewis structure.• Step 1:The molecular formula and theconnectivity are determined byexperiment.Table 1.4 How to Write LewisStructures• Step 1:The molecular formula and theconnectivity are determined byexperiment.• Example:Methyl nitrite has the molecularformula CH3NO2. All hydrogens arebonded to carbon, and the order ofatomic connections is CONO.Table 1.4 How to Write LewisStructures• Step 2:Count the number of valence electrons.For a neutral molecule this is equal tothe number of valence electrons of theconstituent atoms.Table 1.4 How to Write LewisStructures• Step 2:Count the number of valence electrons. Fora neutral molecule this is equal to thenumber of valence electrons of theconstituent atoms.• Example (CH3NO2):Each hydrogen contributes 1 valenceelectron. Each carbon contributes 4,nitrogen 5, and each oxygen 6 for a total of24.Table 1.4 How to Write LewisStructures• Step 3:Connect the atoms by a covalent bondrepresented by a dash.Table 1.4 How to Write LewisStructures• Step 3:Connect the atoms by a covalent bondrepresented by a dash.• Example:Methyl nitrite has the partial structure:Table 1.4 How to Write LewisStructuresCCOONNOOHHHHHH• Step 4:Subtract the number of electrons inbonds from the total number ofvalence electrons.Table 1.4 How to Write LewisStructuresCCOONNOOHHHHHH• Step 4:Subtract the number of electrons inbonds from the total number