3.4 Covalent Bonds And Lewis Structures - Columbia University

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3.4Covalent Bondsand LewisStructures

The Lewis Model of ChemicalBonding 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 betweentwo atoms constitutes a covalent bond.

Covalent Bondingin H2Two hydrogen atoms, each with 1 electron,H.Hcan share those electrons in a covalent bond.H: H Sharing the electron pair gives each hydrogen anelectron configuration analogous to helium.

Covalent Bondingin F2Two fluorine atoms, each with 7 valence electrons,. F:: .F .can share those electrons in a covalent bond. .: .F : .F: Sharing the electron pair gives each fluorinean electron configuration analogous to neon.

The Octet RuleIn forming compounds, atoms gain, lose, orshare electrons to give a stable electronconfiguration characterized by 8 valenceelectrons. .: .F : .F: The octet rule is the most useful in casesinvolving covalent bonds to C, N, O, and F.

ExampleCombine carbon (4 valence electrons) andfour fluorines (7 valence electrons each). C.: .F.to write a Lewis structure for CF4. : .F: .::F: .F: C.: .F:The octet rule is satisfied for carbon andeach fluorine.

ExampleIt is common practice to represent a covalentbond by a line. We can rewrite. : .F: .::F: .F: C.: .F:as.: .F.: F:C: .F:.F:.

3.4Double Bonds and Triple Bonds

Inorganic examples.:O : : C : : O :.:OCCarbon dioxideH : C : :: N:HHydrogen cyanideC.O:N:

Organic examplesH. H.H : C : : C :HHEthyleneHCHHH : C : :: C :HAcetyleneHCCCH

3.4Formal Charges Formal charge is the charge calculatedfor an atom in a Lewis structure on thebasis of an equal sharing of bondedelectron pairs.

Nitric acidFormal charge of HH.O.O:N:O. : We will calculate the formal charge foreach atom in this Lewis structure.

Nitric acidFormal charge of HH.O.O:N:O. : 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.

Nitric acidFormal charge of OH.O.O:N:O. : 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.

Nitric acidFormal charge of OH.O.O:N:O. : Electron count of O is 6. A neutral oxygen has 6 electrons. Therefore, the formal charge of O is 0.

Nitric acidFormal charge of OH.O.O:N:O. : 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.

Nitric acidFormal charge of OH.O.O:N:O. : 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.

Nitric acidFormal charge of NH.O.O:N:O.–: 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.

Nitric acidFormal chargesH.O.O:N :O.–: A Lewis structure is not complete unlessformal charges (if any) are shown.

Formal ChargeAn arithmetic formula for calculating formal charge.Formal charge group numbernumber ofnumber of––in periodic tablebondsunshared electrons

"Electron counts" and formalcharges in NH4 and BF4-1HH4NH H.: F:.– .: .F B .F:: .F:74

3.5Drawing Lewis Structures

Constitution 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.

Table 1.4 How to Write LewisStructures 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:HHCHONO

Table 1.4 How to Write LewisStructures Step 4:Subtract the number of electrons inbonds from the total number ofvalence electrons.HHCHONO

Table 1.4 How to Write LewisStructures Step 4:Subtract the number of electrons inbonds from the total number ofvalence electrons. Example:24 valence electrons – 12 electrons inbonds. Therefore, 12 more electronsto assign.

Table 1.4 How to Write LewisStructures Step 5:Add electrons in pairs so that as manyatoms as possible have 8 electrons.Start with the most electronegativeatom.

Table 1.4 How to Write LewisStructures Step 5:Add electrons in pairs so that as many atoms aspossible have 8 electrons. Start with the mostelectronegative atom. Example:The remaining 12 electrons in methyl nitrite areadded as 6 pairs.HHCH.O.N.:O.

Table 1.4 How to Write LewisStructures Step 6:If an atom lacks an octet, use electron pairs on anadjacent atom to form a double or triple bond. Example:Nitrogen has only 6 electrons in the structureshown.HHCH.O.N.:O.

Table 1.4 How to Write LewisStructures Step 6:If an atom lacks an octet, use electron pairs on anadjacent atom to form a double or triple bond. Example:All the atoms have octets in this Lewis structure.HHCH.O.N.O:

Table 1.4 How to Write LewisStructures Step 7:Calculate formal charges. Example:None of the atoms possess a formal charge in thisLewis structure.HHCH.O.N.O:

Table 1.4 How to Write Lewis Structures Step 7:Calculate formal charges. Example:This structure has formal charges; is less stableLewis structure.HHCH O.N. –O. :

Condensed structural formulas Lewis structures in which many (orall) covalent bonds and electronpairs are omitted.HHHHCCCH : O: HHHcan be condensed to:CH3CHCH3 or (CH3)2CHOHOH

3.5Constitutional Isomers

Constitutional isomers Isomers are different compounds thathave the same molecular formula. Constitutional isomers are isomersthat differ in the order in which the atomsare connected. An older term for constitutionalisomers is “structural isomers.”

A Historical NoteNH4OCNAmmonium cyanateOH2NCNH2Urea In 1823 Friedrich Wöhler discovered thatwhen ammonium cyanate was dissolved in hotwater, it was converted to urea. Ammonium cyanate and urea areconstitutional isomers of CH4N2O. Ammonium cyanate is “inorganic.” Urea is“organic.” Wöhler is credited with an importantearly contribution that helped overturn thetheory of “vitalism.”

Examples of constitutionalisomersHHCH.O:N –::O.NitromethaneHHC.O.N.O:HMethyl nitrite Both have the molecular formula CH3NO2 but theatoms are connected in a different order.

3.5Resonance

Resonancetwo or more acceptable octet Lewis structuresmay bewritten for certain compounds (or ions)

Table 1.4 How to Write LewisStructures Step 6:If an atom lacks an octet, use electron pairs on anadjacent atom to form a double or triple bond. Example:Nitrogen has only 6 electrons in the structureshown.HHCH.O.N.:O.

Table 1.4 How to Write Lewis Structures Step 6:If an atom lacks an octet, use electron pairs on anadjacent atom to form a double or triple bond. Example:All the atoms have octets in this Lewis structure.HHCH.O.N.O:

Table 1.4 How to Write Lewis Structures Step 7:Calculate formal charges. Example:None of the atoms possess a formal charge in thisLewis structure.HHCH.O.N.O:

Table 1.4 How to Write Lewis Structures Step 7:Calculate formal charges. Example:This structure has formal charges; is less stableLewis structure.HHCH O.N. –O. :

Resonance Structures of MethylNitrite same atomic positions differ in electron positionsHHC.O.N.Hmore stableLewisstructure.O:HHC O.N.Hless stableLewisstructure. –:O.

Resonance Structures of MethylNitrite same atomic positions differ in electron positionsHHC.O.N.Hmore stableLewisstructure.O:HHC O.N.Hless stableLewisstructure. –:O.

Why Write ResonanceStructures? Electrons in molecules are often delocalizedbetween two or more atoms. Electrons in a single Lewis structure areassigned to specific atoms-a single Lewis structureis insufficient to show electron delocalization. Composite of resonance forms more accuratelydepicts electron distribution.

Example Ozone (O3)–Lewis structure ofozone shows onedouble bond andone single bond O Expect: one short bond and onelong bondReality: bonds are of equal length(128 pm) O –O

Example Ozone (O3)–Lewis structure ofozone shows onedouble bond andone single bond O O –O Resonance: O O –O – O O O

3.7The Shapes of Some SimpleMolecules

Methane tetrahedral geometry H—C—H angle 109.5

Methane tetrahedral geometry each H—C—H angle 109.5

Valence Shell Electron PairRepulsions The most stable arrangement of groupsattached to a central atom is the one that hasthe maximum separation of electron pairs(bonded or nonbonded).

Water bent geometry H—O—H angle 105 HHO:.but notice the tetrahedral arrangementof electron pairs

Ammonia trigonal pyramidal geometry H—N—H angle 107 HHN:Hbut notice the tetrahedral arrangementof electron pairs

Boron Trifluoride F—B—F angle 120 trigonal planar geometryallows for maximum separationof three electron pairs

Multiple Bonds Four-electron double bonds and six-electrontriple bonds are considered to be similar to atwo-electron single bond in terms of their spatialrequirements.

Formaldehyde: CH2 O H—C—H and H—C—Oangles are close to 120 trigonal planar geometryHCHO

Figure 1.12: Carbon Dioxide O—C—O angle 180 linear geometryOCO

3.7:Polar Covalent Bondsand Electronegativity

ElectronegativityElectronegativity is a measure ofan element to attract electronstoward itself when bonded toanother element. An electronegative element attracts electrons. An electropositive element releases electrons.

Pauling Electronegativity 1.21.51.82.12.53.0 Electronegativity increases from left to rightin the periodic table. Electronegativity decreases going down a group.

Generalization The greater the difference in electronegativitybetween two bonded atoms; the more polar thebond.H—H.: .F.F:.:NN:nonpolar bonds connect atoms ofthe same electronegativity

Generalization The greater the difference in electronegativitybetween two bonded atoms; the more polar thebond.d H. dF:.d Hdd .O H.d- d d:O C O. :.polar bonds connect atoms ofdifferent electronegativity

3.7Molecular Dipole Moments

Dipole Moment A substance possesses a dipole momentif its centers of positive and negative chargedo not coincide.m e x d (expressed in Debye units) —not polar

Dipole Moment A substance possesses a dipole momentif its centers of positive and negative chargedo not coincide.m e x d (expressed in Debye units)— polar

Molecular Dipole Momentsd-Od Cd-O molecule must have polar bonds–necessary, but not sufficient need to know molecular shape–because individual bond dipoles can cancel

Molecular Dipole MomentsOCOCarbon dioxide has no dipole moment; m 0 D

Comparison of Dipole MomentsCarbon tetrachloridem 0 DDichloromethanem 1.62 D

Carbon tetrachlorideResultant of thesetwo bond dipoles isResultant of thesetwo bond dipoles ism 0 DCarbon tetrachloride has no dipolemoment because all of the individualbond dipoles cancel.

DichloromethaneResultant of thesetwo bond dipoles isResultant of thesetwo bond dipoles ism 1.62 DThe individual bond dipoles do notcancel in dichloromethane; it hasa dipole moment.

in H2 H. .H Two hydrogen atoms, each with 1 electron, can share those electrons in a covalent bond. H:H Sharing the electron pair gives each hydrogen an electron configuration analogous to helium. Covalent Bonding in F2 Two fluorine atoms, each with 7 valence electrons,