Chapter 9

Chapter 9:Basic Concepts of Chemical Bonding

NaCl versus C12H22O11.

Types of BondsTypes of AtomsType of BondBond Characteristicmetals to nonmetalsIonicelectronstransferrednonmetals tononmetalsCovalentelectrons sharedmetals tometalsMetallicelectronspooledWe can classify bonds based on the kinds of atoms that are bonded together.2Tro: Chemistry: A Molecular Approach, 2/e2Types of Bonding3

Tro: Chemistry: A Molecular Approach, 2/e3 Lewis Dot SymbolsCombines the element symbol plus the valence electrons as dots placed around symbol.Dots are first placed on each of the four sides (N-S-E-W), then paired up after that.Symbols for period 2. Octet RuleAtoms tend to gain, lose, or share electrons until they are surrounded by eight electrons.Ionic = gain or loseCovalent = shareWhile there are exceptions to this rule, it is the important driving force for the formation of compounds.Ionic BondingWhen Na(s) and Cl2(g) are combined, a very violent, exothermic reaction results.LEP #1

Ionic BondingIonic BondingThe energetics of ionic bond formation can be explained by a series of steps.Step 1: Loss of electron by NaNa(s) Na(g) ; DH = +108 kJNa(g) Na+(g) + 1e- ; DH = +496 kJStep 2: Gain of electron by Cl Cl2(g) Cl(g) ; DH = +122 kJCl(g) + 1e- Cl-(g) ;DH = -349 kJIonic BondingLattice energy is the energy required to completely separate a mole of a solid ionic compound into gaseous ions.NaCl(s) Na+(g) + Cl-(g) ; DH = 788 kJReverse this AND add it to the four previous reactions yields:Na(s) + Cl2(g) NaCl(s) ; DH = -411 kJLattice EnergyThe electrostatic attraction of two charged particles is ruled by the equation:

Q1, Q2 are the magnitudes of the charges and d is the distance between the two nuclei.

Lattice EnergyAs the magnitude of the charge increases, the lattice energy will increase.Na+1 Cl-1 U = +788 kJSr+2 Cl-1 U = +2127 kJSr+2 O-2 U = +3217 kJLattice EnergyAs the distance between the ions increases, the lattice energy decreases.

Summary of Lattice EnergiesThe force of attraction between oppositely charged particles is directly proportional to the product of the chargesLarger charge means the ions are more strongly attractedlarger charge = stronger attractionstronger attraction = larger lattice energyOf the two factors, ion charge is generally more importantLEP #2

Lattice Energy =910 kJ/mol

Lattice Energy =3414 kJ/mol13Tro: Chemistry: A Molecular Approach, 2/e13Covalent BondingWhen two electrons are shared by two atoms, this is known as a covalent bond.Formation of H2

Covalent BondingFormation of Cl2 each Cl atom has seven valence electrons. Each has one unpaired electron that can pair up to make a bond.

Lewis StructuresFor molecules or ions containing three or more atoms, we can follow a set of rules to guide in the process.Add up the total valence electrons from all the atoms in the compound. Ex) CF4Make a skeleton structure the first element in the formula is usually the central atom all others are then placed around this atom and connected with a bond.Lewis StructuresFill the external atoms until they have an octet.Compare total number of electrons used to step 1. If all are used, then go to step 5. If some are left, place on central atom as lone pair(s).Check the central atom for an octet. If no octet, may need multiple bond(s) by moving non-bonding pair(s).Lewis StructuresGeneral GuidelinesGroup 7A as an external atom will NOT do multiple bonds.Carbon will almost always have four bonds.Group 2A, 3A as a central atom may be deficient of octet.Group 5A, 6A, 7A, and Xe as a central atom may exceed the octet.Oxygen will do up to two bonds.ElectronegativityNon-polar covalent bond the electrons are shared equally.Ex) F2 , Br2 , I2 , O2 , etc.Polar covalent bond electrons are not shared equally one atom has a greater desire for the electron pair. Electronegativity ScaleElectronegativity

Bond PolarityIf the electronegativity difference is zero, then the bond is non-polar covalent.If there is a difference AND the two elements are non-metals, then the bond is polar covalent.If there is a difference AND one element is a metal and the other is a non-metal, then the bond is ionic.Bond PolaritySimple molecules like HCl have a polar covalent bond.The more electronegative element will have a partial negative charge and the less electronegative element will have a partial positive charge.Dipole MomentThis is the quantitative measurement of the polar bond.m = Q x dm is measured in a unit called the Debye or Coulomb x meter.LEP #4

Resonance StructuresIn some cases, a molecule or ion may be described by more than one Lewis Structure.Ex) O3

LEP #5Bond StrengthThe energy required to break a covalent bond is its strength.Some are fairly simple.Cl2(g) 2 Cl(g) ; DH = 242 kJOthers are more complicated.CH4(g) C(g) + 4 H(g) ; DH = 1660 kJNote: Energy is ALWAYS required to break a bond.

Using Bond EnthalpiesOne method for estimating the enthalpy of a reaction is:DH = S(Bonds Broken) S(Bonds Made)LEP #6Chemistry of ExplosivesMany explosives are solids or liquids that contain the nitro (NO2) or nitrate (NO3) group. The nitrogen atoms are typically weak (singly) bonded to carbon (293 kJ).They then become N-N strong triple bonds (941 kJ).Chemistry of Explosives

Bond OrderBond order is the number of bonds connecting two atoms.Can be 1, 2, or 3.

C2H2Bond LengthAs bond order increases, the bond lengths decrease.Ex) N-N N=NNN 1.47 1.241.10Ex) C-CC=CCC 1.54 1.341.20Resonance StructuresThe bond orders and lengths for resonance structures must be averaged.Ex) SO2 , NO3-1 , C6H6

Formal ChargesFormal Charge is a fictitious charge assigned to each atom in a Lewis Structure.It helps to evaluate the validity of competing structures.Formal Charge = #Valence Electrons #Nonbonding electrons #Bonding Electrons

Formal ChargesGeneral Rules:The sum of all formal charges in a neutral molecule will equal zero.The sum of all formal charges in an ion must equal the charge of the ion.Small (+1 or -1 or 0) formal charges are preferred over larger ones.When formal charges cannot be avoided, a negative formal charge will preferentially go on the more electronegative atom.Formal ChargesHCN molecule = 10 electrons.Is skeleton structure H C N or H N C?Which Lewis Structure for the cyanate (OCN-) ion is best?