Exceptions to the Octet Rule Rule More Than an Octet • There Are Three Classes of Exceptions to the Octet Rule: • This Is the Largest Class of Exceptions

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Exceptions to the Octet Exceptions to the Octet Rule Rule More than an Octet • There are three classes of exceptions to the octet rule: • This is the largest class of exceptions. • Molecules with an odd number of electrons; • Atoms from the 3rd period onwards can accommodate • Molecules in which one atom has less than an octet; more than an octet. • Molecules in which one atom has more than an octet. • Beyond the third period, the d-orbitals are low enough in Odd Number of Electrons energy to participate in bonding and accept the extra

• Few examples. Generally molecules such as ClO2, NO, electron density. and NO2 have an odd number of electrons. • Should we look at some of these exceptions – problem NO NO 8.58?

Exceptions to the Octet Strengths of Covalent Rule Bonds Less than an Octet • Relatively rare. • The energy required to break a covalent bond is called the bond dissociation enthalpy, D. That is, for the Cl • Molecules with less than an octet are typical for 2 compounds of Groups 1A, 2A, and 3A. molecule, D(Cl-Cl) is given by ∆H for the reaction: Cl2(g) → 2Cl(g). • Most typical example is BF3. • When more than one bond is broken: • Formal charges indicate that the Lewis structure with an CH4(g) → C(g) + 4H(g) ∆H = 1660 kJ incomplete octet is more important than the ones with • the bond enthalpy is a fraction of ∆H for the double bonds. atomization reaction: D(C-H) = ¼∆H = ¼(1660 kJ) = 415 kJ.

1 Strengths of Covalent Bonds Bond Enthalpies and the Enthalpies of Reactions

• Mathematically, if ∆Hrxn is the enthalpy for a reaction, then ∆Hrxn = ∑ D(bonds broken)− ∑ D(bonds formed) • We illustrate the concept with the reaction between

methane, CH4, and chlorine:

CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) ∆Hrxn = ?

Strengths of Covalent Strengths of Covalent Bonds Bonds Bond Enthalpies and the Enthalpies of Reactions • We can use bond enthalpies to calculate the enthalpy for a chemical reaction. • We recognize that in any chemical reaction bonds need to be broken and then new bonds get formed. • The enthalpy of the reaction is given by the sum of bond enthalpies for bonds broken less the sum of bond enthalpies for bonds formed.

2 Strengths of Covalent Strengths of Covalent Bonds Bonds Bond Enthalpies and the Enthalpies of Reactions • In this reaction one C-H bond and one Cl-Cl bond gets broken while one C-Cl bond and one H-Cl bond gets formed. ∆Hrxn = {}[]D()()C - H + D Cl - Cl − [D(C - Cl)+ D(H - Cl)] = −104 kJ • The overall reaction is exothermic which means than the bonds formed are stronger than the bonds broken. • The above result is consistent with Hess’s law. • Let’s try 8.66 and relate it to chapter 5. Prentice Hall © 2003 Chapter 8

Strengths of Covalent Bonds Bond Enthalpy and Bond Length • We know that multiple bonds are shorter than single bonds. • We can show that multiple bonds are stronger than End of Chapter 8: single bonds. Basic Concepts of Chemical • As the number of bonds between atoms increases, the atoms are held closer and more tightly together. Bonding