The Octet Rule Atoms tend to lose, gain, or share electrons until they have eight valence electrons. Chapter 3: Chemical Bonding

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1 Chapter 3: Chemical Bonding Compounds are formed from chemically bound atoms or ions The ctet Rule Atoms tend to lose, gain, or share electrons until they have eight valence electrons Bonding involves only the valence electrons Ionic compounds ionic radii and lattice energies Molecular compounds covalent (polar or non-polar) bond bond order bond strength Lewis structures Lewis ymbols show the valence electrons as dots around the atomic symbol

2 Bond orders and bond distances ingle bond distances Bond strength Bond dissociation energy (the energy required to break a bond) Bond distances of other bond types 5 6 /mol 7 8 2

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4 13 14 ne more example: H 3 (H 2 ) H 24e Exceptions to the octet rule Elements in the 2 nd period can never have more than 8 electrons Elements in the 3 rd or higher periods can have more than 8 electrons H or H H Concept of Resonance More than one possible Lewis structures. The actual structure of a molecule is taken as a blend of all the feasible Lewis structures do not confuse with structural isomers Incomplete octet Expanded octet dd-electron molecules

5 What does writing resonance structures accomplish? Electrons are often delocalized between two or more atoms. Electrons in a single Lewis structure are assigned to a specific atom. Therefore, a single Lewis structure is insufficient to electron delocalization. Composite of resonance structures more accurately depicts electron distribution and describes a molecule Concept of formal charge Take C- as an example illustrating the importance of the formal charge concept (Don't confuse the formal charge with the oxidation state or the charge on an atom) More examples to show the application (formal charge and expanded octet

6 3 ctet Lewis tructure Atom ormal Charge Expanded ctet Lewis ormal Atom Charge tructure Expanded to 12 Expanded octet An atom that has a d subshell in the valence electron shell can accommodate more than an octet of electrons 2 Cl 2 Cl Cl Cl Cl , , 1-1 Xe 3 Xe Xe I 5 I I Xe I Xe I Hypervalence: Hypervalent molecules: species having more than an octet around at least one atom 22 Molecular geometry VEPR model (Valence hell Electron Pair Repulsions) The properties of a compound are very much determined by the size and shape of its molecules. The most stable arrangement of groups attached to a central atom is the one that has the maximum separation of electron pairs (bonded or non-bonded)

7 Electron-pair geometry Molecules in which the central atom has no lone pairs Molecules in which the central atom has one or more lone pairs Using the examples to illustrate the VEPR model 4, Br 3, Xe 2, Xe 4, 4 CH 4 H 3 H

8 Using the examples to illustrate the VEPR model 4, Br 3, Xe 2, Xe 4, 4 Br Br Br Xe Xe Xe Xe Xe 4 adopts a seesaw structure 29 3 Guidelines for applying the VEPR model

9 Valence Bond Theory describes covalent bond in terms of the overlap of atomic orbitals Valence bond model of H Valence bond theory and molecular geometry Hybridization -- Hybrid orbitals Hybrid orbitals are mixtures of atomic orbitals with intermediate energy The number of s, p and d orbitals in the mixture equals the number of hybrid orbitals

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11 Examples with an sp 3 hybridization central atom Hybridization of s, p and d orbitals Examples: PCl 5, 4, 4 Examples: 6, Xe 4 Pentagonal bipyramidal Geometry (sp 3 d 3 ) I Hybridization in ethylene, acetylene, etc. Concept of σ and π bonds Bonds that result from σ overlap are called σ bonds. Bonds that result from π overlap are called π bonds

12 tructure of Ethylene Bond polarity and dipole moment

13 49 5 Molecular Dipole Moments Molecule must have polar bonds (necessary but not sufficient) eed to know molecular shape because individual bond dipoles can cancel Examples 51 13