sharing or transferring electrons between atoms covalent ionic polar covalent Quantitative description: Quantum mechanics

Transcription

1 Chapter. 3 Chemical Bonding: The Classical Description Two or more atoms approach -> their electrons interact and form new arrangements of electrons with lower total potential energy than isolated atoms sharing or transferring electrons between atoms covalent ionic polar covalent Quantitative description: Quantum mechanics F F S F Classical description: F F F LEWIS electron dot diagram -> formula (SF 6 ) VSEPR (valence-shell electron-pair repulsion) theory Molecular Structure (w/o quantum mechanics)

2 3.1 Periodic Table The Properties of the elements display certain regularities: classification is possible. Group, period, main-group elements (representative elements), transition metal elements, lanthanides, actinides, metal, non-metal, semi-metal

3 3.2 Ionization Energies and the Shell Model of Atom IONIZATION ENERGY : a measure of the stability of the electron configuration of the free atom X(g) X + (g) + e - X + (g) X 2+ (g) + e - Periods-2 atoms have very stable He-like inner core Existence of the SHELL And then, less tightly bound electrons Periods-3 atoms have very stable Ne-like inner core And then, less tightly bound electrons

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5 Now, we can classify electrons into core and valence electrons. Valence Shell (partially filled shell) valence electrons form chemical bonds Chemical reaction Inner shell - core electrons Lewis dot symbol Nucleus.. Si.. Group number = the number of valence electrons

6 3.2 Ionization Energies and the Shell Model of Atom First Ionization energy X(g) X + (g) + e - ΔE = IE 1 ΔE = [energy of products] [energy of reactants] = E[X + (g) + e - ] E[X(g)] IONIZATION ENERGY : a measure of the stability of the electron configuration of the free atom Periodicity as the periodic table

7 Electron Affinity (EA): the energy released when an electron is attached X(g) + e - X - (g) EA = -ΔE For free atoms, the ability to lose an electron: ionization energy the ability to gain an electron: electron affinity Electronegativity is the average of ionization E and electron affinity, and this indicate the net tendency of the atom to attract eletrons when it forms a chemical bond with another atom. 3.3 Electronegativity : the tendency of atoms to attract electrons Electronegativity (Mulliken) ½(IE 1 + EA)

8 Electronegativity in the periodic Table Electron Acceptor Electron Donor For chemical bonding large difference of electronegativity Ionic bond by Coulomb stabilization Energy small difference of electronegativity Covalent bond by electron sharing

9 Ionic Bonding Coulomb Stabilization Energy K + + F - IE 1 (K) EA(F) > 0 K + F ΔE = [Q 1 Q 2 ]/[4πε 0 R] = ([Q 1 Q 2 ]/[4πε 0 R])*(N A /10 3 ) (J per ion pair) (kj per mole)

10 Structure of Isolated Molecules: covalent chemical bond Bond formation when an electron is shared. related with atomic radii H 2 + Bond dissociation energy

11 Bond Order Bond Length Bond Energy kj/mol Benzene kj/mol kj/mol kj/mol

12 3.6 Lewis Diagrams for molecules Shared Octet rule multiple bonds Lone pairs Formal Charges = normal # of electrons formal # of electrons = Group # - # of electrons in lone pairs - ½(# of electrons in bonding pairs) Drawing Lewis Diagrams (page. 69 & 70) H, F: bonded to only one other atom Resonance Forms O 3 NO 3 -

13 Breakdown of the Octet Rule Case 1: Odd-Electron molecules Case 2: Octet-Deficient molecules (BF 3 ) Case 3: Valence Shell Expansion (SF 6 )

14 Polar Covalent Bonding Pauling s electronegativity covalent contribution to the dissociation energy for A-B [ΔE AA ΔE BB ] ½ Δ = ΔE AB -[ΔE AA ΔE BB ]½ : ionic character strengthen the bond χ A -χ B = Δ ½ Dipole Moment μ = QR What about CO?

15 The Shape of Molecules: VSEPR theory Arrangement that minimizes repulsions Steric number of the central atom (SN) =(# of atoms bonded) + (# of lone pairs on central atom)

16 Fine Tuning of Molecular geometries Lone pair interaction stronger than bonding pair - distortion of the structure For CH 3 Cl, C-Cl is relatively electron deficient, Making C-Cl bond is less repulsive than others.

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18 non-polar polar polar Dipole Moments of Polyatomic Molecules non-polar

19 Oxidation Number 1. Oxidation numbers of the atoms add up to zero for neutrals 2. Alkali atoms +1; alkaline earth F=-1; other halogens also except with O & with halogens 4. H=+1 except in metal hydrides such as LiH 5. O=-2 except preceding cases Oxidation Number is FOR. 1. Nomenclature 2. Identifying oxidation-reduction reactions 3. Exploring trends in chemical reactivity

20 MnO 2 Mn 2 O 3 MnO Mn 3 O 4

21 Naming Binary covalent compounds (page 82 & 83) Chapter 3 is finished here.