Chapter. 3 Chemical Bonding: The Classical Descriptiontime.kaist.ac.kr › lec › Chap3.pdf ·...
Transcript of Chapter. 3 Chemical Bonding: The Classical Descriptiontime.kaist.ac.kr › lec › Chap3.pdf ·...
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Chemical Bonding: The Classical Description
sharing or transferring electrons between atoms
LEWIS electron dot diagram -> formula (SF6)
VSEPR (valence-shell electron-pair repulsion) theory
Molecular Structure (w/o quantum mechanics)
Chapter. 3
Two or more atoms approach -> their electrons interact and form new arrangements of electrons with lower total potential energy than isolated atoms
covalent ionicpolar covalent
Quantitative description: Quantum mechanics
Classical description:
SF
F
FF
F
F
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3.1 Periodic TableThe 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
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IONIZATION ENERGY : a measure of the stability of the electron configuration of the free atom
X(g) X+(g) + e- X+(g) X2+(g) + e-
PeriodsPeriods--22 atomsatomshave very stable have very stable HeHe--like inner corelike inner core
And then, less And then, less tightly bound tightly bound electrons electrons
PeriodsPeriods--33 atomsatomshave very stable have very stable NeNe--like inner like inner corecoreAnd then, less And then, less tightly bound tightly bound electrons electrons
Existence of the SHELLExistence of the SHELL
3.2 Ionization Energies and the Shell Model of Atom
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Nucleus
- core electrons
valence electrons
Inner shell
Valence Shell (partially filled shell)
Lewis dot symbol
form chemical bondsChemical reaction
Now, we can classify electrons into core and valence electrons.
Si.. ..
Group number = the number of valence electrons
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IONIZATION ENERGY : a measure of the stability of the electron configuration of the free atom
X(g) X+(g) + e-
3.2 Ionization Energies and the Shell Model of Atom
First Ionization energy
ΔE = IE1
ΔE = [energy of products] – [energy of reactants]= E[X+(g) + e-] – E[X(g)]
Periodicity as the periodic table
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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 energythe 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 eletronswhen it forms a chemical bond with another atom.
Electronegativity : the tendency of atoms to attract electrons
Electronegativity (Mulliken) ∝ ½ (IE1 + EA)
3.3
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Electronegativity in the periodic TableElectron Acceptor
Electron Donor
Ionic bond by Coulomb stabilization Energy
Covalent bond by electron sharing
large difference of electronegativity
small difference of electronegativity
For chemical bonding…
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Ionic Bonding
K+ + F-
K + F
IE1(K) – EA(F) > 0
ΔE = [Q1Q2]/[4πε0R]= ([Q1Q2]/[4πε0R])*(NA/103)
Coulomb Stabilization Energy
(J per ion pair)(kJ per mole)
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Structure of Isolated Molecules: covalent chemical bond
Bond formation when an electron is shared.
H2+
related with atomic radii
Bond dissociation energy
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Bond Order
Bond Length Bond Energy
1.536 345 kJ/mol
1.337 612 kJ/mol
1.204 809 kJ/mol
Benzene 1.397 505 kJ/mol
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Lewis Diagrams for molecules
Octet rulemultiple bondsLone 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)
Resonance Forms
NO3-O3
H, F: bonded to only one other atom
3.6 “Shared”
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Breakdown of the Octet Rule
Case 1: Odd-Electron molecules
Case 2: Octet-Deficient molecules (BF3)
Case 3: Valence Shell Expansion (SF6)
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Polar Covalent BondingPauling’s electronegativity
covalent contribution to the dissociation energy for A-B[ΔEAAΔEBB]½Δ = ΔEAB - [ΔEAAΔEBB]½ : ionic character strengthen the bondχA-χB = 0.102 Δ½ Dipole Moment μ = QR
What about CO?
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The Shape of Molecules: VSEPR theory
Steric number of the central atom (SN)=(# of atoms bonded) +(# of lone pairs on central atom)
Arrangement that minimizes repulsions
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Lone pair interaction stronger than bonding pair- distortion of the structure
For CH3Cl, C-Cl is relatively electron deficient,Making C-Cl bond is less repulsive than others.
Fine Tuning of Molecular geometries
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Dipole Moments of Polyatomic Molecules
non-polar
polar
polar
non-polar
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Oxidation Number
1. Oxidation numbers of the atoms add up to zero for neutrals2. Alkali atoms +1; alkaline earth +23. F=-1; other halogens also except with O & with halogens4. H=+1 except in metal hydrides such as LiH5. O=-2 except preceding cases
Oxidation Number is FOR….1. Nomenclature2. Identifying oxidation-reduction reactions3. Exploring trends in chemical reactivity
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MnO Mn3O4
Mn2O3MnO2
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Naming Binary covalent compounds…(page 82 & 83)
Chapter 3 is finished here.