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

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

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

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

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

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

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…

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)

Structure of Isolated Molecules: covalent chemical bond

Bond formation when an electron is shared.

H2+

related with atomic radii

Bond dissociation energy

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

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”

Breakdown of the Octet Rule

Case 1: Odd-Electron molecules

Case 2: Octet-Deficient molecules (BF3)

Case 3: Valence Shell Expansion (SF6)

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?

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

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

Dipole Moments of Polyatomic Molecules

non-polar

polar

polar

non-polar

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

MnO Mn3O4

Mn2O3MnO2

Naming Binary covalent compounds…(page 82 & 83)

Chapter 3 is finished here.