chapter 1: orbitals and bonding - colby 1.pdf · 2008. 9. 1. · a !-orbital (bonding) a !*-orbital...

Chapter 1: Orbitals And Bonding

Look back at “Chemistry, The Central Science” by Brown, Lemay, Bursten!

Chapter 1 Topics: Bonding Concepts

Ionization Potential (Ch. 8) Quantum Numbers (Ch. 8)Ionic Bonds (Ch. 8) Electronic Configuration (Ch. 8)Hund’s Rule (Ch. 8) Pauli Exclusion Principle (Ch. 8)Aufbau Principle (Ch. 8) Atomic Orbitals (Ch. 8)Lewis Structures (Ch. 8) Dipole Moment (Ch. 8)Electronegativity (Ch. 8) Valence Electrons (Ch. 8)Octet Rule (Ch. 8) Resonance Structures (Ch. 8)Bond Dissociation Energy (Ch. 8) Formal Charge (Ch. 8)Covalent Bonds (Ch. 8/9) Nodes (Ch. 8/9)Wave Functions (Ch. 9) Molecular Orbitals (Ch. 9)

Organic Chemists usually don’t use quantum numbers – but we have to remember the correlations:

Orbital Nomenclature

Shell 1:

Shell 2:

Shell 3:

Shell 4:

Hig

her

En

erg

y One S orbital

One S, three P

One S, three P, five D

One S, three P, five D, seven F

The three P orbitals are designated Px, Py, Pz (different spacial orientations).

principal quantum number (n) azimuthal quantum

number (l)

magnetic quantum number (m)

(1s)

(2s, 2p)

(3s, 3p, 3d)

(4s, 4p, 4d, 4f)

Ionic Bonding

Ionic bonds: One atom transfers electron to another. Molecule held together by electostatic (magnetic) forces. Formed between two atoms of very different electonegativities (>2.0 electronegativity difference)

Li F

Loss of one electron will lead to a completelyempty valence shell

Addition of one electronwill lead to a completely filled valence shell (fulloctet)

or LiFFLi

Atoms are especially stable when all of the valence orbitals are either completely filled orcompletely empty (the "noble gas" configuration). This has been adapted to the octet rule: (most)atoms are stable when there are 8 electrons in their outermost (valence) shell. For this course: 1st &2nd row atoms can never have more than 8 valence electrons and/or 4 valence orbitals!!!

Electronegativity

Electronegativity and Percent Covalency

Covalent Bonding

H

Covalent bonds: Two atoms share electrons. Both atoms can count the shared electrons toward their octet. This type of bond is formed between two atoms of similar electonegativities (<2.0 electronegativity difference)

H

Sharing one additionalelectron will lead to acompletely filled valence shell

+

Sharing one additionalelectron will lead to acompletely filled valence shell

H H or H H H2or

A line (bond) signifies

2 shared electrons

Both hydrogens have a filled valence shell (shared

electrons count for both atoms)

Identical atoms will share electrons equally: a nonpolar covalent bond.

Nonidentical atoms will not share electrons equally: a polar covalent bond.

H F or H F HFor

H F

Electronegativity 2.1 Electronegativity 4.0

Fluorine has a higher electronegativity, and will "pull" electrons toward itself causing bond polarization. This creates a dipole along the bond axis.

!+ !–

Polar Covalent Bonding

Writing Lewis Structures

Lewis Structures: Represent connectivity of a chemical species. Dots reach represent oneelectron; lines represent a shared electron pair; atomic symbols represent the nucleus andall non-valence electrons. Nonbonding electron pairs are frequently omitted!


NH3

ammonia

H

H

HN N

H

H

H or N

H

H

H




H2CCH2

ethylene

C C H

HH

H C

H

HC

H

H

C

H

HC

H

H orC C

H

H H

H



H3CS(O)CH3

dimethylsulfoxide

SC C H

HH

H

HH

O O

C

H

H

HC

H

H

H S

O

C

H

H

H

S C

H

H

H

C

H

H

H

S C

H

H

HO

Formal Charge = Valence e– – Nonbonding e– – 1/2 Bonding e–

node (nodal plane)

A 1s orbital

A 2p orbital

Atomic Orbitals: A Brief Review

Atomic Orbitals: A Brief Review

In General, electrons are lower in energy if:

1. They are closer to a positive charge2. They are in an orbital with fewer nodes3. They have a larger orbital space to occupy

Molecular Orbitals: A Brief Review

H H

H H

H H H Hor

Mixing orbitals with same phase: leads to a bonding interaction that is stabilizing (lower in energy than the atomic orbitals)

Mixing orbitals of opposite phase: leads to an antibonding interaction that is destabilizing (higher in energy than the atomic orbitals)

Molecular Orbitals: A Brief Reviewnode (nodal plane)

H H

H H

H H

a !-orbital (bonding)

a !*-orbital (antibonding)

! H–H

!* H–H

Electrons are 52 kcal/mol (per electron) more stable in a σ H–H orbital (larger orbital space)than in a hydrogen 1s orbital.

H H

H H

H H



! H–H

!* H–H

52 kcal/mol

Energy



The bond dissociation energy (BDE) is the energy required to break a bond homolytically(into a diradical). BDE (H2) = 104 Kcal/mol

H + H H H exothermic by 104 Kcal/mol (!H = –104 Kcal/mol)

H + HH H endothermic by 104 Kcal/mol (!H = +104 Kcal/mol)

bond formation

bond cleavage(BDE)


C C C Cor

Mixing orbitals with same phase: leads to a bonding interaction that is stabilizing (lower in energy than the atomic orbitals)

Mixing orbitals of opposite phase: leads to an antibonding interaction that is destabilizing (higher in energy than the atomic orbitals)

C C

C C




! C–C

!* C–C

CC

C C

C C

All mutiple bonds that we will encounter will be !-type

Curved Arrow Notation

Curved arrows are used to designate the movement or flow of electrons.

H O H+ H O H

H O H+ H O H

X

correct

incorrect

the electrons start here (a filled orbital)

this atom has the empty orbital to

receive the electrons

Resonance Structures

Resonance Structures allow Lewis Structures to describe multicenter bonding (more than 2atoms sharing electrons). There are also situations where resonance structures are used toshow bond polarization.

O3

ozone

O O O O O O O O O

O O O

A more accurate single representation:

O

O

O O

O

O

O

O

O

+1

–1/2–1/2

chapter 1: orbitals and bonding - colby 1.pdf · 2008. 9. 1. · a !-orbital (bonding) a !*-orbital...

Documents