Lecture 1Chemical Bonds: Atomic Orbital

Theory and Molecular Orbital Theory

Dr. A.K.M. Shafiqul Islam

06.07.09

Summary of Modern Atomic Theory Atoms have an internal structure consisting of one

or more subatomic particles: protons, neutrons, and electrons.

proton positive chargemass = 1.673 x 10-27 kg

neutron no chargemass = 1.675 x 10-27 kg

electron negative chargemass = 9.109 x 10-31 kg

Summary of Modern Atomic Theory Most of the mass of an atom is concentrated in the

nucleus. The nucleus contains one or more positively

charged protons, and one or more neutrons with no electrical charge.

Summary of Modern Atomic Theory One or more negatively

charged electrons are in constant motion somewhere outside the nucleus.

The number of electrons is equal to the number of protons; the atom has no overall electrical charge.

Summary of Modern Atomic Theory An atom is mostly free

space because the volume of the nucleus and the electrons outside the nucleus are extremely small compared to the overall volume of the atom.

Summary of Modern Atomic Theory Heisenberg Uncertainty Principle

• It is not possible to determine both the position and the momentum of an electron.

Wave Function • Describes the energy of an electron and the probability of finding

the electron in a region around the nucleus. Atomic Orbital

• The probability distribution about one atomic nucleus (i.e., wave function = atomic orbital).

– Amplitudes – numerical magnitudes– Signs – positive or negative– Nodes – values of wavefunction equals zero (given by quantum numbers)

Summary of Modern Atomic Theory Heisenberg Uncertainty Principle

• It is not possible to determine both the position and the momentum of an electron.

Wave Function • Describes the energy of an electron and the probability of finding

the electron in a region around the nucleus. Atomic Orbital

• The probability distribution about one atomic nucleus (i.e., wave function = atomic orbital).

– Amplitudes – numerical magnitudes– Signs – positive or negative– Nodes – values of wavefunction equals zero (given by quantum numbers)

Summary of Modern Atomic Theory Heisenberg Uncertainty Principle

• It is not possible to determine both the position and the momentum of an electron.

Wave Function • Describes the energy of an electron and the probability of finding

the electron in a region around the nucleus. Atomic Orbital

• The probability distribution about one atomic nucleus (i.e., wave function = atomic orbital).

– Amplitudes – numerical magnitudes– Signs – positive or negative– Nodes – values of wavefunction equals zero (given by quantum numbers)

Summary of Modern Atomic Theory Heisenberg Uncertainty Principle

• It is not possible to determine both the position and the momentum of an electron.

Wave Function • Describes the energy of an electron and the probability of finding the

electron in a region around the nucleus. Atomic Orbital

• The probability distribution about one atomic nucleus (i.e., wave function = atomic orbital).

– Amplitudes – numerical magnitudes– Signs – positive or negative– Nodes – values of wave function equals zero (given by quantum

numbers)

Atomic Orbitalss orbital

p orbitals

px py pz

Hybridization

In chemistry, hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties.

Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules.

Hybrid Atomic Orbitals

hybrid bonding shape rotation bond

sp3() head-head tetrahedral free single

sp2 () sideways triangular rigid double

sp () sideways linear rigid triple

CarbonAtomic Number = 6

2p

2s

1s

CarbonAtomic Number = 6

2p

2s

1s

OxygenAtomic Number = 8

2p

2s

1s

OxygenAtomic Number = 8

2p

2s

1s

Chemical BondingChemical Bonds

• The forces holding atoms together in compounds.

Valence Electrons • The electrons in the outer shell.

Lewis Dot Representation of Atoms• Dots around the chemical symbol of an atom

represent the valence electrons.

Chemical BondingChemical Bonds

• The forces holding atoms together in compounds.

Valence Electrons • The electrons in the outer shell.

Lewis Dot Representation of Atoms• Dots around the chemical symbol of an atom

represent the valence electrons.

Chemical BondingChemical Bonds

• The forces holding atoms together in compounds.

Valence Electrons • The electrons in the outer shell.

Lewis Dot Representation of Atoms• Dots around the chemical symbol of an atom

represent the valence electrons.

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Examples

AtomElectronicStructure

ElectronicConfiguration

Lewis DotStructure

Boron

Phosphorus

1s22s22p1

[Ne] 3s23p3

3p

3s

2p

2s

1s

3p

3s

2p

2s

1s

B

P

Molecular Orbital TheoryMolecular Orbital Theory

Molecular orbitals – delocalized over entire molecule.

First Principle• The total number of molecular orbitals is

always equal to the total number of atomic orbitals contributed by the atoms that have combined.

Molecular Orbitals for HMolecular Orbitals for H22 Bonding molecular orbital

• addition of two atomic orbitals, leads to the probability of finding an e- between the atoms.

Antibonding molecular orbital• substration of one atomic orbital from the other, leads to reduced

probability of finding an e- between the nuclei, increased in other regions.

Molecular Orbital TheoryMolecular Orbital Theory

Second Principle• The bonding molecular orbital is lower in

energy than the parent orbitals, and the antibonding orbital is higher in energy.

Third Principle• The electrons of the molecule are assigned

to orbitals of successively higher energy (according to Pauli exclusion principle and the Hund’s rule).

The Ionic Bond

Transfer of Electrons from One Atom to Another

Example:• Sodium chloride

Na Na + 1 e

Cl + 1 e Cl

The Ionic BondThe electrostatic attraction between

oppositely charged ions.

The Covalent Bond

Some atoms do not transfer electrons from one atom to another to form ions.

Instead they form a chemical bond by sharing pairs of electrons between them.

A covalent bond consists of a pair of electrons shared between two atoms.

The Covalent Bond

Some atoms do not transfer electrons from one atom to another to form ions.

Instead they form a chemical bond by sharing pairs of electrons between them.

A covalent bond consists of a pair of electrons shared between two atoms.

The Covalent Bond

Some atoms do not transfer electrons from one atom to another to form ions.

Instead they form a chemical bond by sharing pairs of electrons between them.

A covalent bond consists of a pair of electrons shared between two atoms.

Hydrogen, H2

1s1s

*

E

E*

Fluorine, F2

Fluorine, F2

F F F F+

Fluorine, F2

F F F F+

F2 *

F2

F 2p F 2p

Nitrogen, N2

Nitrogen, N2

N N N N+

O C O O C O+ +

Nitrogen, N2

Carbon Dioxide, CO2

N N N N+

O C O O C O+ +

Nitrogen, N2

Carbon Dioxide, CO2

N N N N+

O C O O C O+ +

Molecular Orbitals for HeMolecular Orbitals for He22

He2 has no net stability – two He atoms have no tendency to combine.