Lots of multisyllabic words, I know

Overlap and BondingWe think of covalent bonds forming through the

sharing of electrons by adjacent atoms.In such an approach this can only occur when

orbitals on the two atoms overlap.

HybridizationAtomic orbitals (s, p, d, f) cannot adequately explain

the bonding in molecules

Consider CH4 … how do we get the tetrahedral shape out of the above orbitals

HybridizationAn averaging of atomic orbitals into a new set of

“hybrid orbitals”Consider beryllium:

In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.

HybridizationConsider beryllium:

if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.

Hybrid OrbitalsMixing the s and p orbitals yields two degenerate

orbitals that are hybrids of the two orbitals.These sp hybrid orbitals have two lobes like a p orbital.One of the lobes is larger and more rounded as is the s

orbital.

Hybrid OrbitalsThese two degenerate orbitals would align themselves 180

from each other.This is consistent with the observed geometry of beryllium

compounds: linear.

Hybrid OrbitalsWith hybrid orbitals the orbital diagram for beryllium

would look like this.The sp orbitals are higher in energy than the 1s orbital but

lower than the 2p.

Hybrid OrbitalsThink about Boron

What’s its electron configuration?How many bonds does it normally form?How?

Hybrid Orbitals…three

degenerate sp2 orbitals.

Hybrid OrbitalsFor Carbon:

Hybrid Orbitals…four degenerate

sp3 orbitals.

Hybrid OrbitalsFor geometries involving expanded octets on the central

atom, we must use d orbitals in our hybrids.

Hybrid OrbitalsThis leads to five degenerate sp3d orbitals…

…or six degenerate sp3d2 orbitals.

Hybrid OrbitalsOnce you know the electron-domain geometry, you know

the hybridization state of the atom.

Short cut… count the bonded electron domains… that’s the number of hybridizations.

# Hybrid Orbitals

Hybridization Bond Angles

Electron Pair Geometry

2 sp 180 Linear

3 sp2 120 Trigonal planar

4 sp3 109 Tetrahedral

5 sp3d 120,90 Trigonal bipyramidal

6 sp3d2 90 octahedral

Practice ProblemsSpecify the electron-pair and molecular geometry for

each underlined atom in the following list. Describe the hybrid orbital set used by this atom in each molecule or ion.

a) BBr3

b) CO2

c) CH2Cl2

d) CO3-2

Practice Problems AnswersElectron-Pair Molecular HybridGeometry Geometry Orbital

Set

a)Trigonal Planar Trigonal Planar sp2

b)Linear Linear sp

c)Tetrahedral tetrahedral sp3

d)Trigonal planar trigonal planar sp2

Types of Covalent BondsAccording to the valence bond theory, bond formation

requires that two orbitals on adjacent atoms to overlap….So what about multiple bonds…Draw C2H4

Is there room in between the two atoms for two pairs of electrons to share space? NO

Types of Covalent BondsSigma (σ)

Formed from the overlap of hybrid orbitals. Electron density along the internuclear axis… between the centers of the atoms…

Types of Covalent BondsPi (π)

Formed from the overlap of unhybridized p-orbitals. Electron density is above and below the internuclear axis, between the atomic centers

Types of Covalent BondsDraw Aceylene:C2H2

In triple bonds, two sp orbitals form a bond between the carbons, and two pairs of p orbitals overlap in fashion to form the two bonds.Or there is always one sigma

bond… anything more is a pi bond

Molecular Orbital TheoryMolecular Orbital Theory:Atomic orbitals from all atoms combine to form new orbitals resulting in known geometries

Hybridization: Atomic orbitals from single atoms combine to form new orbitals resulting in known geometries

VESPER: Electrons repel; moleular shapes result.

ATOMS

Electrons in

Orbitals

s, p, d, f, etc

MOLECULES

Experimental Geometries

Linear, trigonal planar, tetrahedral

Molecular Orbital TheoryMolecular orbitals are formed by combinations of

atomic orbitals from different atoms.There are bonding and antibonding orbitals

Without significant electrons density between them, the nuclei will repel each other… creating antibonding orbitals

Molecular Orbital TheoryFirst Principle of MO Theory

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

Second Principle of MO TheoryThe bonding molecular orbital is lower in energy than

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

Molecular Orbital TheoryThird Principle of MO Theory

Electrons of the molecule are assigned to orbitals of successively higher energy

Fourth Principle of MO TheoryAtomic orbitals combine to form molecular orbitals most

effectively when the atomic orbitals are of similar energy

Molecular Orbital Theory# of molecular orbitals = # of atomic orbitals

Equal # of bonding and antibonding orbitals

Antibonding orbitals ALWAYS higher in energy than its BONDING COUNTERPART

Basically, it means that these guys will fill in from the bottom up… from lower energy to higher energy…

Molecular Orbital TheoryCan use this to calcualte bond order

Bond Order = ½ [(# e- in bonding orbitals) – (# of e- in antibonding orbitals)]

What is the bond order of Ne2+?

Bond order = (8-7)/2 = ½