powerpoint to accompany chapter 8 part 2 molecular geometry and bonding theories

PowerPoint to accompany

Chapter 8Part 2

Molecular Geometry

and Bonding Theories

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Covalent Bonding and Orbital Overlap We 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.

Figure 8.14


Covalent Bonding and Orbital Overlap Increased overlap

brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion.

However, if atoms get too close, the internuclear repulsion greatly raises the energy.

Figure 8.15


Hybrid Orbitals

To explain geometries, we often assume that the atomic orbitals of an atom mix to form new orbitals

called hybrid orbitals.

The process of mixing orbitals as atoms approach each other is called hybridisation.


sp Hybrid Orbitals

Consider beryllium: In its ground electronic

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

But 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.


sp Hybrid Orbitals

Mixing 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, this is

the bonding lobe.

Figure 8.16


sp Hybrid Orbitals

These two degenerate orbitals would align themselves 180 from each other.

This is consistent with the observed geometry of beryllium compounds: linear.

Figure 8.17


sp Hybrid Orbitals With 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.


sp2 and sp3 Hybrid Orbitals

Using a similar model for boron leads to sp2

hybridisation, i.e.



…three degenerate sp2 orbitals.

Figure 8.18



With carbon we get sp3 hybridisation, i.e. …

…four degenerate sp3 orbitals.


Hybridisation involving d orbitals

For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.

What element is this?


Hybridisation involving d orbitals

This leads to:

- five degenerate sp3d orbitals

or

- six degenerate sp3d2 orbitals.


Hybrid Orbitals

Once the electron-domain geometry is known, the hybridisation state of the atom is known.

Table 8.4


Multiple Bonds

sigma, - overlap of two orbitals along the internuclear axis.

pi, - sideways overlap of two p orbitals.

Figure 8.21


Molecular Orbital (MO) Theory

Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals.

Figure 8.22


Molecular Orbital of H2

In H2 the two electrons go into the bonding molecular orbital.

The bond order is half the difference between the number of bonding and antibonding electrons, i.e.bond order = ½(no. of bonding electrons - no. of antibonding electrons)

Figure 8.23


Molecular Orbital of H2

For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is:

12 (2 - 0) = 1

Figure 8.23


Molecular Orbital of He2

In the case of He2, the bond order would be:

Therefore, He2 does not exist.

12 (2 - 2) = 0

Figure 8.23


Scotty!There’s no dilithium!


Molecular Orbitals from 2p Orbitals

For atoms with both s and p orbitals, there are two types of interactions:

The s and the p orbitals that face each other overlap in fashion.

The other two sets of p orbitals overlap in fashion.

Figure 8.26


This is the 2nd halfof the p-block

With paired p electons.


Molecular Orbitals for Li2 and Be2

The MO diagram looks like this:

There are both and bonding molecular orbitals and * and * antibonding molecular orbitals.

Figure 8.27


Molecular Orbitals for B2 to Ne2

The smaller p-block elements in the second period have a sizeable interaction between the s and p orbitals.

This flips the order of the s and p molecular orbitals in these elements.

Figure 8.31

Figure 8.32


Molecular Orbitals ofSecond-Row Diatomic Molecules Figure 8.33


Electron Configurations andMolecular Properties

Paramagnetic molecules have one or more unpaired electrons and are attracted into a magnetic field.

Diamagnetic molecules have no unpaired electrons and are weakly repelled by a magnetic field. (It is a much weaker effect than paramagnetism.)


Chapter 8 End of Part 2

Molecular Orbitals Hybrid Orbitals Special Molecular Properties

powerpoint to accompany chapter 8 part 2 molecular geometry and bonding theories

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