PowerPoint to accompany Chapter 8 Part 2 Molecular Geometry and Bonding Theories.
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Transcript of PowerPoint to accompany Chapter 8 Part 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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
sp2 and sp3 Hybrid Orbitals
Using a similar model for boron leads to sp2
hybridisation, i.e.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
sp2 and sp3 Hybrid Orbitals
…three degenerate sp2 orbitals.
Figure 8.18
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
sp2 and sp3 Hybrid Orbitals
With carbon we get sp3 hybridisation, i.e. …
…four degenerate sp3 orbitals.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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?
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Hybridisation involving d orbitals
This leads to:
- five degenerate sp3d orbitals
or
- six degenerate sp3d2 orbitals.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Hybrid Orbitals
Once the electron-domain geometry is known, the hybridisation state of the atom is known.
Table 8.4
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Multiple Bonds
sigma, - overlap of two orbitals along the internuclear axis.
pi, - sideways overlap of two p orbitals.
Figure 8.21
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Scotty!There’s no dilithium!
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
This is the 2nd halfof the p-block
With paired p electons.
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
Molecular Orbitals ofSecond-Row Diatomic Molecules Figure 8.33
Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia
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.)