| Chem 110 |
General Chemistry |
Fall 2003 |
| Lecture Notes::Lec 26_31 October |
© R. Paselk 2003 |
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The Chemistry of the Elements
The Representative Elements
Group III
Chemistry
Group III, Boron, Aluminum, Galium, Indium, and Thallium,
introduces a couple of characteristics that will continue in varying
degrees with the other p-block elements.
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What is a metal and what are metallic properties?
- Generally good conductors of heat and electricity, most are
malleable (can be pounded to thin sheets) and ductile (can be
drawn into wires), and they have high reflectivity and luster.
- Metals tend to form positive ions, and their hydroxides are
basic.
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- The higher oxidation state decreases in stability as we go
down the group. Thus for Group III we see the +3 oxidation state
is important for all Group III elements, and the only oxidation
state seen for example for Al, +1 is also an important oxidation
state for Tl.
- The metallic character of the elements for identical oxidation
states increases as we go down the group.
- Thus B is a semi-metal, and its oxide, boric acid [B(OH)3],
is a weak acid, rather than a base as we saw for the metals in
Groups I & II. Its a bit different than what we've seen before
in that it acts as a Lewis acid, reacting with water
- B(OH)3(aq) + H2O(l)
Æ B(OH)4-(aq)
+ H+(aq)
- Al and Ga are both amphoteric - that is they dissolve in
both strong aqueous acids and bases:
- 2Al(s) + 6H+(aq)
Æ 2Al3+(aq)
+ 3H2(g)
- 2Al(s) + 6H2O(l)
+ 2OH-(aq) Æ
2Al(OH)4-(aq) + 3H2(g)
- In and Tl do not react with strong bases, but react with
strong aqueous acids. Their hydroxides are basic as expected
for a metal.
- Galium, Indium and Thallium are not as metallic as one might
expect due to the filling of the d-shells of the transition elements.
Remember, these are internal shells, so the nucleus is adding
charge, causing the atom to shrink and thus bring the outer electrons
in closer. As a result the valence electrons are not as easily
lost - less reactive and less metallic.
Properties of Group III
| Property |
B |
Al |
Ga |
In |
Tl |
| Outer electron configuration |
2s22p1 |
3s23p1 |
4s23d104p1 |
5s24d105p1 |
6s24f145d106p1 |
| Melting point (°C) |
2300 |
660 |
29.7 |
156 |
304 |
| Density (g/cm3) |
2.37 |
2.70 |
5.90 |
7.3 |
11.9 |
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Ionization energies - 1st & sum
of 1-3 (kJ/mol)
M(s) Æ M3+(aq)
+ 3 e-
|
800.6
6886
|
577.6
5137
|
578.8
5520
|
558
5063
|
589.3
5415
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Standard Reduction Potentials (V, 25°C)
M2+(aq) + 2 e- Æ
M(s)
|
-0.87 |
-1.66 |
-0.53 |
-0.34 |
-0.72 |
| Electronegativity |
2.0 |
1.6 |
1.8 |
1.8 |
1.6 |
- Boron:
- As noted above B is a semi-metal, and shares properties and
chemistry with the diagonally related semi-metal, silicon.
- Most of boron's compounds are covalent.
- The covalent hydrides are called boranes and are particularly
interesting in their bizarreness:
- BH3 is predicted to have a sextet of electrons
around boron giving a trigonal planar molecule.
- However, BH3 is unstable, reacting with itself
to form diborane, B2H6, where the two borons
are connected via hydrogen bridges with three-center bonding
as we saw with beryllium hydride.
- Higher boranes also exist. In some of them we see not only
three-center bonding, we also see five-center bonding.
- Boranes are highly unstable due to their extreme electron
deficiency. Their highly exothermic reaction with oxygen lead
to their consideration as rocket fuels by the space program.
All of the Group III metals (Al, Ga, In, & Tl) form
both covalent and ionic bonds.
- All four metals form halogen compounds with the formula MX3
- The fluorides, MF3 are ionic.
- The chlorides, bromides and iodides are low melting point
compounds with dimeric vapor-phase molecules, however the situation
is a bit more complex than this.
- For example, aluminum chloride crystallizes from aqueous
solution as an ionic salt containing hydrated Al3+:
[Al(H2O)6]Cl3, commonly written
as AlCl3*6H2O.
- On the other hand, anhydrous aluminum chloride forms a 3-D
covalent network with chlorides bridging the aluminums.
- In the vapor phase, aluminum chloride exists as the dimeric
Al2Cl6, with the aluminums bridged by chlorides
(Cl2AlCl2AlCl2).
- Well defined M3+ ions exist as hydrated forms
in acid solution, e.g.: Al(H2O)63+.
- Aluminum is the most abundant metal and the third
most abundant element in the Earth's crust.
- Very metallic in elemental form.
- Bonds to nonmetals have significant covalent character.
- This results in amphoterism noted above.
- Aluminum forms a hard, tough coating of aluminum oxide (Al2O3)
which protects the metal from corrosion. A thicker coating may
be applied electrolytically to give an anodized surface
(dyes added to this coating can give it colors).
- We already discussed aluminum production by the Hall process
in electrochemistry.
- Aluminum reacts with hydroxide to form aluminum hydroxide,
a gelatinous solid: Al3+ + 3OH- Æ
Al(OH)3(s)
- Alumina, Al2O3, forms a crystalline
solid with oxygen in a hexagonal close packed array, and aluminum
in two thirds of the octahedral holes. However, the Al-O bonds
have significant covalent character, making alumina extremely
hard. Alumina is very stable, as demonstrated in the Thermite
reaction:
- 2Al + Fe2O3 Æ
Al2O3 + 2Fe
- Gallium, Indium and Thallium are all rare, found primarily
in zinc, lead and cadmium, ores.
- Gallium has a low melting point but a very high boiling point
making it the element with the widest liquid range known. It
is thus used in some thermometers, and it is used in semiconductors,
such as galium arsenide lasers.
- Thallium is the only element in Group III and the first element
we've looked at to exhibit the so called inert pair effect.
This refers to the fact that the s-electrons would appear not
to participate as readily in ionization, giving a +1 ion as well
as the +3 ion (as you might expect its more complex). Thallium(I)
is extremely toxic.
© R A Paselk
Last modified 31 October 2003
