Group 13: The Boron Family
Table of contents
The Boron Family contains the semi-metal Boron (B) and metals Aluminum (Al), Gallium (Ga), Indium (In), and Thallium (Tl).
Properties & Periodic Trends
These elements are found in group 13 (XIII) of the p block in the Periodic Table of Elements. In their physical properties aluminum, gallium, indium, and thallium are metallic.
Fig 1. Periodic Table with Boron Family in blue.
They have three electrons in their outermost shell (a full S orbital and one electron in the P orbital) with the valence electron configuration: ns2np1. The Boron Family has oxidation states +3 or +1. The +3 oxidation states are favorable except for the heavier elements, such as Tl, which prefers +1 oxidation state due to its stability. This is known as the inert pair effect. All the elements tend to follow Periodic Trends except Tl for particular trends, which is noted:
*Tl does not follow normal trends
Fig 2. Properties & Trends
Boron is the top element of group 13 and is the only semi-metal of this group. It has properties of a metal and a nonmetal. It's chemical symbol is B with an atomic number 5. It has the electron configuration [He] 2s2 2p1 and mainly the oxidation state of +3. This element does not exist alone. It forms compounds which can be widely found in the Earth's crust. Boron is an essential nutrient for plants. There are a few locations where boron ores, known as borax, are found in great concentrations. Due to its lack of a complete octet Boron is a Lewis acid, so it forms hydrides and the simplest boron hydride found is diborane, B2H6. Boron hydrides are used to synthesize organic compounds. One main compounds used to form other boron compounds is boric acid. Boric acid is a weak acid and may be formed by the following reaction:
B2O3 (s) + 3 H2O (l) → 2 B(OH)3 (aq)
B(OH)3 (aq) + 2 H2O(l) → H3O+(aq) + B(OH)4-(aq)
Boron can be crystallized from a solution of hydrogen peroxide and borax to produce sodium perborate, which is a bleach alternative. The reason for the bleaching action is due to the two peroxo groups that bind to the boron atoms.
Aluminum is the most important metal in the Boron Family. It is a metal with the chemical symbol Al and atomic number 13. It is used in lightweight alloys and is an active metal. It has the electron configuration [Ne] 2s2 2p1 and mainly have the oxidation state of +3. This element is the most abundant metal in the Earth's crust (7.5-8.4%). Even though it is very abundant, before 1886 aluminum was a semiprecious metal. Aluminum was hard to isolate due to it's high melting point. Aluminum is very expensive to produce, because the electrolysis of aluminum needs three moles of electrons:
Al3+ + 3e- → Al(l)
Aluminum is a soft malleable metal with a silver or gray color. It is a very reactive element so it is found in nature combined with other elements. One would think aluminum will react with water but in reality aluminum is protected by a layer of Al2O3, which is known as anodizing. The thickness of the layer can vary through galvanic reactions and protect it from further oxidizing. Many alloys are made of aluminum to prevent corrosion.
Aluminum Oxide (Al2O3): Commonly refered to as alumina, has highly desrirable metallic characteristics due to its strong ionic bonding. Excellent thermal insulator, an gives rise to corondum in its crystalline form. Corondum can exist in several ways, including in saphires and rubies. The differences in their colors are due to transition metal impurities in their corondum structure.
Aluminum Sulfate Al2(SO4)3: Very important commercial compound. Used in sizing paper (paper in which waxes and glues are used to make the paper more water resistant.) Aluminum sulfate, however, has a acidic properties that may deteriorate the paper.
Aluminum can dissolve in both acids and bases, making it an amphoteric molecule. For example in an aqueous OH- solution it will produce Al(OH)4- and in an aqueous H3O+ solution it will produce [Al(H2O)6]3+
Another important feature of aluminum is that it is a good reducting agent, because of the +3 oxidation state. It can therefore react with acids to reduce H+(aq) to H2(g). For example:
2Al (s) + 6H+(aq) → 2Al3+(aq) + 3H2(g)
Aluminum can also extract oxygen from any metal oxide. This reaction is known as the thermite reaction, which is very exothermic:
Fe2O3(s) + 2 Al(s) → Al2O3(s) +2 Fe(l)
Gallium has the chemical symbol Ga and atomic number 31. It has the electron configuration [Ar] 2s2 2p1 and +3 oxidation state. The melting point is 29.8º C and therefore melts by increasing room temperature. Gallium has the second lowest melting point (after mercury) and can remain in the liquid phase at a larger range of temperatures than any other substance. Gallium is important because it forms gallium arsenide (GaAs), which can convert light directly into electricity. Due to the thermite reaction, aluminum can extract oxygen from water therefore realeasing hydrogen. However, as mentioned above, aluminum forms a protective coat in the presence of water. By combining gallium and aluminum, this protective layer does not form and aluminum will reduce water to hydrogen. This alloy can provide a great hydrogen source.
Indium has the chemical symbol In and atomic number 49. It has the electron configuration [Kr] 2s2 2p1 and may have +1 or +3 oxidation state. However the +3 oxidation state is more common. It is a soft malleable metal that is similar to gallium, indium can form InAs which is found in photoconductors in optical instruments. The physical properties of indium include its silvery-white color and the "tin cry" it makes when it is bent. Indium also dissolves in acids, but at room temperature it does not react with oxygen. It is obtained by separating it from its zinc ores. Indium is mainly used to make alloys, only a small amount is needed to make metals more stiff. For example, indium can be added to gold or platinum to make the metals more useful industrial tools.
Thallium has the chemical symbol Tl and atomic number 81. It has the electron configuration [Xe] 2s2 2p1 and has the +3 or +1 oxidation state. As stated above, since thallium is heavier, it has a greater stability in the +1 oxidation state (inert pair effect). Hence, it is found more commonly in its +1 oxidation state. Thallium is soft and malleable. It is very poisonous but nevertheless it is still used, such as for high-temperature superconductors. For its toxicity, thallium was widely used in insecticide and rat poison but in 1975, the U.S prohibited such usage. Currently its usage is limited and must be handled with care.
Diagonal Relationship of Beryllium and Aluminum
Both Be 2+ and Al 3+ are hydrated to produce [Be(H20)4]2+ and [Al(H2O)6]3+ respectively. When reacted with water, both compounds produce hydronium ions, making them slightly acidic. Another similarity between aluminum and beryllium is that they are amphoteric, and their hydroxides are very basic. Both metals also react with oxygen to produce an oxide coating that protects other metals from corrosion. Both metals can also react with halides that can act as Lewis acids.
( highlight the blue areas to find the answers)
1. T/F In reality, aluminum forms a protective layer and does not react with water.
True, This is known as anodizing.
2. Which statement about Gallium is false?
a. Melts in your hands T
b. Can combine with Aluminum to reduce water T
c. Mainly found in the oxidation state +1 F
d. Can form a great source of hydrogen T
3. T/F Thallium is highly toxic and therefore it is commonly used for rat poisons and insecticides in the United States.
False Since 1975, thallium is prohibited from such usage since there is no warning if one digested it.
a. has the electron configuration [Ne] 2s2 2p1 F
b. is the first metal of Group 13. F
c. has an atomic number 6. F
d. is an important element that we use in our daily lives. T
e. All of the above is correct. F
5. Aluminum Oxide:
a. has poor corrosion resistance F
b. is not a very good thermal insulator F
c. in its crystalline form it is called corondum T
d. is not a very reactive metal F
6. T/F Aluminum is amphoterric
True, aluminum can dissolve in both acids and bases
7. Which element is the only semi-metal in the Boron Family?
8. When berryllium reacts with a halide, which of the following is true?
a. They act as Lewis bases F
b. They form a covalant bond F
c. They form lewis acids T
d. They form a neural molecule F
9. What is the electron configuration of Thallium?
10. Which statement is False?
a. Thallium is the heaviest element. T
b. Boron has the highest melting point. T
c. Electron potential increases going down the group. T
d. Thallium has the lowest ionization energy. F
e. All of the above is correct. T
This page viewed 111962 times