Trends and Properties of Boron and Aluminium

What do you know about the properties of Boron Family? Have you ever wondered why some properties of Boron are different from those of aluminium? Why are they then clubbed under a single group? In this chapter, we will study the properties of boron and its family. We will look at a few anomalies and also the uses of boron and aluminium.

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The Boron Family

We can witness some critical patterns in the chemical and physical behaviour of the group 13 elements. The trichlorides, iodides as well as bromides of each one of these elements hydrolyse in water. This is because these are all covalent in nature. Species like tetrahedral [M(OH)₄]^– and octahedral [M(OH)₆]³⁺ exist in a fluid medium. An exception to this is boron.

The monomeric trihalides are strong Lewis acids. This is because they are highly electron lacking. Boron trifluoride effortlessly reacts with Lewis bases, for example, NH₃ to finish octet around boron.

F₃B   +   : NH₃ →   F₃B – NH₃

The greatest covalence of Boron is 4. We can attribute this to the non-attendance of d-orbitals. On the other hand, the d-orbitals are accessible with Aluminium and other elements. This is the reason their most extreme covalency can be normal past 4.

A large portion of other metal halides (e.g. AlCl₃) are dimerized through halide bridging (e.g. Al₂Cl₆). The metal species finishes its octet with the help of electrons accepted from halogen in these halogen bridged particles. Let us now look at the anomalous properties of boron and aluminium.

Anomalous Properties of Boron and Aluminium

• The most basic anomaly that we notice in these elements is that Boron is a non-metal. In contrast to this, aluminium is a metal.
• While boron is a non-conductor of electricity, aluminium is a very good conductor of electricity.
• We can find Boron in two structures. These are the amorphous and crystalline structures. On the other hand, aluminium is a delicate metal. It does not exist in various structures.
• The boiling point and melting point of boron are much greater as compared to those of aluminium.
• Boron forms just covalent compounds while aluminium forms even some ionic compounds.
• The oxides and hydroxides of boron are acidic in nature. As compared to this, the oxides and hydroxides of aluminium are amphoteric in nature.
• The trihalides of boron (BX₃) are monomers. On the other hand, aluminium halides exist as dimers (Al₂X₆).
• The hydrides of boron are quite inert while those of aluminium are flimsy.

Diagonal Resemblance of Boron With Silicon

It is really interesting to note that boron and silicon show a lot of similarity in their properties. Both of these elements exhibit the distinctive properties of non-metals. These don’t shape cations. Both of them exist in crystalline and amorphous structures.

Boron Oxide (B₂O₃) and silica (SiO₂) are both acidic in nature. These break up in solutions that are alkaline in nature to form borates and silicates separately.

B₂ O₃ + 6NaOH → 2Na BO₃ + 3H₂O

SiO₂ + 2NaOH → Na₂SiO₃ + H₂O

We can find Silicates and borates having a tetrahedral SiO₄ and BO₄ auxiliary units individually. The chlorides of both Si and B hydrolyse by water. This reaction gives rise to the formation of respective silicic acid and boric acid.

BCl₃ + 3H₂O → H₃BO₃ + 3HCl

SiCl₄ + 3H₂O  → H₂SiO₃ + 4HCl

The Hydrides of Boron and Silicon are very stable. We know a variety of unstable hydrides that burst into flames on introduction to air and are effortlessly hydrolyzed.  Both boron and silicon are semiconductor by nature. Now, we move on to study some of the uses of boron.

Uses of Boron

• Metal borides are a common part of the nuclear reactors. Here, they are used as defensive shields and control rods. This is because of the high capacity of ¹⁰B isotope to ingest neutrons.
• Boron is an important component in the steel industry. It finds its usage in the expansion of the hardness of steel.
• Many times, we utilise boron as a semiconductor for making electronic gadgets.
• Boron compounds are turning out to be progressively vital as rocket fills. This is primarily because of their high energy/weight proportion.
• The fibres from boron are a common usage in the process of making light composite materials for airships.
• Boron is a basic element in the plant digestion system.
• Boron carbide filaments are hard, even though they are extremely light. That is why they are primarily used for making bulletproof vests.

Solved Example for You

Q: Write down some common uses of aluminium.

Ans:

• Aluminium is widely used in various industries and also regular day to day existence.
• It is a major component of the steel and iron industry. Aluminium and its amalgams are widely applied in the construction of pipe, poles, tubes, wires, plates or foils etc.
• They are of utmost importance in the pressing, utensil making, plane, construction, and transportation industry.