Chemical Properties of Boron Family

The boron family, or otherwise Group 13 elements, is a group of elements that all contain three electrons in the outermost energy level. This grouping of elements results in some rather interesting chemical properties: all the other members are metals except boron, which is a metalloid—hence, it shows both metallic and non-metallic properties. Their atomic and ionic radii increase from boron to thallium and have repercussions on their reactivity and bonding nature. Because of its small size and high ionization energy, boron forms covalent compounds, whereas aluminium and other heavier elements form ionic bonds. The oxides run from acidic, such as boron trioxide, through amphoteric, like aluminium oxide, to basic, like thallium oxide)

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This Story also Contains

1. Chemical Behaviours and Compounds
2. Applications and Relevance
3. Some Solved Examples
4. Summary

Chemical Behaviours and Compounds

The chemical behaviour of the elements of the boron family varies greatly. For instance, boron can form borates and boranes, of major importance in the glass and detergent industries. Boria does self-passivate with an oxide film; hence, it doesn't corrode. Other typical properties of the group element include reactions with acids and other bases to form salts and complex compounds. Gallium has a low melting point, and thus it is used as a semiconductor device for forming compounds such as gallium arsenide. Indium and thallium are rather unknown elements; however, they possess unique properties that make them able to form compounds like indium tin oxide and thallium sulphate, respectively. These elements can exist as trihalides, BX₃, AlX₃, and possess some exciting reactivity for halogens and other nonmetals. Each member of the family has several different compounds it can form making the chemical versatility thus the industrial importance of that element all the greater.

Reaction towards air
Boron is unreactive in crystalline form. Aluminium forms a very thin oxide layer on the surface which protects the metal from further attack. Amorphous boron and aluminium metal on heating in air form B₂O₃ and Al₂O₃ respectively. With dinitrogen at a high temperature, they form nitrides.

2E(s)+3O₂( g)⟶2E₂O₃( s)2E(s)+N₂( g)⟶2EN(s)
Where E is an element

The nature of these oxides varies down the group. Boron trioxide is acidic and reacts with basic (metallic) oxides forming metal borates. Aluminium and gallium oxides are amphoteric and those of indium and thallium are basic in their properties.

Reactivity towards acids and alkalis
Boron does not react with acids and alkalis even at moderate temperatures, but aluminium dissolves in mineral acids and aqueous alkalis and thus shows an amphoteric character. Aluminium dissolves in dilute HCl and liberates dihydrogen.

2Al(s)+6HCl(aq)→2Al³⁺(aq)+6Cl⁻(aq)+3H₂( g)

However, concentrated nitric acid renders aluminium passive by forming a protective oxide layer on the surface.
Aluminium also reacts with aqueous alkali and liberates dihydrogen.

2Al(s)+2NaOH(aq)+6H₂O(l)→2Na+[Al(OH)₄]⁻(aq)+3H₂( g)

Reactivity towards halogens
These elements react with halogens to form trihalides (except TlI₃).

Applications and Relevance

The chemical properties of the boron family find applications in a wide range of aspects. The role of boron in strengthening glass and ceramics is known, but in the manufacture of borosilicate glass, it is also indispensable because of its very high hardness and resistance to thermal shock; thus, having wide applications in laboratory equipment and cookware. Aluminium is vital in the aerospace and automotive industries due to its lightness and resistance to corrosion. Gallium in semiconductors changed technology with efficient light-emitting diodes and solar cells. Next in line for use in touch screens and solar cells is indium, but thallium is being used, and it is highly toxic. It finds an application in medical imaging and electronics. The boron family also speaks, at an academic level, to periodic trends and bonding behaviours that have enriched our knowledge of inorganic chemistry. These applications underline the practical realization of these elements in various walks of life, from the materials in use in everyday life to high technology.

Recommended topic video on (Chemical Properties of Boron Family)

Some Solved Examples

Example 1

Question: Which of the following liberates H₂ gas on reaction with HCl?

1. Ag
2. Cu
3. Au
4. Al

Solution:

Aluminium reacts with diluted HCl and liberates H₂ gas.

2Al+6HCl→2AlCl₃+3H₂

Elements with lower reactivity are not able to displace hydrogen from acids, so H₂ gas is not obtained.

Ag, Au, Cu+HCl→No Reaction

Hence, the answer is option (4).

Example 2

Question:

The bond dissociation energy of B-F in BF₃ is 646 kJ mol whereas that of C-F in CF₄ is 515 kJ mol. The correct reason for higher B-F bond dissociation energy as compared to that of C-F is

1. the smaller size of the B atom as compared to that of the C atom
2. stronger σ sigmaσ bond between B and F in BF₃ as compared to that between C and F in CF₄
3. significant π interaction between B and F in BF₃ whereas there is no possibility of such interaction between C and F in CF₄
4. lower degree of π interaction between B and F in BF₃ than that between C and F in CF₄

Solution: B has vacant p orbitals so it can form an π back bond with Fluorine. This back bonding is not possible in CF₄ as C does not have any vacant orbital. Therefore, the B-F bond is stronger than the C-F bond and has a greater bond energy value.

Hence, the answer is option (3).

Example 3

Question: When metal ‘M’ is treated with NaOH, a white gelatinous precipitate ‘X’ is obtained, which is soluble more than NaOH. Compound ’X’ when heated strongly gives an oxide which is used in chromatography as an adsorbent. The metal ‘M’ is:

1. Fe
2. Zn
3. Ca
4. Al

Solution:

Aluminium dissolves in NaOH to form a gelatinous white precipitate of Al(OH)₃ and liberates H₂ gas.

Al(OH)₃ dissolves more than NaOH forming Sodium Aluminate.

Al(OH)₃+NaOH→Na[Al(OH)₄]soluble

Al(OH)₃ gives Alumina on heating, which is used as an adsorbent in chromatography.

Al(OH)₃⟶ΔAl₂O₃

Hence, the answer is option (4).

Summary

The boron family contains elements of interesting chemical properties and an applicability spectrum that is very wide. From the chemical peculiarities of boron as a metalloid to the uses of aluminium in very large industries and electrical uses of gallium, indium, and thallium, such elements display a very diverse spate of chemical behaviours and practical usages. The possibility of forming a number of compounds and reacting with other elements, coupled with the applicability of the element in life processes, has made the study of the boron family very vital. Knowledge of these elements does not merely improve our awareness regarding the chemical properties but also underlines their importance in various life processes that relate to industries and technologies.