What is the significance of boron's ability to form three-center two-electron bonds?

Boron's ability to form three-center two-electron bonds is significant because it allows boron to overcome its electron deficiency and achieve stability in compounds like diborane (B₂H₆). This unique bonding type explains the structure and reactivity of many boron hydrides and contributes to boron's diverse chemistry.

How does the amphoteric nature of aluminum affect its chemical behavior?

The amphoteric nature of aluminum allows it to react with both acids and bases. In acidic solutions, aluminum acts as a base, forming Al³⁺ ions. In basic solutions, it acts as an acid, forming aluminate ions [Al(OH)₄]⁻. This property influences aluminum's reactivity and its applications in various chemical processes.

Why does boron show a tendency to form electron-deficient compounds?

Boron shows a tendency to form electron-deficient compounds because it has only three valence electrons but four available orbitals in its outer shell. This leads to the formation of compounds where boron doesn't have a complete octet, resulting in its Lewis acid behavior and ability to accept electron pairs.

How does the boron-nitrogen bond compare to the carbon-carbon bond?

The boron-nitrogen bond is similar to the carbon-carbon bond in many ways, leading to the concept of "inorganic benzene." Both bonds have similar bond lengths and energies. However, the B-N bond is slightly more polar due to the electronegativity difference between boron and nitrogen, affecting the reactivity and properties of boron-nitrogen compounds.

Why does gallium have a lower melting point compared to aluminum?

Gallium has a lower melting point than aluminum due to its unique crystal structure. Gallium atoms form covalent bonds in pairs, resulting in a structure with weaker intermolecular forces. This leads to a lower melting point (29.8°C) compared to aluminum (660.3°C), despite gallium being heavier.

Why does boron have a lower electronegativity compared to other group 13 elements?

Boron has a lower electronegativity due to its smaller atomic size and higher effective nuclear charge. Unlike other group 13 elements, boron lacks d-orbitals, which results in less shielding of the nucleus and stronger attraction to electrons.

How does the reactivity of boron family elements change as we move down the group?

The reactivity of boron family elements generally increases as we move down the group. This is because the atomic size increases, making it easier for the atoms to lose electrons and form compounds. However, there are some exceptions due to the inert pair effect in heavier elements.

What is the "inert pair effect" and how does it affect the chemistry of heavier boron family elements?

The inert pair effect refers to the tendency of the outermost s electrons to remain unionized in heavier elements. In boron family, this effect becomes prominent in thallium, causing it to form more stable +1 oxidation state compounds compared to the +3 state, unlike lighter elements in the group.

Why does boron form covalent compounds while other group 13 elements tend to form ionic compounds?

Boron forms covalent compounds due to its small size, high ionization energy, and high electronegativity compared to other group 13 elements. These properties make it difficult for boron to lose electrons, favoring the sharing of electrons (covalent bonding) instead of electron transfer (ionic bonding).

How does the electron configuration of boron family elements influence their chemical properties?

The electron configuration of boron family elements (ns² np¹) influences their chemical properties by determining their valence electrons and potential oxidation states. This configuration allows them to form compounds with +1 and +3 oxidation states, with the stability of these states varying across the group due to factors like atomic size and inert pair effect.

How does the reactivity of boron family elements with water change across the group?

The reactivity of boron family elements with water generally increases down the group. Boron doesn't react with water at room temperature, while aluminum reacts slowly to form a protective oxide layer. Gallium and indium react more readily, and thallium reacts vigorously. This trend is due to increasing atomic size and decreasing electronegativity down the group.

What causes the "diagonal relationship" between boron and silicon?

The diagonal relationship between boron and silicon is caused by their similar atomic sizes and electronegativity values. This similarity arises from the interplay of nuclear charge, electron shielding, and atomic radius trends across periods and down groups. As a result, boron and silicon exhibit some comparable chemical properties despite being in different groups.

How does the ability to form double bonds differ between boron and other group 13 elements?

Boron has a greater ability to form double bonds compared to other group 13 elements. This is due to its smaller size and higher electronegativity, which allow for more effective orbital overlap. Heavier elements in the group have larger atomic sizes and are less likely to form stable double bonds due to poor orbital overlap.

Why does boron trifluoride act as a Lewis acid while boron trichloride is a weaker Lewis acid?

Boron trifluoride (BF₃) is a stronger Lewis acid than boron trichloride (BCl₃) due to the higher electronegativity of fluorine. This results in a more electron-deficient boron atom in BF₃, making it more prone to accept electron pairs. Additionally, the smaller size of fluorine allows for better orbital overlap, enhancing the Lewis acidity of BF₃.

How does the stability of +1 oxidation state change across the boron family?

The stability of the +1 oxidation state increases down the boron family. Boron and aluminum rarely form stable +1 compounds. Gallium and indium can form some +1 compounds, but they are less stable than their +3 counterparts. Thallium forms stable +1 compounds due to the inert pair effect, where the 6s² electrons are less likely to participate in bonding.

What is the reason for the high melting point of boron compared to other group 13 elements?

Boron has a high melting point (2076°C) compared to other group 13 elements due to its ability to form strong covalent bonds in its crystalline structure. Boron atoms tend to form icosahedral clusters with strong B-B bonds, resulting in a rigid three-dimensional network that requires high energy to break, hence the high melting point.

How does the tendency to form complexes change across the boron family?

The tendency to form complexes generally increases down the boron family. Boron forms few complexes due to its small size and high charge density. Aluminum forms more complexes, often with six-coordinate octahedral geometry. Gallium, indium, and thallium show increasing tendencies to form complexes, with a wider range of coordination numbers and geometries due to their larger atomic sizes.

Why does boron not follow the octet rule in many of its compounds?

Boron often doesn't follow the octet rule because it has only three valence electrons and four available orbitals in its outer shell. To achieve a full octet, boron would need to gain five electrons, which is energetically unfavorable. Instead, boron forms electron-deficient compounds, often with incomplete octets, leading to its Lewis acid behavior.

How does the reactivity of boron family elements with halogens change across the group?

The reactivity of boron family elements with halogens generally increases down the group. Boron reacts with halogens to form covalent trihalides. Aluminum, gallium, indium, and thallium form increasingly ionic halides. The reaction becomes more exothermic down the group due to decreasing ionization energies and increasing atomic sizes.

What is the significance of the "boron anomaly" in periodic trends?

The "boron anomaly" refers to the unexpected trend in first ionization energies within group 13. Boron has a higher first ionization energy than aluminum, contrary to the general trend of decreasing ionization energy down a group. This anomaly is due to boron's small size and the absence of d-orbitals, resulting in less effective shielding of the nuclear charge.

How does the ability to form organometallic compounds vary across the boron family?

The ability to form organometallic compounds generally increases down the boron family. Boron forms stable organoboron compounds with covalent B-C bonds. Aluminum forms various organoaluminum compounds used in organic synthesis. Gallium, indium, and thallium also form organometallic compounds, with increasing ionic character in the metal-carbon bond as we move down the group.

Why does boron trifluoride form an adduct with ammonia while boron doesn't react directly with nitrogen?

Boron trifluoride forms an adduct with ammonia due to its strong Lewis acid character, accepting the lone pair of electrons from ammonia (a Lewis base). In contrast, boron doesn't react directly with nitrogen because of the strong triple bond in N₂ and the lack of a suitable electron pair for boron to accept. The BF₃-NH₃ adduct formation is driven by the completion of boron's octet.

How does the tendency to form hydroxides change across the boron family?

The tendency to form hydroxides increases down the boron family. Boron forms boric acid (H₃BO₃), a weak acid. Aluminum forms amphoteric aluminum hydroxide Al(OH)₃. Gallium and indium form increasingly basic hydroxides. Thallium forms a strongly basic hydroxide TlOH. This trend is due to increasing atomic size and decreasing electronegativity down the group.

What causes the difference in oxidizing power between boron and aluminum?

Boron is a stronger oxidizing agent than aluminum due to its higher electronegativity and smaller atomic size. Boron's electron affinity is higher, making it more likely to gain electrons and be reduced. Aluminum, with its larger size and lower electronegativity, is less effective at attracting electrons, resulting in weaker oxidizing power.

Why does boron form stronger π-bonds compared to other group 13 elements?

Boron forms stronger π-bonds compared to other group 13 elements due to its smaller atomic size and higher electronegativity. These properties allow for better orbital overlap between boron's p-orbitals and those of other elements, resulting in stronger π-bonds. Heavier elements have larger, more diffuse orbitals, leading to weaker π-bonding.

How does the tendency to form intermetallic compounds change across the boron family?

The tendency to form intermetallic compounds generally increases down the boron family. Boron forms borides with many metals, often with complex structures. Aluminum forms various intermetallic compounds, including important alloys. Gallium, indium, and thallium also form intermetallic compounds, with increasing metallic character and diversity of structures down the group.

What causes the difference in reducing power between aluminum and thallium?

Aluminum is a stronger reducing agent than thallium due to its smaller atomic size and higher ionization energy. Aluminum more readily loses its three valence electrons to form Al³⁺. Thallium, affected by the inert pair effect, tends to form Tl⁺ ions, making it a weaker reducing agent. The stability of the +1 oxidation state in thallium also contributes to its weaker reducing power.

How does the ability to form oxoacids vary across the boron family?

The ability to form oxoacids decreases down the boron family. Boron forms boric acid (H₃BO₃) and various polyborates. Aluminum can form aluminic acid, but it's less stable. Gallium and indium form weak, unstable oxoacids. Thallium doesn't form significant oxoacids. This trend is due to decreasing covalent character and increasing basic nature of the oxides down the group.

Chemical Properties of Boron Family

Chemistry
The P Block Elements (Group 13 and Group 14)
Chemical Properties of Boron Family

Chemical Properties of Boron Family

Shivani PooniaUpdated on 02 Jul 2025, 06:27 PM IST

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)

This Story also Contains

Chemical Behaviours and Compounds
Applications and Relevance
Some Solved Examples
Summary

Chemical Properties of Boron Family

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.

Some Solved Examples

Example 1

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

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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

the smaller size of the B atom as compared to that of the C atom
stronger σ sigmaσ bond between B and F in BF₃ as compared to that between C and F in CF₄
significant π interaction between B and F in BF₃whereas there is no possibility of such interaction between C and F in CF₄
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 CF₄as 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:

Solution:

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

Al(OH)₃dissolves 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.

Frequently Asked Questions (FAQs)

Q: Why does aluminum form a protective oxide layer while boron doesn't?

Aluminum forms a protective oxide layer (Al₂O₃) because it reacts readily with oxygen, and the resulting oxide forms a dense, adher

Q: How does the Lewis acidity of boron trihalides change with different halogens?

The Lewis acidity of boron trihalides increases in the order BF₃ > BCl₃ > BBr₃ > BI₃. This trend is due to two competing factors: electronegativity and size of the halogen. While fluorine is the most electronegative, making boron more electron-deficient, its small size leads to greater p-π bonding with boron, partially compensating for the electron deficiency. Larger halogens provide less effective p-π bonding, increasing Lewis acidity.

Q: What causes the difference in melting points between gallium and indium?

The difference in melting points between gallium (29.8°C) and indium (156.6°C) is primarily due to their crystal structures. Gallium has a unique structure with covalent Ga₂ pairs, resulting in weak intermolecular forces and a low melting point. Indium has a more typical metallic structure with stronger metallic bonding, leading to a higher melting point despite being lower in the group.

Q: How does the tendency to form mixed valence compounds vary across the boron family?

The tendency to form mixed valence compounds increases down the boron family. Boron and aluminum rarely form mixed valence compounds. Gallium and indium can form some compounds with both +1 and +3 oxidation states. Thallium shows the highest tendency to form mixed valence compounds, often existing in both +1 and +3 states due to the inert pair effect.

Q: Why does boron trifluoride have a trigonal planar geometry while ammonia is pyramidal?

Boron trifluoride (BF₃) has a trigonal planar geometry because boron uses sp² hybridization, forming three equivalent B-F bonds with 120° angles. The central boron atom has no lone pairs. Ammonia (NH₃) is pyramidal because nitrogen uses sp³ hybridization, with one orbital occupied by a lone pair. This lone pair repels the bonding pairs, resulting in a pyramidal shape with bond angles less than 109.5°.

Q: How does the ability to form coordination compounds change across the boron family?

The ability to form coordination compounds generally increases down the boron family. Boron forms few coordination compounds due to its small size and high charge density. Aluminum forms more coordination compounds, often with octahedral geometry. Gallium, indium, and thallium show increasing tendencies to form diverse coordination compounds with various ligands and geometries, due to their larger atomic sizes and available d-orbitals.

Q: What causes the difference in hydrolysis behavior between boron and aluminum compounds?

The difference in hydrolysis behavior between boron and aluminum compounds is due to their different electronic structures and bonding types. Boron compounds often undergo hydrolysis to form boric acid, maintaining covalent bonds. Aluminum compounds hydrolyze to form Al(OH)₃, which can further react in acidic or basic conditions due to aluminum's amphoteric nature. This difference arises from boron's smaller size and higher electronegativity.

Q: How does the ability to form cluster compounds vary among boron family elements?

The ability to form cluster compounds is most pronounced in boron and decreases down the group. Boron forms various cluster compounds, like boranes, with complex three-dimensional structures. Aluminum can form some cluster compounds, but they are less common. Heavier elements (Ga, In, Tl) rarely form stable cluster compounds due to their larger atomic sizes and tendency towards ionic bonding.

Q: How does the stability of trihalides change across the boron family?

The stability of trihalides generally decreases down the boron family. Boron trihalides (BX₃) are the most stable and exist as discrete molecules. Aluminum trihalides (AlX₃) exist as dimers in the vapor phase. Gallium, indium, and thallium trihalides show increasing ionic character and are less stable in their +3 oxidation state, especially for thallium due to the inert pair effect.

Q: Why does boron show a greater tendency to form catenated compounds compared to other group 13 elements?

Boron shows a greater tendency to form catenated compounds (compounds with boron-boron bonds) due to its small size and ability to form strong covalent bonds. The electron deficiency of boron also promotes the formation of multicenter bonds, leading to complex borane structures. Heavier group 13 elements prefer ionic bonding and have less tendency to form element-element bonds.

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