Properties of the Boron family - Occurrence, Trends in Periodic Properties, Physical and Chemical properties, Anomalous Properties of Boron, Practice Problems and FAQ

In a joint family, you can easily expect to find people with multi-dimensional personalities and traits. Yet they are bound as one family with some common links of connectivity.

Meet the ‘Bondopadhyay’ family! (aka Boron family), heading for a weekend getaway. They live in the 13^th alley (Group 13) of a district called Purulia (‘p-block’). The youngest member is Bodhi (the little girl), followed by Aloy (the vibrant little boy), Gargi (the mommy), Indro (the father), Tuli (the granny) and Nihar (the grandpa).

Bodhi is 5 years old while Aloy is 13 years old, Gargi is 31 years old and Indro is 49. Tuli is 81 years old and Nihar, the centurion Grandpa is 113 years old!

That’s quite convincing of a proactive and happy family, isn't it? 

Metaphorically, this family is synonymous with the Boron family. They are unique in their own traits and mannerisms. Let us now dig in a bit more and find out the properties of this exciting family!

TABLE OF CONTENTS

• Group 13 Elements – Introduction
• Group 13 Elements – Occurrence 
• Group 13 Elements – Trends in Periodic Properties
• Group 13 Elements – Physical Properties 
• Group 13 Elements – Oxidation States and Trends in Chemical Reactivity
• Group 13 Elements – Chemical Properties
• Group 13 Elements – Inert Pair Effect
• Important Trends and Anomalous Properties of Boron
• Practice Problems
• Frequently Asked Questions - FAQ

Group 13 Elements – Introduction

All the elements are neatly arranged in the periodic table based on their atomic numbers. The elements in the periodic table are arranged in rows (also known as periods) and columns (also known as groups) (in ascending order) in the increasing order of their atomic numbers. 

The periodic table is divided into four blocks, namely, s, p, d, and f. This segregation is based on the valence electron. If the valence electron falls on the p subshell, the element is assigned to the p-block, and so on. The first group in the p-block of the periodic table is Group 13 or the boron family. 

The boron family refers to group 13 and is called so as its first member is ‘Boron’. It belongs to the periodic table's 13th group, with a defined electronic configuration of ns² np¹. Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Thallium (Tl), and Nihonium (Nh) are the elements that make up the boron family or Group 13.

The physical and chemical features of the 13th group, often known as the boron family, appear to follow a pattern in the periodic table. Boron (B) is distinct from the other elements in the group due to its tiny size and lack of d-orbitals. Anomalous features of boron (B) are produced as a result of these discrepancies in the periodic trends.

The outermost electron shell of the elements of the boron family, or the 13th group, has just three electrons. Except for boron (B) and Nihonium (Nh), all of the other elements are metals. Nihonium is a radioactive element. B is a typical non-metal, Al is a metal but shows many chemical similarities to boron. Ga, In, and Tl are almost exclusively metallic in character. Group 13 elements, also known as the boron family, have primarily two oxidation states: +1 and +3. 

In the boron family, the outermost shell of the elements contain only three electrons. All the elements are metals except for boron (B) and nihonium (Nh). Boron is a non-metal, aluminium is a metal that shows similarities in chemical properties to that of boron. Gallium, indium and thallium are metallic and nihonium is radioactive. The elements of this group have two primary oxidation states namely, +1 and +3.

Group 13 Elements – Occurrence

• Boron (B) is only found in trace amounts in its natural condition.

• Boron is typically formed when subatomic particles are bombarded during radioactive decay.

• B is a rare element that occurs as orthoboric acid (H₃BO₃), borax (Na₂B₄O₇.10H₂O), kernite (Na₂B₄O₇.4H₂O) etc,. It exists in two isotopic forms i.e.,¹⁰B (19%) and¹¹B (81%).

• B is a rare element that occurs as borax (Na₂B₄O₇.10H₂O), orthoboric acid (H₃BO₃), kernite (Na₂B₄O₇.4H₂O) etc,. It exists in two isotopic forms i.e.,¹⁰B (19%) and¹¹B (81%).

• Aluminium is one of the most widely available elements both on and off the planet.

• Aluminium occurs as bauxite (Al₂O₃.2H₂O) and cryolite (Na₃AlF₆).

• Gallium is a rare element that can not be found in nature. It can be found in the earth's crust in a concentration of 16.9 ppm (parts per million). It's extracted from bauxite and, on rare occasions, sphalerite.

• Indium is a rare metal, having an estimated abundance of 0.1 ppm in the Earth's crust. An ore of In is roquesite (CuInS₂). It is found in trace amounts in ores of sphalerite and chalcopyrite.

• Thallium is found in minor amounts all over the world. 

• Thallium can be found in a variety of ores. Pyrite is one of these, and it's used to make sulfuric acid. Pyrites provide some thallium, although it is mostly derived as a by-product of copper, zinc, and lead refining. Manganese nodules found on the ocean floor also contain thorium.

• Nihonium (Nh) is not naturally occurring but produced in a laboratory. It is a radioactive element. It is extremely radioactive, with a half-life of roughly 10 seconds for its most stable isotope, nihonium-286.

Group 13 Elements – Trends in Periodic Properties

• The valence shell electronic configuration of the Boron family is ns² np¹.
  

• Atomic and Ionic Radii: On moving down the group, for each successive member, one extra shell of electrons is added, and therefore, the atomic radius is expected to increase. However, a deviation can be seen. The atomic radius of Ga is less than that of Al. The atomic radius is expected to increase when one more electron shell is introduced to each subsequent member as one moves along the group. There is, however, a deviation. Ga has a smaller atomic radius than Al.

This can be understood from the variation in the inner core of the electronic configuration. The presence of additional 10 d-electrons in gallium offers only a poor screening effect for the outer electrons from the increased nuclear charge. As a result, the atomic radius of gallium (135 pm) is less than that of aluminium (143 pm). The difference in the electronic configuration in the inner core helps to explain this. Gallium's additional 10 d-electrons has a poor screening effect on the outer electrons due to the increased nuclear charge. Gallium's atomic radius (135 pm) is therefore smaller than that of aluminium (143 pm).

• Electronegativity: Down the group, electronegativity first decreases from B to Al, and then increases marginally due to the discrepancies in the atomic size of the elements.

• Density: On moving from gallium to thallium, density increases. Boron and aluminium, on the other hand, have low values. This is because they have lower atomic weights than gallium, indium, and thallium.
   

• Ionisation Enthalpy: The sum of the first three ionisation enthalpies for each of the elements is very high. Irregular trends in the I.E. values are observed between Al and Ga, and between In and Tl.

The observed discontinuity in the ionisation enthalpy values between Al and Ga, and between In and Tl are due to the inability of d and f-electrons, which have a low screening effect to compensate for the increase in nuclear charge. The apparent discrepancy in the ionisation enthalpy values between Al and Ga and between In and Tl is caused by poor screening of the d and f electrons' to make up for the rise in nuclear charge.

• In the aqueous state, the boron family's tetrahedral and octahedral compounds exist.

• Trihalides are formed when group 13 elements react with halogens (or group 17 elements like Fluorine, Chlorine, Bromine and Iodine). One boron molecule reacts with three halogen atoms to form trihalides. In water, trihalides hydrolyse and form covalent bonds. 

• Because of their electron shortage, trihalides are good Lewis acids.

• From boron to aluminium, the electronegativity declines at first, then increases slightly as we move lower.

• As we progress down the boron family, the metallic character becomes more prominent.

• Melting and boiling points: From B to Ga, the melting point drops and subsequently rises. Boiling point decreases gradually
   

Because boron exists as a huge covalent polymer in both solid and liquid forms, it has a high melting point. The metal structures of the elements Al, In, and TI are all closely packed.

Gallium is a metal with a unique structure. It is made up entirely of gas molecules. Hence, it has a low melting point. It can be utilised in high-temperature thermometry since it exists as a liquid up to 2000^°C.

The boiling points of the elements of the 13th group, on the other hand, fall in order from boron to thallium. From boron to thallium, the strength of the links that keep the atoms in a liquid state weakens.

Group 13 Elements – Physical Properties

• Boron is non-metallic in nature.
• It is an extremely hard and black coloured solid.
• It exists in many allotropic forms.
• B has an unusually high melting point due to a very strong crystalline lattice.
• Rest of the members of group 13 are soft metals with low melting points and high electrical conductivities.
• Ga which has a lower melting point (303 K), and could exist in a liquid state during summer. The density of the elements increases down the group from boron to thallium.
• Boron has an unusually high melting point due to its icosahedral shape.
• Certain mild mineral acids, as well as aqueous sodium hydroxide (NaOH), cause aluminium to crumble. Because of its amorphous qualities, this is the case. As a result, aluminium is amphoteric in nature. 
• Gallium (Ga) has the lowest melting point of all the elements in the 13th group, or the boron family.
• Indium (In) has a smaller nuclear radius than Thallium (Th) due to lanthanide contraction (Th).
• At high temperatures, the elements of the 13th group combine with oxygen to generate oxides with the conventional chemical formula M₂O₃
• As one moves down the boron family, the acidity of the hydroxides decreases.

• Boron is a non-metal
• It is a hard solid that is black in colour.
• It has various allotropic forms.
• Due to its strong crystalline lattice, the melting point of boron is unusual.
• The other elements in the boron family have high electrical conductivities and low melting points.
• Gallium exisits in liquid state during summer due to is low meting point (303 K). 
• As we move down the boron family, density increases.
• Owing to its amorphic properties, aluminium crumbles on reaction with some mineral acids and aqueous sodium hydroxide. Due to this property, aluminium is amphoteric.
• Among the elements of the boron family, gallium has the lowest melting point.
• As a result of lanthanide contraction, the nuclear radius of indium is lesser than that of thallium.
• The elements of group 13 combine with oxygen at high temperatures to form oxides of general formula M₂O₃.
• The acidity of hydroxides of the boron family decreases down the group. 

Group 13 Elements – Oxidation States and Trends in Chemical Reactivity

• Due to the small size of B, the sum of its first three ionisation enthalpies is very high. This prevents it from forming +3 ions and compels it to form only covalent compounds. The total of the first three ionisation enthalpies of B is extremely high due to its small size. This forces it to only form covalent compounds and inhibits it from generating +3 ions.

• As we move from B to Al, the sum of the first three ionisation enthalpies of Al considerably decreases, so Al forms Al³⁺ ions. The sum of Al's first three ionisation enthalpies significantly drops as we proceed from B to Al, resulting in the formation of Al³⁺ ions.

• In Ga, In, and Tl, both +1 and +3 oxidation states are observed. Relative stability of +1 oxidation states progressively increase for heavier elements. Al < Ga < In < Tl . Both +1 and +3 oxidation states are seen in Ga, In, and Tl. For larger elements, the relative stability of +1 oxidation states gradually rises. Al < Ga < In < Tl.

• In thallium (Tl⁺¹), +1 oxidation state is predominant and so Tl³⁺ oxidation state is highly oxidising in character due to the reluctance of participation of the last two electrons present in the s-orbital of Thallium.

• The compounds in +1 oxidation states are more ionic than those in +3 oxidation state. 

• In a trivalent state, the number of electrons around the central atom in a molecule will be only six like, B in BF3. Such electron deficient molecules have a tendency to accept a pair of electrons to achieve stable electronic configuration and behave as Lewis acids. The tendency to behave as a Lewis acid decreases with the increase in the size, down in the group. In a trivalent state, a molecule's core atom will only have six electrons surrounding it, like B in BF₃. In order to acquire a stable electronic state and behave as Lewis acids, such electron-deficient molecules have a propensity to receive two electrons. With an increase in group size, there is a decrease in the inclination to behave as a Lewis acid.

• AlCl₃ achieves stability by forming a dimer. 
  

• In the trivalent state (+3), most of the compounds being covalent are hydrolysed in water. Trichlorides on hydrolysis form tetrahedral species [M(OH)₄]^-. The majority of compounds that are covalent are hydrolyzed in water when they are in the trivalent state (+3). On hydrolysis, trichlorides produce the tetrahedral species [M(OH)₄]^-.

• AlCl₃ in acidified aqueous solution, forms an octahedral [Al(H₂O)₆]³⁺ ion. In an acidified aqueous solution, AlCl₃ forms an octahedral ion of the formula [Al(H₂O)₆]³⁺.
  

Group 13 Elements – Chemical Properties

Reaction with Hydrogen

• Group 13 elements react with hydrogen and form hydrides. Moving down the group, the elements produce trihydrides (MH₃), and the stability declines. They are compounds that are lacking in electrons. Boron forms boranes, a group of volatile hydrides.

• Hence, they form good Lewis acids and exist in anionic hydride form also like, BH₄^-, AlH₄^-.

• BH₃ undergoes polymerisation to form diborane (B₂H₆). 

• Organo-boranes are formed when diborane is added to alkenes and alkynes in ether at room temperature.

Reactivity Towards Air

• Boron is unreactive in crystalline form. Aluminium forms a very thin protective oxide layer on its surface. Amorphous forms of B and Al metal on heating in air form respective oxides. B and Al react with nitrogen at high temperatures to form nitrides. When it is in crystalline form, boron is inert. On the surface of aluminium, a very thin oxide layer serves as protection. When heated in air, the amorphous forms of B and Al metals transform into their corresponding oxides. Nitrides are created when B and Al react with nitrogen at high temperatures.

• Nitrogen is inert, so to break the bond between the nitrogen atoms in a nitrogen molecule, a very high temperature is required.

• Aluminium does not react with dry air. In moist air, a thin oxide layer forms on the surface of aluminium metal. It shields the metal from further deterioration. At high temperatures, it also forms nitride with nitrogen.

Nature of Oxides of Group 13 Elements

• Boron trioxide (B₂O₃) is acidic in nature. Down the group, acidity of oxides decreases, and basicity increases.

• Aluminium and gallium oxides are amphoteric in nature as they react with both acids and bases. Due to their ability to react with both bases and acids, the oxides of gallium and aluminium are amphoteric.

• Indium and Thallium oxides (In₂O₃ and Tl₂O₃) are basic in nature.

• All oxides of the boron family disintegrate when reacted with weak mineral acids.

Reactivity Towards Acids and Alkalis

• Boron does not react with acids and alkalis even at moderate temperature, but aluminium dissolves in mineral acids and aqueous alkalis and thus shows amphoteric character. Even at room temperature, boron is inert towards acids or alkalis, whereas aluminium dissolves in both and exhibits amphoteric properties.

Sodium tetrahydroxoaluminate (III) forms when aluminium and sodium hydroxide react.

• Boron however reacts with strong oxidising agents like a mixture of HNO₃ and H₂SO₄ at elevated temperatures. With base it forms borates but only above 773 K.

B(s)+3HNO₃ (aq) → H₃BO₃ (aq)+3NO₂ (g)

2B(s) + 6KOH(s) → 2K₃BO₃(s) + 3H₂(g)

• All other members react with non-oxidising as well as oxidising acids to liberate hydrogen gas.

• Concentrated nitric acid renders aluminium passive by forming a protective oxide layer on the surface. Aluminum is made passive by concentrated nitric acid by creating a barrier of protective oxide on the surface.

Reactivity Towards Water 

Boron does not react with water or even steam. However, at elevated temperatures, boron can react with steam. If there is no oxide layer on the surface of aluminium, it decomposes in cold water.

2B(s)+6H₂O(g) →2B(OH)₃(s)+3H₂(g)

2Al(s)+6H₂O(aq) →2Al(OH)₃(s)+3H₂(g)

Gallium and Indium abstain from reacting with water unless oxygen gas is present. Thallium forms TlOH in moist air.

Reactivity Towards Halogens

• Group 13 elements react with halogens to form trihalides, except Tl (because of the inert pair effect in Tl, it prefers +1 oxidation state). Except for Tl (which prefers +1 oxidation state due to the inert pair effect in Tl), the elements in the boron group react with halogens to generate trihalides.

• The reactions are:

2E(S)+3X₂(g) →2EX₃(s) ;Where E= F,Cl,Br,I

2Al (s)+3Cl₂(g) →2AlCl₃(s)

Here, Cl2 gas is passed on an Al foil and yellow solid AlCl₃ powder is formed. 


 

• The inclination to accept electron pairs can also be seen in aluminium halides and other members (which operate as Lewis acids). However, as the cation grows larger, this tendency diminishes. They generate complex halides of the type [MX₆]^3-, in which the coordination number is increased to 6 using the d-orbitals.

Reaction with Metals

Borides are formed when boron reacts with metals. The rest of Group 13 elements are hesitant to combine with metals. This depicts the non-metallic property of Boron.

3Mg + 2B → Mg₃B₂

Group 13 Elements – Inert pair Effect

In p-block, on moving down the group, the non-participation of the two s-electrons in bonding due to the high energy needed for unpairing them is known as the Inert-pair effect.

• The group 13 elements in the group have a general oxidation state of +3, and +1. The tendency to create +1 ion rises as we move down the Boron family. The inert pair effect is to blame for this.

• Considering B³⁺ and B⁺, B³⁺ is more stable than B⁺. Considering Tl³⁺ and Tl⁺, Tl⁺ was shown to be more stable than Tl³⁺.

• The inert pair effect can be used to explain this. Because of the insufficient shielding of the intervening electrons, the outermost s-orbital does not participate in chemical bonding.

Group 13 Elements – Important Trends and Anomalous Properties of Boron

The tri-chlorides, tri-bromides, and tri-iodides of all these elements, being covalent in nature, are hydrolysed in water. Tetrahedral [M(OH)₄]^- and octahedral [M(H₂O)₆]³⁺, except in boron, exist in aqueous medium. The tri-iodides, tri-bromides and tri-chlorides of the group 13 elements hydrolyse in water due to their covalent nature. In an aqueous medium, Tetrahedral [M(OH)₄]^- and octahedral [M(H₂O)₆]³⁺ exist.

Reason: Due to the absence of d-orbitals in boron, maximum covalency of B is 4. However, in other elements of group 13 (M = Al, Ga, In, Tl), d-orbitals are available. So, the maximum covalency can be expanded beyond 4. The monomeric trihalides, being electron deficient, are strong Lewis acids. Borontrifluoride easily reacts with Lewis bases such as NH₃ to complete an octet around boron.

Metal halides (E.g. AlCl3 ) are dimerised with the help of halogen bridging (E.g. Al₂Cl₆). The metal species completes its octet by accepting electrons from halogen in these halogen-bridged molecules. Electron deficient aluminium completes its octet by accepting a lone pair of electron from chlorine of another AlCl₃ molecule and forms a 3-centred-4-electron banana bond of Al−Cl−Al.

Owing to the lack of d-orbitals, the maximum covalency of B is 4. The rest of the elements of group 13 (M = Al, Ga, In, Tl) have d-orbitals. This is precisely the reason why the other elements can expand their covalency beyond 4. The monomeric trihalides are electron deficient and are therefore strong Lewis acids. Lewis bases like NH₃ complete the octet around boron in boron trifluoride.

Due to halogen bridging, metal halides like AlCl₃ dimerise (E.g. Al₂Cl₆). In these halogen-bridged molecules, the metals accept electrons from the halogens and complete their octet. The octet of aluminium, which is electron-deficient is completed by accepting a lone pair of electrons from the chlorine of another AlCl₃ molecule. This results in the formation of a 3-centred-4-electron banana bond of Al−Cl−Al.

Practice Problems

Q1. The most basic hydroxide among the elements of the boron family is

A. B(OH)₄
B. TlOH
C. Al(OH)₄
D. InOH

Answer: Because the electropositive nature of elements increases down the group, the hydroxides of group 13 become more basic. So, the hydroxide of thallium is the most basic. 

So, option B) is the correct answer.

Q2. Define pyroborates and metaborates.
Answer: Borates are basically compounds of boron containing BO₃^3- units that are sp² hybridised. When two such discrete units are linked by a common oxygen atom, pyroborate is formed which is denoted as B₂O₅^4-. 

Example: Mg₂B₂O₅

Metaborates are those where each borate unit shares two oxygen atoms. Hence they are either cyclic or chain shaped. The general formula is (BO₂)_n^n-.

Q3. The oxidation state +1 is most stable for 

A. Boron
B. Indium
C. Thallium
D. Gallium

Answer: In thallium (Tl⁺¹), +1 oxidation state is predominant due to the inert pair effect. So, Tl³⁺ oxidation state is highly oxidising in character due to the reluctance of participation of the last two electrons present in the s-orbital of Thallium.

So, option C) is the correct answer.

Frequently Asked Questions - FAQ

Question 1. The borohydride ion BH₄^- can form coordinate bonds with metal centres. Predict whether it behaves as a monodentate, bidentate or tridentate ligand.
Answer:  It can actually behave as any or all these three types of ligands, depending on changes in symmetry of the B-H bond lengths..

Question 2. Why are trihalides of the boron family good Lewis acids?
Answer: Trihalides of elements of the boron family are electron deficient as the central ion consists of only six electrons. So they have a tendency to gain electron pairs to fulfil octet. Thus, they are good Lewis acids.

Question 3. Why is the radius of gallium less than aluminium?
Answer: On moving down the group, for each successive member, one extra shell of electrons is added, and therefore, the atomic radius is expected to increase. However, a deviation can be seen. The atomic radius of Ga is less than that of Al. This can be understood from the variation in the inner core of the electronic configuration. The presence of additional 10 d-electrons in gallium offers only a poor screening effect for the outer electrons from the increased nuclear charge. As a result, the atomic radius of gallium (135 pm) is less than that of aluminium (143 pm). The difference in the electronic configuration in the inner core helps to explain this. Gallium's additional 10 d-electrons has a poor screening effect on the outer electrons due to the increased nuclear charge. Gallium's atomic radius (135 pm) is therefore smaller than that of aluminium (143 pm). The atomic radius is expected to increase when one more electron shell is introduced to each subsequent member as one moves along the group. There is, however, a deviation. Ga has a smaller atomic radius than Al.

Question 4. What happens when aluminium is made to react with nitric acid?
Answer: Concentrated nitric acid renders aluminium passive due to the formation of an oxide layer on aluminium surface (Al₂O₃).

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