Group-13 Elements: Boron Family | Chemistry Class 12 - NEET PDF Download

What are Group 13 Elements?

The group 13 elements are the first group in the p-block of the periodic table. 

All the elements of group 13 are also called the boron family. The periodic table is segregated into s, p, d and f-blocks. This segregation is done based on the valence electron, if the valence electron falls on the p subshell, it comes in p-block and so on.

Group 13 Elements

The members of Group 13 elements are:

• Boron
• Aluminium
• Gallium
• Indium
• Thallium

The general electronic configuration for the group 13 elements is ns²np¹. Al is the most abundant metal and third most abundant element in the earth’s crust.

Physical Properties & Trends of Group 13 Elements

1. Electronic Configuration

• Their valence shell electronic configuration of this group is ns²np¹. Here is the detailed electronic configuration of all the elements.
  

  

2. Atomic Radii and Ionic Radii

• Group 13 elements have a smaller size than those of alkaline earth metals due to greater effective nuclear charge, Z_eff’
  
• Atomic radii increase on going down the group with an anomaly at gallium (Ga). Atomic radius increases down the group (Tl has the largest atomic radius.) An unexpected decrease in the atomic size of Ga is due to the presence of electrons in d-orbitals which do not screen the attraction of the nucleus effectively. The ionic radii regularly increase from B³⁺ to TI³⁺.

3. Density

It increases regularly on moving down the group from B to Tl.

4. Melting and Boiling Points

• The Melting point and boiling point of group 13 elements are much higher than those of group 2 elements.
  
• The melting point decreases from B to Ga and then increases, due to structural changes in the elements. Boron has a very high melting point because of its three-dimensional structure in which B atoms are held together by strong covalent bonds.
• The low melting point of Ga is due to the fact that it consists of Ga₂ molecules, and Ga remains liquid upto 2276 K. Hence, it is used in a high-temperature thermometer.
  
  Question for Group-13 Elements: Boron Family

  Try yourself:Which of the following element has the highest melting point?

  • A.

    thallium
  • B.

    gallium
  • C.

    aluminium
  • D.

    boron

  Explanation

  Boron has a very high melting point when compared to the other elements in group 13, this is because of its three-dimensional structure in which boron atoms are held together by strong covalent bonds, it also has a higher boiling point comparatively.

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5. Ionisation Enthalpy (IE)

• The initial ionization enthalpy values of Group 13 elements are comparatively lower than those of the corresponding alkaline earth metals. This can be attributed to the ease of electron removal, especially considering the [ns² np¹ configuration]. 
• The trend of decreasing ionization energy persists as one moves down the group. This is primarily due to the electrons being farther from the core, making them easier to remove. However, it's worth noting that thallium (Tl) does not conform to this trend.

Trends & Properties of Group 13 elements

• Upon descending the group, the ionization enthalpy decreases from boron (B) to aluminum (Al). Nevertheless, gallium (Ga), the subsequent element, exhibits a slightly higher ionization enthalpy than Al. 
• This anomaly can be attributed to the inefficient shielding provided by intervening d-electrons. The trend then resumes with a decrease in ionization enthalpy for indium (In) before concluding with an increase in the last element, thallium (Tl).

6. Oxidation States

• B and Al show an oxidation state of +3 only while Ga, In and Tl exhibit oxidation states of both +1 and +3.
• As we move down in group 13, due to the inert pair effect the tendency to exhibit +3 oxidation state decreases and the tendency to attain +1 oxidation state increases. Stability of +1 oxidation state follows the order Ga < In < Tl.
• Inert pair effect is the reluctance of the s-electrons of the valence shell to take part in bonding. It occurs due to poor shielding of the ns² – electrons by the intervening d and f – electrons. It increases down the group and thus, the lower elements of the group exhibit lower oxidation states.

7. Electropositive (Metallic) Character

• These elements are less electropositive than the alkaline earth metals due to their smaller size and higher ionisation enthalpies.
• On moving down the group, the electropositive character first increases from B to Al and then decreases from Ga to Tl, due to the presence of d and f-orbitals which causes poor shielding.

8. Reducing Character

• It decreases down the group from Al to Tl because of the increase in electrode potential value for M³⁺ / M.
• Therefore, it follows the order: AI > Ga > In > Tl

9. Complex Formation

Due to their smaller size and greater charge, these elements have a greater tendency to form complexes than the s-block elements.

10. Nature of Compounds

The tendency of the formation of ionic compounds increases from B to Tl. Boron forms only covalent compounds whereas Al can form both covalent as well as ionic compounds. Gallium forms mainly ionic compounds, although anhydrous GaCI₃ is covalent.

Chemical Properties of Group 13 Elements

Group 13 elements, also known as the boron group, exhibit a variety of chemical properties that are influenced by their electronic configurations and position on the periodic table. Here are some notable chemical properties of Group 13 elements:

1. Action of Air 

• Crystalline boron is unreactive whereas amorphous boron is reactive. It reacts with air at 700°C as follows-
  4B + 3O₂ → 2B₂O₃
  2B + N₂ → 2BN
• Al is stable in air due to the formation of the protective oxide film.
  4Al + 3O₂ → 2Al₂O₃
• Thallium is more reactive than Ga and In due to the formation of unipositive ion, Tl⁺.
  4Tl + O₂ → 2Tl₂0

2. Reaction with Nitrogen

The chemical reaction of boron and Aluminium with nitrogen at a high temperature is as follows :

3. Action of Water 

• Both B and Al do not react with water but amalgamated aluminium reacts with H₂O evolving H₂.
  2Al(Hg) + 6H₂O → 2AI(OH)₃ + 3H₂ + 2Hg
• Ga and In do not react with pure cold or hot water but Tl forms an oxide layer on the surface.

4. Reaction with Alkalies 

• Boron dissolves in alkalies and gives sodium borates.
  
• Aluminium also reacts with alkali and liberates hydrogen.

5. Reaction with Carbon

Aluminium carbide is ionic and forms methane with water.

6. Hydrides 

• Elements of group 13 do not combine directly with H₂ to form hydrides, therefore their hydrides have been prepared by indirect methods, e.g
• Boron forms a number of hydrides, they are known as boranes. Boranes catch fire in the presence of oxygen.B₂H₆ + 3O₂ → B₂O₃ + 3H₂O; & Δ_cH° = – 1976 kJ mol^-l
• Boranes are hydrolysed by water.
  B₂H₆ + 6H₂O → 2H₃BO₃ + 6H₂
• Boranes are stable but the stability of hydrides of Al, Ga, In, and Tl decreases on moving down the group because the strength of the M-H bond decreases.

Structure of diborane: 

• BH₃ does not exist as such, but exists as a dimer, i.e; B₂H₆(diborane].

• In the above structure, B atoms are in sp³ – hybrid state. There are six B-H bonds out of which four B-H bonds are normal bonds present in the same plane while rest two B-H bonds behave as bridge bonds, ie; 3c – 2e (three centre-two electrons, also known as a banana bond) and present above and below the plane of the molecules which do not have a sufficient number of electrons to form covalent bonds.
• Aluminium (Al) forms a polymeric hydride of general formula (AlH₃)_x which decomposes into its elements on heating.

7. Oxides 

• Except for Tl, all the elements of group 13 form oxides or general formula M₂O₃ on heating with oxygen.
  
• Tl forms thallium (l) oxide. Tl₂O is more stable than thallium (III) oxide TI₂O₃ due to the inert pair effect.

8. Nature of Oxides and Hydroxides 

• B(OH)₃ or H₃BO₃ is soluble in water, while other hydroxides are insoluble in water.
• On moving down the group, there is a change from acidic to amphoteric and then to the basic character of oxides and hydroxides or group 13 elements.

9. Halides

• All the elements of the boron family (except Tl) form trihalides of type MX_3.
  
• All the boron trihalides [(BX₃) and aluminium trihalides AlX₃ (except AIF₃ which is ionic) are covalent compounds. AlX₃ exists as a dimer while BX₃ is a monomer because the boron atom is too small to coordinate with four large halide ions. 
• The energy released during the formation of the bridge structure is not sufficient for the cleavage of the typical pπ – pπ bond in BF₃.
  
• BF₃ is a colourless gas, BCl₃ and BBr₃ are colourless fuming liquids and BI₃ is a white solid at room temperature.
• Trihalides of group 13 elements behave as Lewis acids because of their strong tendency to accept a pair of electrons. The relative strength of Lewis acids of boron trihalides is: BF₃ < BCI₃, < BBr₃, < BI₃.
  This is due to pπ – pπ back bonding in BF₃ which makes it less electron deficient.
• The halides of group 13 elements behave as Lewis acids and the acidic character is BX₃ > AIX₃ > GaX₃ > InX₃ (where, X = Cl, Br or I)
  TICI₃ decomposes to TICl and Cl₂ and hence acts as an oxidising agent.

Anomalous Behaviour of Boron

Boron shows anomalous behaviour with the other members of the group, due to the following reasons:

• The smallest size in the group.
• High ionisation energy.
• Highest electronegativity in the group.
• Absence of vacant d-orbital.

Few Points of Difference

• It is non-metal while other members of the group are metallic.
• It shows allotropy while other members do not.
• It has the highest melting point and boiling point in group 13.
• It forms only covalent compounds while other members form both ionic and covalent compounds.
• The halides of boron exist as monomers while AlCl₃ exists as a dimer.
• The oxides and hydroxides of boron are weakly acidic while those of aluminium are amphoteric and those of other elements are basic.
• It can be oxidised by concentrated HNO₃ while aluminium becomes passive due to the formation of an oxide layer on the surface.
  

Diagonal Relationship between Boron and Silicon

Boron exhibit resemblance with its diagonal element silicon of group 14.

Diagonal Relationship

• Both Band Si are non-metals.
• Both are semi-conductors.
• Both Band Si form covalent hydrides, i.e.. boranes and silanes respectively.
• Both form covalent, and volatile halides which fume in moist air due to the release of HCI gas.
  BCI₃ + 3H₂O → H₃ BO₃ + 3HCl
  SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl
  
  Question for Group-13 Elements: Boron Family

  Try yourself:Which of the following property is not a similarity between Boron and silicon?

  • A.

    metals
  • B.

    non-metals metals
  • C.

    semiconductors
  • D.

    formation of covalent hydrides

  Explanation

  Boron of group 13 and silicon of group 14 share a diagonal relationship and have many similarities. Both boron and silicon are non-metals, semiconductors and they also form covalent hydrides, therefore, they are not metals.

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• Both form solid oxides which get dissolve in alkalies forming borates and silicates respectively.
• Both react with electropositive metals and give binary compounds, which yield a mixture of boranes and silanes on hydrolysis.
  
  Question for Group-13 Elements: Boron Family

  Try yourself: Boron shows a diagonal relationship with ____________

  • A.

    Sillicon
  • B.

    lithium
  • C.

    magnesium
  • D.

    aluminium

  Explanation

  Boron and silicon are both semiconductors, form halides that are hydrolysed in water and form acidic oxides. Boron and silicon have diagonal relationship.

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Metallurgy of Boron

Occurrence

It does not occur in a free state. Its important minerals are

• Borax (or Tineal), Na₂B₄O₇ * 1OH₂O
• Kernite, Na₂B₄O₇ * 4H₂O
• Orthoboric acid, H₃BO₃

Isolation

Elemental boron is obtained by the following methods:

• By reduction of boric oxide with highly electropositive metals like K, Mg, AI, Na etc, in the absence of air.
  
• By the reaction of boron halides with hydrogen.
  

Uses of Boron

• As a semi-conductor.
• Boron steel rods are used to control nuclear reactions.
  ₅B¹⁰ + ₀n¹ → ₅B¹¹
  
  
  Question for Group-13 Elements: Boron Family

  Try yourself:Which of the following is not a use of Boron?

  • A.

    silver paints
  • B.

    bulletproof vest
  • C.

    in nuclear reactors
  • D.

    making of boron fibres

  Explanation

  There are many applications of boron like heat resistant glasses, glass wool, fibreglass, flux for soldering metals, for heat, scratch and stain resistant glazed coating to earth and wears and as a constituent of medical soaps. It is also used and the making of Boron fibres, bulletproof vest making and also and nuclear reactions in order to absorb neutrons.

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Compounds of Boron

1. Borax or Sodium Tetraborate Decahydrate[Na2B4O7.1OH2O]

Preparation
It occurs naturally as tineal in dried up lakes. It is obtained by boiling mineral colemanite with a solution of Na2CO3.
NaBO2 can be removed by passing CO2 through it.
4NaBO2 + CO2 → Na2CO3 + Na2B4O7
Properties
(i) Its aqueous solution is basic in nature.
Na2B4O7 + 7H2O → 2NaOH + 4H3BO3
(ii) On heating with ethyl alcohol and conc. H2SO4. It gives volatile vapours of triethyl borate which burn with a green flame.

(iii) Action of heat:
Borax bead is used for the detection of coloured basic radicals under the name borax bead test e.g.,

2. Boric Acid or Orthoboric Acid [H3BO3 or B(OH)3]

Preparation
By treating borax with dil. HCl or dil. H2SO4.
Na2B4O7 + 2HCl + 5H2O → 2NaCI + 4H3BO3
Properties
(i) It is a weak monobasic acid (Lewis acid).
H3BO3 + 2H2O → [B(OH)4]– + H3O+
(ii) With C2H5OH and cone H2SO4, it gives triethyl borate.
(iii) Heating Effect:
Uses
It is used as an antiseptic and eye lotion under the name ‘boric lotion’, and as a food preservative.3. Borazine or Borazole, [B3N3H6]It is a colourless liquid having a six-membered ring of alternating B and N atoms. It is also called ‘inorganic benzene’. It is prepared by B2H6 as follows:
The π electrons in borazine are only partially delocalised. It is more reactive than benzene.

Compounds of Aluminium

1. Anhydrous Aluminium Chloride [AlCl₃ or Al₂Cl₆]

Preparation

(i) It cannot be prepared by heating AICI₃. 6H₂O.(ii) By passing dry chlorine or HCl gas over heated Al.(iii) By heating a mixture of alumina and carbon in a current of dry chlorine.

Properties

(i) AlC1₃ fumes in moist air due to hydrolysis.
AlC1₃ + 3H₂O → Al(OH)₃ + 3HCI
(ii) It behaves as Lewis acid.

Uses

It is used as a catalyst in Friedel-Craft reaction and as a mordant dye.

2. Aluminium Oxide or Alumina [AI₂O₃]

It is the most stable compound of aluminium and occurs in nature as colourless corundum and several coloured oxides, (it present in combination with different metal oxides) like ruby (red), topaz (yellow), sapphire (blue), and emerald (green), which are used as precious stones (gems).

Alum:

The term alum is given to double sulphates of the type X₂SO₄ * Y₂(SO₄)₃ * 24H₂O where, X represents a monovalent cation such as Na⁺, K⁺ and NH⁺₄, while Y is a trivalent cation such a Al³,Cr³⁺, Fe³⁺ and Co³⁺(Li⁺ does not form alum).

Potash Alum

Some important alums are:

(i) Potash alum K₂SO₄ * Al₂(SO₄)₃ * 24H₂O

(ii) Sodium alum Na₂SO₄ * A1₂(SO₄)₃. 24H₂O

(iii) Ammonium alum (NH₄)₂SO₄ * AI₂(SO₄)₃ 24H₂O

(iv) Ferric alum (NH₄)₂SO₄ * Fe₂(SO₄)₃ 24H₂O

Potash alum is prepared in the laboratory by mixing hot equimolar quantities of K₂SO₄ and Al₂(SO₄)₃. The resulting solution on concentration and crystallisation gives potash alum.

Note:

(i) A mixture of Al powder NH₄NO₃ is called ammonal and is lUed in bombs.

(ii) Al is the chief constituent of silver paints.

(iii) A1₂(SO₄)₃ 1.8 used for making fireproof clothes.

3. Thermite

A mixture of aluminium powder and ferric oxide in the ratio 1: 3.

4. Aluminium Sulphate, Al₂(SO₄)₃

It is used for obtaining H2S in pure form and for making fireproof clothes.

5. Alloys of Aluminium

Alums

The term alum is given to double sulphates of the type X2SO4, Y2(SO4)3. 24H2O where X represents a monovalent cation such as Na+,K+ and NH Rb+, Cs+, Ag+ while Y is a trivalent cation such a Al3+, Cr3+ and Fe3+, Co3+, Ga3+, V3+, Ti3+. (Li+ is too small to be accommodated in the lattice).

General formula
 or 
Some important alums are:
1. Potash alum K2SO4. Al2(SO4)3. 24H2O
2. Sodium alum Na2SO4. Al2(SO4)3. 24H2O
3. Ammonium alum (NH4)2SO4. Al2(SO4)3. 24H2O
4. Ferric alum (NH4)2SO4. Fe2(SO4)3. 24H2O
5. Chrome alum K2SO4. Cr2(SO4)3. 24H2O

Pseudo Alums

Properties

• Potash alum is a white crystalline compound. 
• The aqueous solution of all alums is acidic due to hydrolysis of Al2(SO4)3, Cr2(SO4)3 or Fe2(SO4)3 as given below

• On heating all alums lose water of crystallization 
  and swell up. The anhydrous alum is known as burnt alum.
• Ionisation of aqueous solution of a double salt is as

Uses

• In purification of water.
• For sizing of paper.
• As a styptic to stop bleeding.
• As a mordant in dyeing and tanning of leather.