Chapter 4
B.sc 1st year Book
(Page 8)

Group IIIA (13) Elements: B, Al, Ga, In, TI Boron Family

General Properties Boron Family :

The elements comprising the IIIA group are boron (B), aluminium (AI), gallium (Ga), indium (In) and thallium (TI). In the second period of the periodic table when we move from left to right, the metallic character of elements decreases continuously. Boron is a nonmetal whereas the next element below the boron in the group is aluminium decidedly has a metallic character. The elements of this group form compounds which are electron deficient because they do not complete their octet during its formation of it. These compounds show a characteristic property to accept lone pair of electrons from other atoms, ions or molecules. These compounds are of considerable theoretical interest and involve multi-centred bonds.
The other four elements Al, Ga, In and TI of group IIIA form monovalent and trivalent compounds where monovalent compounds of Ga, In and TI are stable due to the inert pair effect. These elements are more metallic and their salts are more ionic than boron. Their compounds are covalent in an anhydrous state but become ionic in solution.

1 – Electronic configuration Boron Family :

Table 4.5: Electronic configuration of IIIA group elements
Element Symbol Atomic No. Electronic configuration with an inert gas core
Boron B 5 [He]2s22p1
Aluminium Al 13 [Ne]3 s2,3p1
Gallium Ga 31 [Ar]3 d10,4 s2,4p1
Indium In 49 [Kr]4 d10,5 s2,5p1
Thallium T 81 [Xe]4f14,5 d10,6 s2,6p1

 

All the elements of group IIIA have three electrons in their outermost shell which is represented by the configuration ns2npi⋅B is a non-metal and has only two electrons before the valence shell whereas Al has 8 electrons, Ga, In and TI has 18 electrons before the outermost valence shell. Hence, boron differs from Al in several respects.

2- Atomic and ionic radii :

On moving from top to bottom in a group the atomic and ionic radii of IIIA group elements increases due to the increase in the number of shells. However, there is an abnormality in the value of atomic and ionic radii as we move from Ai to Ga. e.g. the atomic radii of Al and Ga are 1.43Å and 1.35Å respectively. This abnormality can be explained on the basis of their electronic configuration as:
  13Al: [Ne], 3s2, 3p1
           31Ga: [Ar], 3d10, 4s2, 4p1
It is clear from the above electronic configuration that Ga has a 3d10 electron whose orbital size is large and screens the nucleus poorly hence effective nuclear charge (Z∗) of Ga is more than Al. Therefore the atomic or ionic radius of Ga is smaller than Al.

3- Melting point and Boiling point of Boron Family :

In the group, the value of m.p. decreases from B to Ga and after that, there is a regular increase in this trend. The deviation in mp of Ga is attributed to the structural change. The value is related to the formation of Ga2a covalent molecules. But the value of b.p. decreases continuously from B to Tl.

4- Ionization energies of  Boron Family :

In the case of IIIA group elements, the value of ionization energies decreases sharply from B to Al and then it shows a marginal increase in the last element TI. In another word, we can say that the value of IE changes in an oscillating manner as shown in the table. This oscillating behaviour of IE can be explained on the following basis.
Boron has the smallest size, the high value of IE and less shielding effect of nuclear charge whereas Al has greater atomic radii and very effective shielding of nuclear charge which results in a low value of IE. Ga has3d10 an electron in an inner shell whose shielding effect is less than s and p electron hence it has the nearly same value of IE as that of AI. The value of IE decreases from Ga to In because of a similar reason as found in the case of Al and Ga. There is an increase in the atomic radius of Ga and a decrease in the shielding effect of d-electron. TI has a 4fpower14 electron whose shielding effect is smaller than the electron of d orbitals and there is a slight increase in atomic radii, Hence TI has a greater value of I.E. as compared to In.

5- Metallic and nonmetallic character of Boron Family :

Although B is a non-metal and has the highest value of IE than others. It also behaves as a semi-metal or metalloid. B is a poor conductor of heat and electricity. The metallic character of Al and other elements are more or less nearly the same because of a slight difference in IE amongst all of them.

6- Oxidation State of Boron Family :

The common value of the oxidation state of group IIIA elements is +3 in their ionic compounds. The elements Ga, In and TI also show a +1 oxidation state which is due to the inert pair effect. The +1 oxidation state becomes more and more stable as we move downward in the group e.g. Ga+ < In+ < TI+whereas the stabilities of the +3 oxidation state amongst these three are in the reverse orderGa3+ > ln3+> T3+. TI resists the conversion of T++TOT∣3+ and Tl+is the most stable oxidation state.
Gallium chloride (GaCl3)dimerises into Ga2Cl6 and exists as Ga+[GaCl4)−, Gallium shows the oxidation states +1 and +3 both. Recently, it was found that Al, Ga and In form compounds of a +1 oxidation state but they are not stable.
Table 4.6: Some important physical properties of group IIIA elements
7. Inert pair effect and group oxidation state :

The elements of the IIIA group have electronic configuration ns2np1 of the outermost valence shell and can lose all three electrons during chemical combination. Thus, they show a +3 oxidation state which is called a group oxidation state and represented by G All the elements of IIIA, IVA arid VA groups have group oxidation states +3,+4and +5respectively. But the heavier elements of these groups also show oxidation state G-2 i.e. for the IIA group it is +1, for the IVA group it is +2 and for VA it is +3.

This lower oxidation state becomes more stable with the increase of the atomic number moving from top to bottom in a group. In these cases, only np electron are involved in bond formation and ns? electron becomes passive or inert. This phenomenon is called the inert pair effect. It increases from Ga to Tl in a group.

8 Standard reduction electrode potential and reducing property:

All the elements of group Lila have positive values of standard reduction electrode potential and possess a large value of hydration energy. Boron does not form 83+ in an aqueous solution. reducing property increases from B to Al and decreases from Ga to in.

9- Electron affinity of Boron Family :

The value of electron affinity of the IIIA group does not show an expected trend of decrease when we move from top to bottom in a group. There is a reasonable explanation for this trend of variation.

10- Electronegativity of Boron Family :

All the members of group III are more electronegative as compared to IA and IIA group elements. This is due to their smaller atomic size and low values of standard electrode potential, in a group from top to bottom, the values of electronegativity do not show an expected decrease with the increase in the number of shells, as shown in Table 4.6.

11- Formation of oxides of Boron Family :

All the elements of the IIIA group form sesquioxides of the formula M2O3 where M=BAl, Ga, In and T1. These oxides are prepared by the direct action of metals with O2 at high temperatures.
2M + 3O2 ⟶ 2M2O3
Boron oxides are prepared by exposure of B2H6 to air.
B2H6 + 3O2 ⟶ B2O3 + 3H2O
The oxides of Al, Ga and In are prepared by the thermal decomposition of nitrates and hydroxides. T2O3is prepared by heating T(NO3)3. All the sesquioxides form hydroxides when dissolved in water
M2O3⋅3H2O⟶2M(OH)3
The hydroxides of B are acidic in nature [B(OH)3] whereas Al((OH)3 and Ga(OH)3 are amphoteric in nature. The rest of the hydroxides or oxides are basic in nature. Tl also forms oxides of the type Tl2O.

12- Formation of hydrides :

The IIIA group elements form hydrides of the general formula MH3 which exist in polymeric forms. Boron forms a number of hydrides which are covalent and known as boranes: They are of two types having the general formula BnHn+4 and BnHn+6 where ‘ n ‘ is the number of B and H atoms. BnHn+4⟶B2H6, B5H9, B6H10, B8H12, B10H14 etc.B4H4+6H10, B8H14, B9H15, B10H16 etc. They are named Diborane-6, tetra borane-10, Deca borane-14 or Deca borane-16 where the digit represents the number of H-atoms. Boron hydrides are prepared by the actions of HCl on Mg3B2.
The hydrides of IIIA group elements are electron deficient molecules
It is also prepared by reduction of halides with LIAIH4
4BCl3+3LiAlH4→2 B2H6+2LICl+3AlCl3
The hydrides of IIIA group elements are electron-deficient molecules and form adducts with the molecules which are electron-pair donors e.g. NH3, PF3, CO etc. Due to this nature, the hydrides act as Lewis acid and donor molecules as Lewis base.
[H3 N:→BH3]2

Polymeric or Polynuclear hydrides:

AI, Ga and in the form of polymeric or polynuclear hydrides of the formula (MH3)n. For example (AlH3)n which is a solid and covalent in nature linked together through bridging H bonds e.g. (AlH3)n is insoluble in water and decomposes on heating at 200∘C to give Al-complex hydrides. B, Al and Ga also form hydrides of the general formula [MH4]where M=B, A]and Ga. These hydrides exist asL[MH4]Be[MH4]2 and Al[MH4]3.

Structure of (AmH3)n

They can be prepared by the action of metal hydrides with the hydrides of alkali, alkaline and IIIA group metals e.g.

B2H6+2MH⟶2M6[BH4]BF3+4NaH⟶Na2BH
The properties of complex hydrides depend upon the nature of cations. They do not contain hydride ions and show Ionic reactions. They act as strong reducing agents.
The properties of complex hydrides depend upon the nature of cations

13- Formation of halides :

All the elements of the IIIA group form various types of halides which are categorised as :

(a) Monohalides:

They are formed only by TI e.g. TIF, TICl, TIBr and TII. Boron also forms several stable polymeric mono-halides (BXnB4Cl4, B8Cl8 and B9Cl9 are crystalline solid and have a structure like a closed cage or polyhedron of B atoms.

(b) Trihalides:

They are formed by all elements of the IIIA group the formula of trihalide is MX3 M=B to Al,  X=F, Cl, Br, I.BX3 is of covalent nature due to its small size and high charge density. Fluorides of Al, Ga, in and TI are ionic in nature and have high m.p. whereas chlorides, bromides and iodides are covalent in an anhydrous state. Anhydrous AlCl3 is covalent but in water, it gives AB + ion. MX3 is planet and triangular in shape due to sp2 hybridization of the central atom. Boron forms [BF4]−when combined with fluoride ion whereas another member of its family form [MXA]−and [MX6]−showing the coordination numbers 4 and 6 respectively. Boron belongs to the second period hence it shows maximum coordination number 4 only. AlCl3 in anhydrous conditions is used as a catalyst in a variety of Friedel-Crafts types of reactions.

(c) Dinuclear Tetrahalide

Dinuclear Tetrahalide Such types of halides ate formed by B only having formula B2X4 a.g.B2F4, B2Cl4, B2Br4, B2I4.

(d) Trihalides

Trihalides which exist in the dimeric form are called dimeric trihalides having the general formula [MiX3 L2=M2X6, where M=Al and Ga. They exist in a vapour state and in non-polar. solvent like C6H6

(e) Complex halides:

Complex halides are formed by all the elements of the IIIA group having the general formula [MX4]−[MX6]3− where M=B, Al. Ga, In and T1, X=F and Cl etc.

Anomalous properties of boron:

In many properties boron differs from the other elements of its own sub-group. The main points of difference are.
(1) Boron has a very small atomic radius. The hypothetical B3+ ion has a very small size and high charge density. This value is so high that the B3+ ion does not exist. All the compounds of boron are seen as therefore, covalent.
(2) Boron has less than four valence electrons and this gives it a great electron acceptor. e: its compounds behave as strong Lewis acids and forms a large number of complex compounds exhibiting its tendency to acquire a stable octet.
(3) Eoron shows a maximum covalency of four while all other elements show a covalency of six or more.
(4) It does not exhibit an inert pair effect.

Comparison between Boron and Aluminium :

The outermost electronic configuration and size of an atom mainly determine the properties of that element. In a group, particularly the main group, all the members have similar outer electronic configurations and hence several properties are common. Since the atomic size of the elements changes gradually with the increasing atomic number a gradual change in properties is observed. Consequently, the last member of a group differs considerably from the first. Boron and aluminium, are members of the same group (III-A). They have a number of properties common. The number of electrons in their outermost shell is the same and the penultimate shell of both the elements is also filled to their maximum capacity.

Similarities:

The point of similarities are given below :

1-Valancy:

Electronic configurations of boron and aluminium are 2, 3 and 2,8,3 respectively which show that they should form trivalent compounds.

2- Reaction with oxygen:

When heated In the air, both form amphoteric oxides of the type M2O3. which react with acids as well as with alkalies to form salts.
(a) Basic nature of oxides:
B2O3 + 6HCl⟶ 2BCl3 + 3H2O
Al2O3 + 6HCl ⟶ 2AlCl3 + 3H2O
(b) Acidic nature of oxides:
B2O3 + 6NaOH ⟶ 2Na3BO3 + 3H2O
Al2O3 + 2NaOH ⟶ 2NaAlO2 + H2O

3- Reaction with nitrogen:

On heating, both the elements combine with nitrogen to form nitrides BN and AIN, which on hydrolysis evolve ammonia.
Reaction with nitrogen

4- Reaction with chlorine:

Both boron and aluminium combine directly with chlorine on heating to form trichlorides, Chlorides of boron and aluminium are covalent and hygroscopic in nature. They are readily hydrolysed by water.
BCl3+3HOH⟶B(OH)3+3HClAlCl3+3HOH⟶Al(OH)3+3HCl

5- Reaction with sulphur:

They react with sulphur at high temperatures (red hot conditions) to form their sulphides.
2B + 3SB2S3
2AI + 3S ⟶ Al2S3
6- Reaction with alkalies:
Boron and aluminium both are attacked by alkalies and evolve hydrogen gas.

7- Boron and aluminium:

both have a great affinity toward oxygen and act as strong reducing agents. The reduction of metal oxides with aluminium is of great industrial importance.
3SiO2+4 B⟶2 B2O3+3Si14Cr2O3+2Al⟶Al2O3+2Cr1∇
  1. Oxy compounds:

Oxy compounds called ‘borates’ and ‘aluminates’ are known for boron and aluminium respectively. Borates are more stable than aluminates.

9- Both form electron-deficient compounds.

Dissimilarities of Boron with Aluminium

Dissimilarities of Boron with Aluminium

Dissimilarities of Boron with Aluminium

 

Comparison of Boron with Silicon:

Silicon is a member of the IVA group in the third period of the periodic table. It is diagonally related to boron. The chemistry of boron, therefore, resembles that of silicon much more closely than that of its congeners. They have some dissimilarities as well. The important similarities are given below:

1- Occurrence:

Both occur in nature as their oxysalts, borates and silicates, which are substances of very much complicated structures.

2- Melting points:

Boron and silicon are non-metals which melt at high temperatures, 2300∘C and 1410∘C respectively.

3- Allotropy:

Both exhibit allotropy. The amorphous forms, which are more common, are brown powders and the crystalline terms, formed with much more difficulty, are hard and blackish. The amorphous form is more reactive than the crystalline form.

4- Covalency:

Both the elements form covalent compounds only and do not form cations.

5- Action with nonmetals:

 (i) Oxygen:

when heated both the elements combine with oxygen to give oxides,
4 B+3O2700⋅C400⋅C−2 B2O3Si+SiO2

(ii) Sulphur:

When boron and silicon are heated with sulphur the sulphides are formed.
2 B + 3S⟶B2S3
Si + 2S⟶SiS2

(iii) Halogens:

Both the elements react spontaneously on heating with fluorine and other halogens :
2B + 3X2—-1200∘C—>2BX3
Si + 2X2—-1300∘C—>SiX4

(iv) Nitrogen:

Nitrogen when combined with boron and silicon gives readily hydrolysable nitrides.
2B + N2—-400∘C—->2BN
3Si + 2N2—-400∘C—->Si3N4
  1. When boron and silicon are heated strongly with a large number of metallic elements, both the elements form binary compounds, called ‘borides’ and ‘silicides’.
3Mg + 2B⟶Mg3B2
2Mg + Si⟶Mg2S1

7- Acids:

Both remain unaffected by dilute acids.

8-Alkalies:

Strong alkalies react with these elements forming oxysalts and evolving hydrogen.
           2B + 6NaOH⟶2Na3BO3 + 3H2↑
Si + 2NaOH + H2O⟶Na2SiO3 + 2H2↑

9- Oxides:

Oxides and hydroxides of these elements are of a weak acid in nature and combine with alkalies to form borates and silicates. The transparent crystalline allotropes of the oxides of boron and silicon react difficulty with alkalies.

10- Halides:

Halides (except fluorides) of the elements are readily hydrolysed into boric and silicic acids.
BCl3 + 3HOH⟶B(OH)3 + 3HCl
SiCl4 + 3HOH⟶H2SiO3 + 4HCl
11-Several covalent hydrides are known for both boron and silicon called boranes and silanes. These compounds are volatile, spontaneously inflammable in air and are readily hydrolysed.

Electron Deficiency and Acceptor Behaviour of Boron Compounds:

In BXX3 type of compounds, the octet of boron is not complete with three s-hands. Boron/B has three/3 valence electrons and four/4 valence orbitals. With three s-bonds, it has only six electrons (instead of an octet) while one p-orbital remains vacant. This electron deficiency in BX3 compounds makes them powerful electron acceptors. As a result, they behave as very good Lewis acids and accept electron pairs in the empty p-orbital (which is at right angles to the plane of the BX3 molecule) on the boron atom. In this way, boron forms tetrahedral compounds involving one coordinate bond.
The empty p-orbital of boron in BX3 can overlap with the filled p-orbital of X, giving double bond character to the bond. Now if BX3 forms a coordinate bond, then it is necessary that the p-orbital of boron is available to accept electron pair from the donor atom and is not used up in s-bond formation with X. Hence, the acid strength of such a molecule depends upon the extent to which the vacant p-orbital is available for coordination.
BF3 molecule is a powerful electron acceptor. Because of the small size of the fluorine atom, it is likely that its orbitals do not overlap effectively with the p-orbital of boron to form an s-bond. Besides, due to the very high electronegativity of fluorine, it has little tendency to part with its electrons to boron. Hence in BF3, the p-orbital of boron is fully available to accept electron pairs. Contrary to this, Bl3 possesses very little tendency to accept electron pairs so it does not form addition compounds with very good electron donors namely, ammonia. Iodine atoms possibly, completely fill the vacant p-orbitals of boron in p-bond formation. This may be associated with a big atomic size and low electronegativity of iodine.
In BX3 type of compounds, the tendency to form tetrahedral addition compounds decreases. in the following order.
BH3 > BF3 > BCl3 > BBr3> (CH3)3B> (CH3O)3B > Bl3
In BX3 type of compounds after accepting an electron pair, the boron atom possesses sp2 to sp3 hybridization state. Thus it forms tetrahedral compounds. This rearrangement reduces the repulsion between the newly formed bond and the already existing bonds.
BX3 – the type of compounds that form tetrahedral addition compounds with electron donors, such as ammonia, amines, phosphine, ether, CN, F− etc. The reaction of BF3 with F−ion is given below.

Tetrahedral structure of BF4−

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