UNIT 08 THE p-BLOCK ELEMENTS
Syllabus
· Group 13 Elements
· Group 14 Elements
· Group 15 Elements
· Group 16 Elements
· Group 17 Elements
· Group 18 Elements
In the long form the periodic table , the elements have been classified into four blocks : s , p , d and f depending upon the subshell in which the last electron enters. The elements belonging to s and p-blocks in the periodic table are called representative elements or the main group elements. They belong to groups 1 , 2 and from 13 to 18. The elements belonging to groups 1 and 2 belong to s-block and have the general configuration n s1-2. The elements belonging to groups 13 to 18 belong to p-block and have the general configuration n s2n p1- 6. The properies of the s- and p-block elements follow the systematic gradation in both the periods and groups. However, the elements of the second row show a number of differences in properties from other members of their families
GROUP 13 ELEMENTS
Group 13 of the periodic table consists of the elements boron(B), aluminium (Al), gallium(Ga), Indium (In) and thallium (Tℓ). Except boron, which is classified as non-metal, all other elements of this group are metals. Aluminium is third most abundant element (8.3% by weight) in the earth’s crust after oxygen (45.5%) and silicon (25.7%). It is the most abundant metal in earth’s crust. It is also very important element of this group because of its various industrial uses.
Occurrence and uses
Among the elements of Group 13 , aluminium is most abundant metal. It is the third most abundant element (after oxygen and silicon) by weight in the earth’s crust. It is a major constituent of the common aluminosilicate rocks such as feldspars and micas. The most abundant ore of aluminium is bauxite Al2O3. H2O . It also occurs in many well known , though rare minerals such as cryolite (Na3AlF6), spinel (MgAl2O4), beryl (Be3Al2Si6O18), garnet [Ca3Al2(SiO4)3] etc.
Boron is quite rare element but is well known because it occurs as concentrated deposits of borax [Na2[B4O5(OH)4] 8 H2O and kernite Na2[B4O5(OH)4] 2 H2O. Gallium , indium and thallium are very much less abundant and occurs in traces in sulphide minerals rather than oxides. Small amounts of gallium are also found in the ores of elements adjacent to it in the periodic table (Al, Zn and Ge). Traces of In and Tℓ are found in ZnS and PbS ores. The abundance of these elements in the earth’s crust are given below :
Element | Abundance in earth’s crust (ppm) |
B | 9 |
Al | 8.3 x 104 |
Ga | 19 |
In | 0.24 |
Tℓ | 0.5 |
The elements of group 13 has many important uses. The main use of boron is to make boron steel or boron carbide control rods for nuclear reactors. Boron increases the hardness of steel, it is used to make impact resistant steels.
Aluminium metal is light, soft , non-toxic , corrosion resistant and has high thermal and electrical conductance. Therefore it is extensively used in household utensils, in chemical plants and as structural metal for aircrafts, ships , cars, heat exchangers, etc. It is also used for making electric power cables. Aluminium foil is used for packing choclates, cigerettes, food items etc. Aluminium is also used for making cans for drinks, beer, diary products and containers for chemicals and tooth paste tubes. Alumina is used in making refractory bricks and ultramarines and alums are used in deying and paints. Many of the mechanical properties of pure aluminium are greatly improved by alloying it with Cu, Mn, Sn, Ni, Mg, Zn, etc. These alloys have many uses in different fields.
The other metals Ga, In and Tℓ have no large scale uses. Small amounts of Ga are used to dope crystals for making transistors. It find uses in solid state devices as GaAs. It is also used in low melting solder and other low melting alloys. Thallium as a metal does not find worthwhile uses, though its salts find a number of uses. Its compounds are used in preparing optical glasses because of their high refractive indices. TℓBr and TℓI have been used in spectroscopy and as infrared detectors and as photosensitive diodes. Thallium carboxylates have been used in organic synthesis.
General Characteristics of Group 13 Elements
The elements in Group 13 have the general configurations ns2np1 i.e., one electron in the outermost p-orbitals and two electrons in the s-orbital. Their electronic configurations are given in the Table.
Element | Symbol | Atomic Number | Electronic configuration |
Boron | B | 5 | [He] 2s2 2p1 |
Aluminium | Al | 13 | [Ne] 3s2 3p1 |
Gallium | Ga | 31 | [Ar] 3d10 4s2 4p1 |
Indium | In | 49 | [Kr] 4d10 5s2 5p1 |
Thallium | Tℓ | 81 | [Xe] 5d10 6s2 6p1 |
Atomic and Physical Properties
The important atomic parameters and physical constants of Group 13 elements are shown in TABLE .
Property | Boron | Aluminium | Gallium | Indium | Thallium |
Atomic number | 5 | 13 | 31 | 49 | 81 |
Atomic mass | 10.81 | 26.98 | 69.72 | 114.82 | 204.38 |
Ele. configuration | [He] 2s2 2p1 | [Ne] 3s2 3p1 | [Ar] 3d10 4s2 4p1 | [Kr] 4d10 5s2 5p1 | [Xe] 5d10 6s2 6p1 |
Atomic radius (pm) | 85 | 143 | 135 | 167 | 170 |
Ionic radius M3+ (pm) | 27 | 53.5 | 62.0 | 80.0 | 88.5 |
Ionic radius M+ (pm) | - | - | 120 | 140 | 150 |
Ionisation enthalpy (kJ/mol) I | 800 | 577 | 578 | 558 | 590 |
Ionisation enthalpy (kJ/mol) II | 2427 | 1816 | 1979 | 1820 | 1971 |
Ionisation enthalpy (kJ/mol) III | 3659 | 2744 | 2962 | 2704 | 2877 |
Electronegativity | 2.0 | 1.5 | 1.6 | 1.7 | 1.8 |
Density ( g cm-3 , 298 K) | 2.35 | 2.70 | 5.90 | 7.31 | 11.85 |
Melting point (K) | 2453 | 933 | 303 | 430 | 576 |
Boiing point (K) | 3923 | 2740 | 2676 | 2353 | 1730 |
E° (V) for (M3+ +3e ® M ) | - | - 1.66 | - 0.56 | - 0.34 | + 1.26 |
E° (V) for (M+ + e ® M ) | - | 0.55 | - | - 0.18 | - 0.34 |
1. Atomic and ionic radii
The atomic and ionic radii of group 13 elements are smaller as compared to corresponding elements of group 2. This is due to the increase in nuclear charge when we move from element of group 2 to group 13 in the same period. As we move from left to right in the period, the magnitude of nuclear charge increases but electrons are added to the same shell. Since the electrons in the same shell do not screen each other, therefore , the electrons experience greater nuclear charge. In other words, effective nuclear charge increases and thus, size decreases. Therefore the elements of this group have smaller size than the corresponding elements of second group. On moving down the group , both the atomic and ionic radii are expected to increase due to addition of new shells. However, the observed atomic radius of Al (143 pm) is slightly more than that of Ga ( 135 pm).
Explanation : When going from Al (Z 13) to Ga ( Z = 31) there are ten elements of the first transition series of d-block from ( Z = 21 to 30 ) which have electrons in the inner d-orbitals. The d-orbitals do not screen the nucleus effectively because of their shapes and poor penetration power. As a result, the effective nuclear charge in Ga becomes more than in aluminium and its atomic radius , therefore decreases slightly.
2. Ionisation Enthalpies
The first ionisation enthalpies of group 13 elements are less than the corresponding members of alkaline earth metals.
Explanation : The first electron in the case of group 13 elements (ns2np1) is to be removed from p-orbital while in the case of elements of group 2 , the electron has to be removed from the s-orbital. Since p-orbitals are at slightly higher energy than the s-orbitals, the electron in the atoms of group 13 elements is weakly held by the nucleus and therefore , the first ionisation enthalpy is less. However, second (IE2) and third (IE3) ionisation enthalpies are quite high. When one electron is removed from outermost p-sub-shell, the resulting ion has completely filled s-orbital (ns2). Therefore, it becomes difficult to remove the second electron.
On moving down a group, the ionisation enthalpies in general decreases.
Explanation : This is due to increase in atomic size and screening effect which is more than to compensate the effect of increase in nuclear charge. Consequently, the electron becomes less and less tightly held by the nucleus as we move down the group. Hence , ionisation enthalpy decreases down the group.
The inspection of the above table shows an anomalous behaviour. The ionisation enthalpy decreases sharply from B to Al and then ionisation enthalpy of Ga is unexpectedly higher than that of Al.
Explanation : The sharp decrease in ionisation enthalpy from B to Al is due increase in size. In the case of Gallium, there are ten d-electrons in its inner electronic configuration. Since the d-electrons shield the nuclear charge less effectively than the s- and p-electrons, the outer electron is held fairly strongly by the nucleus. As a result, the ionisation enthalpy increases slightly because of the increase in atomic size as we move from Al to Ga.
3. Melting and boiling points
The melting and boiling points decrease on moving down the group. However, the decrease in melting points is not regular as in boling points.
4. Electropositive ( or metallic ) character
Due to high ionisation enthalpies the elements of Group 13 are less electropositive as compared to elements of Group 2. On moving down the group, the electropositive (metallic character) increases because ionisation energy decreases. For example, boron is a non-metal, while the other elements are typical metals.
5. Oxidation state
The atoms of these elements have three valence electrons, two in s-subshell and one in p-subshell. Therefore all these elements can show a maximum of + 3 oxidation state.
Boron shows only +3 oxidation state in its compounds. Except boron , other elements also show +1 oxidation state. The +1 oxidation state becomes more stable as we move down the group from boron to thallium. In case of last element , thallium , +1 oxidation state has been found to be more stable than +3 oxidation state.
Gallium , the third element of the group appears to show +2 oxidation in some of its compounds such as GaCl2. However, it has been shown that this compound has the structure Ga+ [GaCl4]- which contains gallium in +1 (Ga+
Explanation : This is explained on the basis of inert pair effect. The elements of Group 13 have three electrons in their valence shell (ns2np1) and therefore , exhibit oxidation state of +3. However, it has been observed that in addition to +3 oxidation state, they also exhibit oxidation state of +1. The +1 oxidation state becomes more and more stable as we go down the group from B, Al , Ga
TRENDS IN CHEMICAL REACTIVITY
The important trends observed in the chemical behaviour of Group 13 elements are :
1. Except boron, the other elements of group 13 show meallic character which increases down the group.
2. Al , Ga
3. The Group 13 elements form hydrides of the type MH3 type whose thermal stability decreases as we moves down the group. AlH3 is a solid, polymerised via Al-H-Al bridging units. These hydrides are weak Lewis acids and readily form adducts with strong Lewis bases (B:) to give compounds of the type MH3 : B ( M = Al or Ga). They also form tetrahydridoanions , e.g., [MH4]-. The most important tetrahydrido compound is lithium tetrahydrido aluminate (III). LiAlH4 is obtained by the action of LiH with AlCl3 in ether as :
LiAlH4 , a white crystalline solid and soluble in ether , is a versatile reducing agent used in organic synthesis.
4. Al , Ga
In these halides, boron uses sp2 hybridisation using one (2s) and two (2p) orbitals. The sp2 hybrid orbitals adopt trigonal planar structure. Each of the sp2 hybrid orbital overlaps with 2p-orbital of halogen to form B-X bonds. A 2p-orbital is left vacant in the valence shell.
Structure of BF3
In these compounds , the octet of boron is not complete , because they have only six electrons around boron. Therefore , these halides act as Lewis acids . The relative Lewis acid strength of boron halides decreases as :
BBr3 > BCl3 > BF3
This order is just the reverse of what may be expected on the basis of electronegativities of the halogen atoms. Since fluorine is most electronegative among halogens, it will withdraw electrons from boron atom strongly. As a result it will make BF3 as the most electron deficient and strongest Lewis acid. However, BF3 is least acidic among trihalides of B. This anomalous behaviour can be explained on the basis of the tendency of halogen atom to back donate its electrons to the boron atom. This is explained below :
There is one 2p-vacant orbital on B atom in BF3 and 2p-orbital of each fluorine atom is fully filled. Since the energies of these two 2p-orbitals ( of B and F) are almost similar, one of the 2p-filled orbital of F overlaps sidewise with the vacant 2p-orbital of B atom resulting in the transference of electron from F atom to vacant 2p-orbitals. Thus, B-F bond acquires some double bond or pp-pp bond. This type of bond is called dative or back bonding.
Formation of pp-pp back bonding in one of the B-F bonds in BF3.
The back bonding in BF3 molecules also get support from the fact that the observed B-F bond length in BF3 (130 pm) is significantly shorter than the sum of the covalent radii ( B = 80 pm and F = 72 pm) or normal B-F single bond.
As a result of back donation of electrons from fluorine to boron, the electron deficiency of boron atom gets compensated and therefore, the Lewis acid character of BF3 decreases. The tedency to form pp-pp is maximum in the case of BF3 and decreases rapidly as we move to BCl3 and BBr3. This is because of the inability of the vacant 2p-orbital of boron to overlap effectively with the 3p-orbital of chlorine in B-Cl bond and the 4p-orbital of bromine in B-Br bond due to the appreciable differences in their energy levels.
The halides of other elements of Group 13 also behave as Lewis acids. The Lewis acid strength decreases as :
B > Al > Ga > In
In contrast to boron halides, aluminium halides exist as dimeric molecules. The halides of aluminium in vapour phase as well as in inert organic solvents such as benzene exist as dimers. For example, AlCl3 exists as Al2Cl6. The formation of dimer is illustrated below :
In AlCl3 , there are six electrons around aluminium atom and these are two less than the octet. In dimeric structure, each aluminium atom completes its octet by accepting a lone pair from chlorine atom of another aluminium chloride molecule. The two co-ordinate covalent Cl -Al bonds are shown in the figure, in which the arrow indicates the donation of electrons from chlorine to aluminium.
However, in polar solvents such as water, the dimer dissociates due to high heat of hydration as :
The dimer structure exists only in the vapour state and at low temperature ( 473 K) but at higher temperature , it dissociates to trigonal planar AlCl3 molecules.
Anhydrous AlCl3 is covalent in nature but hydrated aluminium chloride is ionic. In anhydrous aluminium chloride , aluminium atom is linked to three Cl atoms by covalent bonds. This is due to the fact that a large amount of energy is needed to convert aluminium atom to aluminium ion (Al3+). Therefore it prefers to form covalent bonds with Cl atoms. However , when aluminium chloride is dissolved in water, it undergoes hydration as :
Hydration of Al2Cl6 is exothermic process and a large amount of energy is released. Thus energy liberated during hydration process is responsible for the removal of three electrons from Al and form Al3+ ion. In fact, in dissolved state, Al exists as [Al(H2O)6]3+ . Therefore, hydrated aluminium chloride is ionic in nature.
5. Oxides and Hydroxides : All elements of this group 13 form oxides of general formula M2O3 and hydroxides M(OH)3. The oxides of Boron and Aluminium are obtained by heating the metal in oxygen.
4 B + 3 O2 ® 2 B2O3
4 Al + 3 O2 ® 2 Al2O3
The hydroxides are generally obtained by dissolving the oxides of the element in water. However, boron oxide does not react with water even in the form of steam. The oxides and hydroxides of Boron are weakly acidic and therefore, react with alkalies./
B2O3 + 2 NaOH ® 2 NaBO2 + H2O
Sodium metaborate
B(OH)3 + 3 NaOH ® Na3BO3 + 3 H2O
Sodium borate
Aluminium oxide and hydroxide are amphoteric in nature. They dissolve both in acids as well as in alkalies.
Al2O3 + 2 NaOH ® 2 NaAlO2 + H2O
Sodium meta aluminate
Al2O3 + 3 H2SO4 ® Al2(SO4)3 + 3 H2O
Similarly,
Al(OH)3 + NaOH ® NaAlO2 + 2 H2O
Sodium meta aluminate
A(OH)3 + 3 HCl ® AlCl3 + 3 H2O
Like Al(OH)3 , Ga(OH)3 is also amphoteric. Indium hydroxide , In(OH)3 and thalous hydroxide TℓOH are basic. Therefore , the basic character of oxides and hydroxides increases as :
Explanation : As we move down the group, the magnitude of the ionisation energy decreases. As a result, the strength of M-O bond also decreases accordingly. Therefore its cleavage becomes easy resulting in the increased basic strength down the group. The acidic strength of hydroxides , therefore decreases.
In constrast to Tℓ(OH)3 which is insoluble in water, Tℓ(OH) is soluble and is a strong base. Many of the Tℓ(I) compounds are similar to the corresponding alkali metal compounds.
5. Aℓ, Ga , In and Tℓ ions exists as octahedral aqua ions, [M(OH2)6]3+ in aqueous solution and many salts like halides, sulphates, nitrates and perchlorates exist as hydrates. Aluminium sulphate forms double salts with sulphates of other metals and are called alums, having the general formula M Al(SO4)2 . 12 H2O, where M is a uni-valent cation, Na+ or K+. Alums are extensively used in the softening of hard water and as a mordant in dyeing and printing of textiles. A mordant helps to bind the dye to the fabric.
6. The +1 oxidation state gets stabilised progressively from Ga to Tℓ . The monohalides, GaX, InX and TℓX are known for X = Cl, Br and I. GaX and InX disproportionate in water :
3 MX (s) ® M(s) + M3+(aq) + 3 X-(aq)
Tℓ (I) is, however, stable.
Problem
01. Which type of cations are capable of replacing aluminium in alums ? Give some examples.
ALUMINIUM METAL EXTRACTION
Occurrence : Aluminium occurs widely as a constituent of rocks and soils. The main ores of aluminium are given below :
(i) Bauxite : Al2O3 . 2 H2O
(ii) Cryolite : Na3AlF6
(iii) Feldspar : KAlSi3O8
(iv) Mica : KAlSi2O10(OH)2
Extraction of aluminium
Aluminium is normally extracted from bauxite ore, Al2O3 . 2 H2O. It involves two steps :
(i) Purification of Bauxite
(ii) Electrolysis of alumina
In the first stage, pure alumina(Al2O3) is obtained from bauxite and in the second stage , electrolysis of Al2O3 in molten cryolite (Na3AlF6) is carried out to obtain aluminium metal.
(i) Purification of Bauxite : Bauxite contains SiO2, iron oxides and titanium (IV) oxides as impurities. The bauxite ore is digested with a concentrated solution of sodium hydroxide at 473-523 K and 35 – 36 bar pressure. Aluminium oxide and silica dissolve to form sodium aluminate and sodium silicate respectively leaving behind iron oxide and TiO2 which are filtered off.
Al2O3(s) + 2 NaOH(aq) + 3 H2O ® 2 Na[Al(OH)4](aq)
The filtrate containg sodium aluminate and sodium silicate is diluted and seeded with freshly precipitated aluminium hydroxide leaving behind sodium silicate in solution.
The aluminium hydroxide is filtered, dried and calcined at 1473 K to yield pure alumina.
(ii) Electrolysis of fused alumina : Aluminium is obtained from alumina by electrolysis ; this is known as Hall – Heroult Process. The modern electrolysis process uses synthetic cryolite , Na3AlF6. Typical electrolyte composition ranges are Na3AlF6 (80 – 85%), CaF2 ( 5 – 7%). AlF3 ( 5 – 7%) , Al2O3( 2 – 8% intermittenly recharged). The electrolysis of this mixture is carried out in an electrolytic cell (Fig) using carbon electrodes.
Electrolysis of fused alumina
The oxygen liberated at the anode reacts with carbon anode producing CO and CO2. The overall reactions may be written as :
Cathode : Al3+(aq) + 3 e- ® Al(ℓ)
Anode : C(s) + O2- (melt) ® CO(g) + 2 e-
C(s) + 2 O2- (melt) ® CO2(g) + 4 e-
For each kilogram of aluminium produced , over 0.5 kg of carbon anode is burnt away. Because of this anodes need to be replaced periodically.
Properties of Aluminium
Aluminium ia a light silvery- white metal, with high tensile strength , a high electrical and thermal conductivity. On a weight -to-weight basis, the electrical conductivity of aluminium is twice that of copper. It is highly electropositive and readily reacts with oxygen to form a hard protective layer of Al2O3, which renders it passive. Aluminium dissolves in aqueous hydrochloric acid to give :
2 Al(s) + 6 HCl(aq) + 12 H2O(ℓ)® 2 [Al(OH)6]Cl3(aq) + 3 H2(g)