In p-block elements, the last electron enters in the outermost p-orbital. There are six groups of p-block elements in the Periodic Table, numbering from 13 to 18. Their valence shell electronic configuration is ns2np1 – 6 (except for He).
It is also called boron family. It includes B, Al, Ga, In, Tl. Al is the most abundant metal and third most abundant element in the earth’s crust.
General Physical Properties of Group 13 Elements:
(i) Electronic configuration: Their valence shell electronic configuration is ns2np1.
(ii) Atomic radii and ionic radii: Group 13 elements have smaller size than those of alkaline earth metals due to greater effective nuclear charge, Zeff’
Atomic radii increases on going down the group with an anomaly at gallium (Ga). 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 nucleus effectively. The ionic radii regularly increases from B3+ to TI3+.
(iii) Density: It increases regularly on moving down the group from B to Tl.
(iv) Melting and boiling points: 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.
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 high temperature thermometer.
(v) Ionisation enthalpy (IE)- The first ionisation enthalpy values of group 13 elements are lower than the corresponding alkaline earth metals, due to the fact that removal of electron is easy. [ns2 np1 configuration] .
On moving down the group, IE decreases from B to Al, but the next element Ga has slightly higher ionisation enthalpy than Al due to the poor shielding of intervening d-electrons. It again decreases in In and then increases in the last element Tl.
(vi) 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 the group 13, due to 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 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.
(vii) 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.
(viii) 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
(ix) Complex formation: Due to their smaller size and greater charge, these elements have greater tendency to form complexes than the s-block elements.
(x) 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 Ga CI3 is covalent.
Chemical Properties of 13 Group Elements:
(i) 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 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
(ii) Reaction with nitrogen:
(iii) Action of water: Both B and Al do not react with water but amalgamated aluminium react 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.
(iv) Reaction with alkalies: Boron dissolves in alkalies and gives sodium borates.
Aluminium also reacts with alkali and liberates hydrogen.
(v) Reaction with carbon:
Aluminium carbide is ionic and forms methane with water.
(vi) 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; & ΔcH° = – 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 banana bond) and present above and below the plane of the molecules which do not have 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.
(vii) Oxides Except Tl, all the elements of group 13 form oxides or general formula M2O3 on heating with oxygen.
Tl forms thallium (l) oxide. Tl2O which is more stable than thallium (III) oxide TI2O3 due to inert pair effect.
(viii) 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 basic character of oxides and hydroxides or group 13 elements.
(ix) Halides: All the elements of boron family (except Tl) form trihalides of type MX3.
All the boron trihalides [(BX3) and aluminium trihalides AlX3 (except AIF3 which is ionic) are covalent compounds. AlX3 exists as dimer while BX3 is monomer because 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 backbonding 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:
(i) Smallest size in the group.
(ii) High ionisation energy.
(iii) Highest electronegativity in the group.
(iv) Absence of vacant d-orbital.
A few points of difference are:
1. It is a non-metal while other members of the group are metallic.
2. It shows allotropy while other members do not.
3. It has the highest melting point and boiling point in group 13.
4. It forms only covalent compounds while other members form both ionic and covalent compounds.
5. The halides of boron exist as monomers while AlCl3 exists as a dimer.
6, The oxides and hydroxides of boron are weakly acidic while those of aluminium are amphoteric and those of other elements are basic.
7. It can be oxidised by concentrated HNO3 while aluminium becomes passive due to the formation of oxide layer on the surface.
Diagonal Relationship between Boron and Silicon:
Boron exhibit resemblance with its diagonal element silicon of group 14.
1. Both Band Si are non-metals.
2. Both are semi-conductors.
3. Both Band Si form covalent hydrides, i.e.. boranes and silanes respectively.
4. Both form covalent, and volatile halides which fume in moist air due to release of HCI gas.
BCI3 + 3H2O → H3 BO3 + 3HCl
SiCl4 + 4H2O → Si(OH)4 + 4HCl
5. Both form solid oxides which get dissolve in alkalies forming borates and silicates respectively.
6. Both react with electropositive metals and give binary compounds, which yield mixture of boranes and silanes on hydrolysis.
Boron and Its Compounds:
It does not occur in free state. Its important minerals are
(i) Borax (or Tineal), Na2B4O7 * 1OH2O
(ii) Kernite, Na2B4O7 * 4H2O
(iii) Orthoboric acid, H3BO3
Elemental boron is obtained by following methods :
(i) By reduction of boric oxide with highly electropositive metals like K, Mg, AI, Na etc, in the absence of air.
(ii) By the reaction of boron halides with hydrogen,
Uses of Boron
(i) As a semi-conductor.
(ii) Boron steel rods are used to control the nuclear reactions.
5B10 + 0n1 → 5B11
1. Borax or Sodium Tetraborate Decahydrate [Na2B4O7 * 1OH2O]
It occurs naturally as tineal in dried up lakes. It is obtained by boiling of mineral colemanite with a solution of Na2CO3.
NaBO2 can be removed by passing CO2 through it.
4NaBO2 + CO2 → Na2CO3 + Na2B4O7
1. Its aqueous solution is basic in nature.
Na2B4O7 + 7H2O → 2NaOH + 4H3BO3
2. On heating with ethyl alcohol and conc. H2SO4. It gives volatile vapours of triethyl borate which burn with a green flame.
3. 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]
By treating borax with dil. HCl or dil. H2SO4.
Na2B4O7 + 2HCl + 5H2O → 2NaCI + 4H3BO3
1. It is a weak monobasic acid (Lewis acid).
H3BO3 + 2H2O → [B(OH)4]– + H3O+
2. With C2H5OH and cone H2SO4, it gives triethyl borate.
3. Heating Effect:
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]
It cannot be prepared by heating AICI3. 6H2O.
It can be prepared-
(i) By passing dry chlorine or HCl gas over heated Al.
(ii) By heating a mixture of alumina and carbon in a current of dry chlorine.
1. AlC13 fumes in moist air due to hydrolysis.
AlC13 + 3H2O → Al(OH)3 + 3HCI
2. It behaves as Lewis acid.
1. 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).
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).
Fig: Potash AlumSome 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 1. A mixture of Al powder NH4NO3 is called ammonal and is lUed in bombs.
2. Al is the chief constituent of silver paints.
3. A12(SO4)3 1.8 used for making fire proof clothes.