Boron Family

(i) All exhibit +3 oxidation state and thus complete their

Boron Family octet either by covalent or ionic union.
Group 13 of long form of periodic table (previously (ii) Boron being smaller in size cannot lose its valence
reported as group III A according to Mendeleefs periodic table) electrons to form B3+ ion and it usually show +3 covalence. The
includes boron (B) ; aluminium (Al) , gallium (Ga), indium (In) and tendency to show +3 covalence however decreases down the
thallium (Tl) Boron is the first member of group 13 of the periodic group even Al shows +3 covalence in most of its compounds.
table and is the only non-metal of this group. The all other (iii) Lower elements also show +1 ionic state e.g Tl +, Ga+.
members are metals. The non-metallic nature of boron is due its This is due to inert pair effect. The phenomenon in which outer
small size and high ionisation energy. The members of this family shell ‘s’ electrons (ns2) penetrate to (n-1) d-electrons and thus
are collectively known as boron family and sometimes as become closer to nucleus and are more effectively pulled the
aluminium family. nucleus. This results in less availability of ns2 electrons pair for
(1) Electronic configuration bonding or ns2 electron pair becomes inert. The inert pair effect
begins after n ³ 4 and increases with increasing value of n.
Element Electronic configuration (ns2 np1) (iv) The tendency to form M+ ion increases down the gp.
5B [He]2s2 2p1 Ga+1 < Tl+1
(10) Hydrated ions : All metal ions exist in hydrated state.
13Al [Ne]3s2 3p1
(11) Ionisation energy
31Ga [Ar]3d10 4s2 4p1 (i) Inspite of the more charge in nucleus and small size, the
first ionisation energies of this group elements are lesser than the
49In [Kr]4d10 5s2 5p1
corresponding elements of s block. This is due to the fact that
81Tl [Xe]4f14 5d10 6s2 6p1 removal of electron from a p-orbitals (being far away from nucleus
and thus less effectively held than s-orbitals) is relatively easier
(2) Occurrence : The important of this group elements are
than s-orbitals.
given below,
(ii) The ionisation energy of this group element decrease
Boron : Borax (Tincal) (Na2B4O7.10H2O), Colemanite down the group due to increases in size like other group elements.
(Ca2B6O115H2O) (iii) However, ionisation energy of Ga are higher than that
Boracite (2Mg3B8O15.MgCl2), Boronatro calcite (CaB4O7.NaBO2.8H2 O), of Al because of smaller atomic size of Ga due to less effective
shielding of 3d electrons in Ga. Thus valence shell exert more
Kernite (Na2B4O7.4H2O), Boric acid (H3BO3) effective nuclear charge in Ga to show higher ionisation energies.
Aluminium : Corundum (Al2O3), Diaspore (Al2O3.H2O), (12) Electropositive character
Bauxite (Al2O3. 2H2O), and Cryolite (Na3AlF6). (i) Electropositive character increases from B to Tl.
(ii) Boron is semi metal, more closer to non-metallic nature
Physical properties
whereas rest all members are pure metals.
(1) A regular increasing trend in density down the group is (iii) Furthermore, these elements are less electropositive
due to increase in size. than s-block elements because of smaller size and higher
(2) Melting points do not vary regularly and decrease from ionisation energies.
B to Ga and then increase. (13) Oxidation potential
(3) Boron has very high melting point because it exist as (i) The standard oxidation potentials of these element are
giant covalent polymer in both solid and liquid state. quite high and are given below,
B Al Ga In Tl
(4) Low melting point of Ga (29.80C) is due to the fact that
E0op for M ® M3++ 3e – +1.66 +0.56 +0.34 +1.26
consists of only Ga2 molecule; it exist as liquid upto 20000C and
E0op for M ® M+ + e – +0.55 – +0.18 +0.34
hence used in high temperature thermometry.
(ii) However Boron does not form positive ions in aqueous
(5) Boiling point of these elements however show a regular solution and has very low oxidation potential.
decrease down the group.
(iii) The higher values of standard oxidation potentials are
(6) The abrupt increase in the atomic radius of Al is due to due to higher heats of hydration on account of smaller size of
greater screening effect in Al (it has 8 electrons in its penultimate trivalent cations.
shell) than in B (it has 2 electrons in its penultimate shell) (iv) Aluminium is a strong reducing agent and can reduce
(7) The atomic radii of group 13 elements are smaller than oxides which are not reduced even by carbon. This is due to lower
the corresponding s-block elements. This is due to the fact that ionisation energy of aluminium than carbon. The reducing character of
when we move along the period, the new incoming electron these elements is Al > Ga > In > Tl.
occupy the same shell whereas the nuclear charge increases (14) Complex formation : On account of their smaller size
regularly showing more effective pull of nucleus towards shell and more effective nuclear charge as well as vacant orbitals to
electrons. This ultimately reduces the atomic size. accept elements, these elements have more tendency to form
complexes than-s block elements.
(8) The atomic radius of Ga is slightly lesser than of Al
because in going from Al to Ga, the electrons have already Chemical properties
occupied 3d sub shell in Ga. The screening effect of these (1) Hydrides
intervening electrons being poor and has less influence to (i) Elements of group 13 do not react directly with
decrease the effective nuclear charge, therefore the electrons in Ga hydrogen but a number of polymeric hydrides are known to exist.
experience more forces of attractions towards nucleus to result in (ii) Boron forms a large no. of volatile covalent hydrides,
lower size of Ga than Al known as boranes e.g. B2H6,B4H10,B5H11,B6H10 Two series of
borones with general formula BnHn+4 and BnHn+6 are more
(9) Oxidation state
important.
 Boron Family

(iii) Boranes are electron deficient compounds. It is (ii) All the trihalides of group 13 elements are known except
important to note that although BX3 are well known, BH3 is not Tl (III) iodide.
known. This is due of the fact that hydrogen atoms in BH3 have no (iii) Due to small size and high electronegativity of boron,
free electrons to form pp–pp back bonding and thus boron has all boron halides are covalent and Lewis acids. These exist as
incomplete octet and hence BH3 molecules dimerise to form B6H6 monomeric molecules having plane triangular geometry (sp2
having covalent and three centre bonds. hybridization).
(iv) Al forms only one polymeric hydride (AlH3)n commonly (iv) All Boron trihalides except BF3 are hydrolysed to boric
known as alane It contains Al…..H……Al bridges. acid.
(v) Al and Ga forms anionic hydrides e.g. LiAlH4 and LiGa H4, BX3+ 3H2O ®B(OH)3 + 3HX; [X=Cl, Br, I]
ether
4 LiH + AlCl 3 ¾¾ ¾® Li[ AlH 4 ] + 3 LiCl However, BF3 forms as addition product with water,
H2 O
(2) Reactivity towards air BF3 + H2O®H+ [BF3OH]- H3O+ [BF3OH]- .
(i) Pure boron is almost unreactive at ordinary BF3 having less tendency for hydrolysis as well as Lewis
temperature. It reacts with air to form B2O3 when heated It does acid nature, is extensively used as a catalyst in organic reactions
react with water. Al burns in air with evolution of heat give Al2O3. e.g. Friedel- Crafts reaction.
(ii) Ga and In are not effected by air even when (v) Boron atom, in BX3, has six electrons in the outermost
heated whereas Tl is little more reactive and also form an oxide orbit and thus it can accept a pair of electrons form a donor
film at surface. In moist air, a layer of Tl (OH) is formed. molecule like NH3 to complete its octet. Hence boron halides act as
(iii) Al decomposes H2O and reacts readily in air at ordinary very efficient Lewis acids. The relative Lewis acid character of
temperature to form a protective film of its oxides which protects boron trihalides is found to obey the order ; BI3>BBr3>BCl3>BF3.
it from further action. However, the above order is just the reverse of normally
(3) Oxides and hydroxides expected order on the basis relative electronegativities of the
(i) The members of boron family form oxide and hydroxides halogens. Fluorine, being the most electronegative, should create
of the general formula M2O3 and M (OH)3 respectively. the greatest electron deficiency on boron and thus B in BF3 should
(ii) The acidic nature of oxides and hydroxides changes accept electron pair from a donor very rapidly than in other boron
from acidic to basic through amphoteric from B to Tl. trihalides. But this is not true.
B2O3 and B(OH)3> Al2O3 and Al(OH)3 > This anomalous behaviour has been explained on the basis
(acidic) (amphoteric) of the relative tendency of the halogen atom to back-donate its
unutilised electrons to the vacant p orbitals of boron atom. In
Ga2O3 and Ga(OH)3> In2O3 In (OH)3> Tl2O3 Tl(OH)3 boron trifluoride, each fluorine has completely filled unutilised 2p
(amphoteric) (basic) (strong basic) orbitals while boron has a vacant 2p orbital. Now since both of
B(OH)3 or H3BO3 is weak monobasic Lewis acid. these orbitals belong to same energy level (2p) they can overlap
(iii) Boric acid, B(OH)3 is soluble in water as it accepts lone effectively as a result of which fluorine electrons are transferred
into the vacant 2p orbital of boron resulting in the formation of an
pair of electron to act as Lewis acid. Rest all hydroxides of group
additional pp–pp bond. This type of bond formation is known as
13 are insoluble in water and form a gelatinous precipitate.
back bonding or back donation. Thus the B- F bond has some
B(OH)3 + H2O ®B(OH)41–+H+ double bond character. Back bonding may take place between
(iv) Al2O3 being amphoteric dissolves in acid and alkalies boron and of the three fluorine atoms and thus boron trifluoride is
both. regarded as a resonance hybrid of some structures.
Al2O3 + 3H2SO4® Al2 (SO4)3 + 3H2O Resonance in boron trifluoride is also evidenced by the fact
fuse that the three boron-fluorine bonds are indentical and are shorter
Al 2 O 3 + 2 NaOH ¾¾ ¾® 2 NaAlO 3 + H 2O
Sodium meta aluminate than the usual single boron-fluorine bond As a result of back
bonding, the electron deficiency of boron is reduced and hence
(v) One of the crystalline form of alumina (Al2O3) is called
Lewis acid nature is decreased. The tendency for the formation of
corrundum. It is very hard and used as abrasive. It is prepared by
heating amorphous form of Al2O3 to 2000 K. back bonding (pp- pp bond) is maximum in BF3 and decreases very
rapidly from BF3 to BI3 This is probably due to the fact that
(4) Action of Acids
overlapping of the vacant 2p orbitals of boron cannot take place
(i) Boron does not react with non oxidizing acids, however, easily with the p-orbitals of high energy levels (3p in Cl, 4p in Br
it dissolves in nitric acid to form boric acids. and 5p in iodine). Thus BI3Br3 and BCl3 are stronger Lewis acids
(ii) Al, Ga and In dissolve in acids forming their trivalent than the BF3.
cations; however, Al and Ga become passive due to the formation
(vi) Lewis acid character of halides of the group 13
of protective film of oxides.
elements decreases in the order, B > Al > Ga > In.
(iii) Thallium dissolves in acids forming univalent cation
(vii) Boron halides form complex halides of the type, [BF4–],
and becomes passive in HCl due to the formation of water
in which boron atom extends its coordination number to four by
insoluble TICl.
utilising empty p-orbital. It cannot extend its coordination number
(5) Action of Alkalies beyond four due to non availability of d-orbitals. However, the
(i) Boron dissolves only in fused alkalies, other trihalides of this group form complex halides of the type
2B + 6NaOH (fused)® 2Na3BO3 + 3H2 (AlF6)3–, (GaCl6)3– and (InCl6)3–, etc where the central atom extends
its coordination number to 6 by the use of d-orbitals.
(ii) Al and Ga dissolves in fused as well as in aqueous
(viii) The fluorides of Al, Ga In and Tl are ionic and have
alkalies, 2Al + 2 NaOH + 2H2O ®2NaAl O2 + 3H2 high melting points. The high melting points of metal fluorides can
(iii) Indium remains unaffected in alkalies even on heating. be explained on the basis that their cations are sufficiently large
(6) Halides and have vacant d-orbitals for attaining a coordination number of
(i) All the group 13 elements from the trihalides, MX3 on six towards the relatively small fluorine atom.
directly combining with halogens. (ix) Other halides of Al, Ga, In and Tl are largely covalent in
M + X2 ® MX3 anhydrous state and possess low melting point. These halides do
 Boron Family

not show backbonding because of increases in the size of the (13) Boron combines with metals to give borides e.g. Mg3B2.
element. However, the make use of vacant p-orbitals by co- Other members form simply alloys.
ordinate bond i.e. metal atoms complete their octet by forming (14) Concentrated nitric acid oxidises boron to boric acid
dimers. Thus aluminium chloride, aluminium bromide and indium but no such action is noticed other group members.
iodide exist as dimers, both in the vapour state and in non-polar B + 3HNO3 ®H3BO3 + 3NO2
solvents.
Diagonal relationship between Boron and Silicon
The dimer structure for Al2Cl6 is evidenced by the following
Due to its small size and similar charge/mass ratio, boron
facts,
differs from other group 13 members, but it resembles closely
(a) Vapour density of aluminium chloride measured at with silicon, the second element of group 14 to exhibit diagonal
4000C corresponds to the formula Al2Cl6.
relationship. Some important similarities between boron and
(b) Bond distance between aluminium chlorine bond silicon are given below,
forming bridge is greater (2.21Å) than the distance between
(1) Both boron and silicon are typical non-metals, having
aluminum-chlorine bond present in the end (2.06 Å). The dimeric high m.pt. b.pt nearly same densities (B=2.35gml–1 S=2.34 g//ml).
structure disappears when the halides are dissolved in water This
low atomic volumes and bad conductor of current. However both
is due to high heat of hydration which split the dimeric structure
are used as semiconductors.
into [M(H2O)6]3+ and 3X– ions and the solution becomes good
(2) Both of them do not form cation and form only covalent
conductor of electricity.
compounds.
Al2Cl6 + 2H2O ®2[Al(H2O)6]3++6Cl– ; Therefore Al2Cl6 is ionic
(3) Both exists in amorphous and crystalline state and
in water.
exhibit allotropy.
The dimeric structure may also split by reaction with donor
(4) Both possess closer electronegativity values (B=2.0;
molecules e.g. R3N. This is due to the formation of complexes of the
Si=1.8).
type R3NAlCl3 The dimeric structure of Al2Cl6 exist in vapour state
below 473K and at higher temperature it dissociates to trigonal (5) Both form numerous volatile hydrides which
planar AlCl3 molecule. spontaneously catch fire on exposure to air and are easily
hydrolysed.
Boron halides do not exist as dimer due to small size of
boron atom which makes it unable to co-ordinate four large-sized (6) The chlorides of both are liquid, fume in most air and
halide ions. readily hydrolysed by water.
(x) BF3 and AlCl3 acts as catalyst and Lewis acid in many of BCl3 + 3H2O ® B(OH)3 + 3HCl
the industrial process. SiCl4 + H2O ®Si(OH)4 + 4HCl
Anomalous Behaviour of Boron (7) Both form weak acids like H3BO3 and H2SiO3.
Like Li and Be, Boron – the first member of group 13 also (8) Both form binary compounds with several metals to
shows anomalous behaviour due to extremely low size and high give borides and silicide. These borides and silicide react with
nuclear charge/size ratio, high electronegativity and non- H3PO4 to give mixture of boranes and silanes.
availability of d electrons. The main point of differences are, 3Mg+2B®Mg3B2; Mg3B2+H3PO4 ® Mixture of boranes
(1) Boron is a typical non- metal whereas other members (Magnesium boride)

are metals. 2Mg + Si ® Mg2Si ; Mg2Si + H3PO4 ®Mixture of silanes

(2) Boron is a bad conductor of electricity whereas other (magnesium silicide)
metals are good conductors. (9) The carbides of both Boron and silicon (B4C and SiC) are
(3) Boron shows allotropy and exists in two forms – very hard and used as abrasive.
crystalline and amorphous. Aluminium is a soft metal and does not (10) Oxides of both are acidic and can be reduced by limited
exist in different forms. amount of Mg In excess of Mg boride and silicide are formed.
(4) Like other non-metals, the melting point and boiling B2O3+3Mg ® 3MgO+2B ; SiO2+2Mg ®2MgO+Si
point of boron are much higher than those of other elements of (11) Both the metals and their oxides are readily soluble in
group 13. alkalies.
(5) Boron forms only covalent compounds whereas 2B + 6NaOH ® 2Na3BO3 + 3H2 ­
aluminium and other elements of group 13 form even some ionic (borate)

compounds. Si + 2NaOH + H2O ® Na2SiO3 + 2H2 ­

(silicate)
(6) The hydroxides and oxides of boron are acidic in nature
whereas those of others are amphoteric and basic. B2O3 + 6NaOH ® 2Na3BO3 + 3H2O
SiO2 + 2NaOH ® Na2SiO3 + H2O
(7) The trihalides of boron (BX3) exist as monomers On the
Both borates and silicates have tetrahedral structural units
other hand, aluminium halides exist as dimers (Al2X6).
BO 4n - and SiO 4n - respectively. Boro silicates are known in which
(8) The hydrides of boron i.e. boranes are quite stable while
those of aluminium are unstable. boron replaces silicon in the three dimensional lattice. Boron can
however form planar BO3 units.
(9) Dilute acids have no action on boron Others liberate H2
(12) Acids of both these elements form volatile esters on
from them.
heating with alcohol in presence of conc. H2SO4.
(10) Borates are more stable than aluminates.
B(OH)3 + 3ROH®B(OR)3 + 3H2O
(11) Boron exhibit maximum covalency of four e.g., BH–4 ion Si(OH)4 + 4ROH® Si(OR)4 + 4H2O
while other members exhibit a maximum covalency of six e.g.,
Boron and its compounds
[Al(OH)6]3-.
Boron is the first member of group –13 (IIIA) of the periodic
(12) Boron does not decompose steam while other table. Boron is a non- metal . It has a small size and high ionization
members do so. energy due to which it can not lose its valence electrons to form
 Boron Family

B +3 ion. Its compounds especially the hydrides and halides are (b) All boranes catch fire in the presence of oxygen to
electron deficient and behave as Lewis acid. liberated a lot of heat energy. Thus, they can also be used as high
energy fuels.
(1) Ores of boron
(i) Borax or tincal : Na2 B4O7 . 10H2O B 2 H 6 + 3 O 2 ¾¾® 2 B 2 O 3 + 3 H 2 O; DH = -1976 KJ / mole
(ii) Kernite or Rasorite : Na2 B4O7 . 4H2O (c) Boranes are readily hydrolysed by water.
(iii) Colemanite : Ca2 B6O11 . 5H2O
B 2 H 6 + 6 H 2 O ¾¾® 2 H 3 BO 3 + 6 H 2
(iv) Orthoboric acid : H3BO3 (It occurs in the jets of steam
called soffioni escaping from ground in the volcanic region of the (d) With carbon monoxide
Tuscany). Boron is present to a very small extent (0.001%) in B 2 H 6 + 2 CO ¾¾®(BH 3 ¬ CO )2
earth’s crust.
(2) Isolation : Elemental boron in the form of dark brown (e) Boranes are used for formation of hydroborates or
powder is obtained either by reduction of boric oxide with highly borohydrides such as LiBH 4 or NaBH 4 , which are extensively
electropositive metals like K, Mg, Al, Na, etc. in the absence of air used as reducing agents in organic synthesis.
and boron halides with hydrogen at high temperature eg. ¾® 2 Li + [BH 4 ]-
Diethyl ether
2 LiH + B 2 H 6 ¾¾ ¾¾¾
Heat
B2O3 + 6K ¾¾ ¾® 2B + 3K2O
1270 K 2 NaH + B 2 H 6 ¾¾ ¾ ¾¾® 2 Na + [BH 4 ]-
Diethyl ether
2BCl3 + 3H2 ¾¾ ¾¾® 2B + 6HCl.
By thermal decomposition of boron triiodide over red hot Structure of diborane : B2 H 6 has a three centre electon
tungsten filament and boron hydrides for example, pair bond also called a banana shape bond.
W , heat Heat
2BI3 ¾¾ ¾ ¾® 2B + 3I2 ; B2H6 ¾¾ ¾® 2B + 3H2 (a) B - H t : It is a normal covalent bond (two centre
(3) Properties : It exists in mainly two allotropic forms i.e. electron pair bond i.e., 2c - 2 e ).
amorphous dark brown powder and crystalline black very hard
(b) B - H b : This is a bond between three atoms,
solid. It occurs in two isotopic forms, i.e., 5 B 10 (20% abundance)
B - H b - B, (three centre electron pair bond i.e., 3 c - 2 e ).
and 5 B 11 (80% abundance). With air, boron forms B2 O3 and BN Bridged
at 973K, with halogens, trihalides (BX 3 ) are formed, with metals Terminal
Hydrogen
borides are formed. eg. Hydrogen Ht Hb Ht
Heat
4B+ 3O 2 ¾¾¾® 2 B2 O3 120o
Boron trioxide
B 97o B
Heat
2B + N 2 ¾¾ ¾® 2BN
Ht
Boron nitride Hb Ht
2B + 3 X 2 ¾¾® 2BX 3
Boron trihalide
Heat Ht Hb Ht
3Mg + 2B ¾¾¾® Mg 3 B 2
Magnesium boride
..
or B B
Water, steam and HCl have no action on B. oxidising acids ..
(HNO 3 , H 2 SO 4 ) convert boron to H 3 BO 3 . Ht Hb Ht
B + 3 HNO 3 ¾¾® H 3 BO 3 + 3 NO 2
Structure of diborane (B2H6)
2B + 3 H 2 SO 4 ¾¾® 2 H 3 BO 3 + 3 SO 2
The other boron hydrides are B5 H 9 , B 4 H 10 , B5 H 11 etc.
Fused alkalies (NaOH, KOH) dissolve boron forming borates, (ii) Boron Halides
liberating hydrogen. Boron reacts with halogens on strong heating to form
Fused
2B + 6KOH ¾¾ ¾
¾® 2 K 3 BO 3 + 3H 2 boron halides .
Heat
(4) Uses of Boron : Boron is used in atomic reactors as 2B + 3 X 2 ¾¾ ¾® 2B X 3 ( X = F, Cl, Br , I)
protective shields and control rods, as a semiconductors for BF3 and BCl 3 are gases, BBr3 is a volatile liquid while
making electronic devices in steel industry for increasing the
hardness of steel and in making light composite materials for air BI 3 is a solid.
crafts. In these halides, the central boron atom has three shared
(5) Compounds of Boron pairs of electrons with the halogen atoms. Therefore, these have
(i) Boron Hydrides two electrons less than the octet and are electron deficient
compounds. They acts as Lewis acids.
Boron forms hydrides of the types Bn H n + 4 and Bn H n + 6
F H F H
called boranes. Diborane is the simplest boron hydride which is a | | | |
dimer of BH 3 . F - B + : N - H ¾¾® F - B ¬¾¾ N - H
| | | |
Preparation
F H F H
450 K Lewis acid
(a) 8 BF3 + 6 LiH ¾¾ ¾® B 2 H 6 + 6 LiBF4 Lewis base

The relative acidic strength of boron trihalides decreases as

(b) 4 BCl 3 + LiAlH 4 ¾¾® 2 B 2 H 6 + 3 AlCl 3 + 3 LiCl
: BI 3 > BBr3 > BCl 3 > BF3 .
(c) In the laboratory, it is prepared by the oxidation of sod. (iii) Borax ( Na 2 B 4 O 7 . 10 H 2 O )
Borohydride with I2 .
It occurs naturally as tincal (Suhaga) which contains about
Polyether
2 NaBH 4 + I2 ¾¾ ¾ ¾® B 2 H 6 + 2 NaI + H 2 50% borax in certain land, lakes. It is also obtained from the
Properties : (a) Since Boron in boranes never complete its mineral colemanite by boiling it with a solution of Na 2 CO 3 .
octet of electrons hence all boranes are called as electron-deficient Ca 2 B6 O11 + 2 Na 2 CO 3 ¾¾® Na 2 B 4 O7 + 2 CaCO 3 + 2 NaBO 2
compounds or Lewis acids. Colemanite Borax

Properties : (a) Its aqueous solution is alkaline due to

hydrolysis,
 Boron Family

Na 2 B 4 O7 + 7 H 2 O ¾¾® 2NaOH+4 H 3 BO 3 . (v) Borazine or Borasole or Triborine triamine

( B3 N 3 H 6 )
(b) On heating borax loses its water of crystallization and
It is a compound of B, N and H. It is a colourless liquid and
swells up to form a fluffy mass. On further heating, it melts to give a
is also called inorganic benzene.
clear liquid which solidifies to a transparent glassy bead consisting o
180 C
of sodium metaborate ( NaBO 2 ) and boric anhydride ( B 2 O 3 ), 2 B 2 H 6 + 6 NH 3 ¾¾ ¾¾® 2 B 3 N 3 H 6 + 12 H 2 .
It has a six membered ring of alternating B and N atoms,
D D
Na 2 B 4 O7 . 10 H 2 O ¾¾® Na 2 B 4 O7 ¾¾® 2 NaBO 2 + B 2 O 3 each is further linked to a H- atom.
- 10 H 2 O Borax bead
H
Borax bead is used for the detection of coloured basic
radicals under the name borax bead test. B
Borax bead test : Borax bead is a mixture of NaBO 2 and H—N N—H
B 2 O 3 . B 2 O 3 on heating combines readily with a number of H—B B—H
N
coloured transition metal oxides such as Co, Ni, Cr, Cu, Mn, etc. to
form the corresponding metaborates which possess characteristic H
colours, Borazine
D (vi) Boron nitride (BN)
CoSO 4 ¾¾® CoO + SO 3 ; CoO + B 2 O 3 ¾¾® Co (BO 2 )2
Cobalt meta borate It is prepared by treating BCl 3 with an excess of NH3 and
(Blue)
pyrolysing the resulting mixture in an atmosphere of NH3 at 750°C,
Colours of some important metaborates are : Cupric metaborate, o
750 C
Cu (BO 2 )2 is dark blue, chromium metaborate, Cr (BO 2 )2 is green, BCl 3 + NH 3 ¾¾® [ H 3 N ¾¾® BCl 3 ] ¾¾ ¾¾® BN + 3HCl .
Excess NH 3
nickel metaborate, Ni(BO 2 )2 is brown and manganese
It is a colourless, good insulator, diamagnetic and almost
metaborate, Mn(BO 2 )2 is pink violet. unreactive solid
(c) When heated with C 2 H 5 OH and conc. H 2 SO 4 it gives Aluminium and its compounds
volatile vapours of triethyl borate which burns with a green edged (1) Ores of Aluminium : Bauxite ( Al 2 O 3 . 2 H 2 O), Cryolite
flame.
( Na 3 AlF6 , Felspar (KAlSi 3 O 8 ), Kaolinite ( Al 2 O 3 . 2 SiO 2 . 2 H 2 O) ,
Na 2 B 4 O7 + H 2 SO 4 + 5 H 2 O ¾¾® Na 2 SO 4 + 4 H 3 BO 3
Mica (K 2 O. 3 Al 2 O 3 . 6 SiO 2 . 2 H 2 O), Corundum ( Al 2 O 3 ) , Diaspore
H 3 BO 3 + 3 C 2 H 5 OH ¾¾® B(OC 2 H 5 )3 + 3 H 2 O ( Al 2 O 3 . H 2 O), Alunite or alum stone [K 2 SO 4 . Al 2 (SO 4 )3 .
Triethyl b orate

This reaction is used as a test for borate radical in 4 Al(OH )3 ].

qualitative analysis. (2) Extraction : Aluminium is obtained by the electrolysis
Uses : (a) In making optical and hard glasses. (b) In the of the oxide (alumina) dissolved in fused cryolite. This involves
laboratory for borax bead test. (c) In softening of water. (d) In the following steps,
preparation of medicinal soaps due to its antiseptic character. Purification of ore
(iv) Boric acid or orthoboric acid ( H 3 BO 3 )
(i) Baeyer's process
It is obtained from borax by treating with dil. HCl or dil. Roasted + Caustic soda solution
H 2 SO 4 , Bauxite ¾¾ ¾¾® Roasted ore ¾¾ ¾ ¾ ¾ ¾ ¾¾®
Finely powdered FeO ® Fe 2 O 3 High pressure (150 o C, 80 atm)
(red) filtered, Fe 2 O 3 as residue
Na 2 B 4 O7 + 2 HCl + 5 H 2 O ¾¾® 2 NaCl + 4 H 3 BO 3
Filtered Heat
It can also be obtained from the mineral colemanite by Filtrate ¾¾ ¾¾® Pure Al 2 O 3 ¾¾ ¾® Al(OH )3
(Sod. Aluminate) CO 2
passing SO 2 through a mixture of powdered mineral in boiling
(ii) Hall's process
water,
CO
Ca 2 B6 O11 + 4 SO 2 + 11 H 2 O ¾¾® 2 Ca(HSO 3 )2 + 6 H 3 BO 3 Bauxite ¾¾+ Na
¾2¾CO 3
¾ ¾® Solution ¾¾ ¾2¾ ¾®
(Finely powdered) Fused, extracted with 50 - 60 o C and filtered.
water. Residue Fe 2 O 3
Properties : (a) It is a very weak monobasic acid, does not (red) Filtrate (Na 2 CO 3 )

act as a proton doner but behaves as a Lewis acid i.e. it accepts a Heat
Precipitat e Al(OH)3 ¾¾ ¾® Pure Al 2 O 3
pair of electrons from OH - ion of H 2 O ,
(iii) Serpek's process
H 3 BO 3 + H 2 O ¾¾®[B(OH )4 ]- + H + + Coke + N
Bauxite ¾¾ ¾ ¾ ¾ 2
¾® Silica reduced to + Alumina form AIN
It acts as a strong acid in presence of polyhydroxy (Finely powdered)
(white)
Heated to
o
1800 C
Si which volatalis es aluminium nitride
compounds such as glycerol, mannitol etc. and can be titrated Hydrolysis Heated
against strong alkali . ¾¾ ¾ ¾
¾® Pure Al 2 O3 ¾¾ ¾ ¾® Al(OH )3
(b) With NaOH it forms, sodium metaborate,
(iv) Hall and Heroult process : It is used for extraction of
H 3 BO 3 + NaOH ¾¾® NaBO 2 + 2 H 2 O aluminium. In this process a fused mixture of alumina (20%),
(c) With C 2 H 5 OH and conc. H 2 SO 4 , it gives triethyl cryolite (60%) and fluorspar (20%) is electrolysed using carbon
borate electrodes. Whereas cryolite makes Al 2 O 3 conducting fluorspar
H 3 BO 3 +3 C 2 H 5 OH ¾¾ ¾ ¾
2
¾
¾4 Conc . H SO
® B(O C 2 H 5 )3 + 3 H 2 O decreases the melting point of alumina.
Aluminium is refined by Hoope's electrolytic process.
(d) Action of heat : The complete action of heat on boric
acid may be written as, (3) Compounds of Aluminium
373 K
H 3 BO 3 ¾¾ ¾® HBO 2 433 K
¾¾ Red hot
¾® H 2 B 4 O7 ¾¾ ¾¾® B 2 O 3 (i) Aluminium oxide or Alumina ( Al 2 O 3 ) : It occurs in
Boric acid Metaboric acid Tetra boric acid Boron oxide
nature as colourless corundum and several coloured minerals like
Structure : In boric acid, planar units are joined by BO 3-3 ruby (red), topaz (yellow), Sapphire (blue), amethyst (violet) and
hydrogen bonds to give a layer structure. emerald (green). These minerals are used as precious stones
Uses : (a) As a food preservative. (b) As a mild antiseptic (gems).
for eye wash under the name boric lotion. (c) For the preparation (ii) Aluminium chloride ( Al 2 Cl 6 ) : It is prepared by
of glazes and enamels in pottery. passing dry chlorine over aluminium powder.
 Boron Family

Al 2 O 3 + 3 C + 3 Cl 2 ® 2 AlCl 3 + 3 CO (g)
(anhydrous )
It exists as dimer Al 2 Cl 6 , in inert organic solvents and in
vapour state. It sublimes at 100 o C under vacuum. Dimeric
structure disappears when AlCl 3 is dissolved in water. It is
hygroscopic in nature and absorbs moisture when exposed to air.
(iii) Thermite : A mixture of aluminium powder and Fe 2 O 3
in the ratio 1:3. It is used for welding of iron. The reaction between
Al and Fe 2 O 3 is highly exothermic,
Al + Fe 2 O 3 ® Al 2 O 3 + Fe + Heat
(iv) Aluminium sulphate [Al2(SO4)3] : It is used for the
preparation of alums e.g., potash alum Al 2 (SO 4 )3 . K 2 SO 4
. 24 H 2 O . It is also used for making fire proof clothes.
(iv) Alums : In general, the term alum is given to double
sulphates of the type M 2 SO 4 .M ¢2 (SO 4 ) 3 . 24 H 2 O where M is a
univalent cation like Na + , K + and NH 4+ , M ¢ is a trivalent cation
like Al 3 + , Fe 3 + and Cr 3 + .
Some important points to be noted about the alums are
(a) General formula is M 2 SO 4 . M ¢2 (SO 4 ) 3 . 24 H 2 O
M = Monovalent metal; M ¢ = Trivalent metal
In alum crystals, 6 water molecules are held by monovalent
ion, 6 water molecules are held by trivalent ion, 12 water
molecules are held in the crystal structure.
(b) All alums are isomorphous. Aqueous solutions of alums
are acidic due to cationic hydrolysis of trivalent cation.
(c) Double sulphates of divalent ions and trivalent ions with
24 water molecules in their crystals are known as Pseudoalums.
General formula is MSO 4 . M ¢2 (SO 4 ) 3 . 24 H 2 O
M = Bivalent metal; M ¢ = Trivalent metal
(d) Pseudoalums are not isomorphous with alums.
(e) Feather alum or ‘Hair-salt’ Al 2 SO 4 . 18 H 2 O is a native
form of aluminium sulphate.
(f) Potash alum is used for tanning of leather, as mordant in
dyeing and calico printing, for sizing paper, as a syptic to stop
bleeding and purification of water.
Some important alums are
Potash alum K 2 SO 4 . Al 2 (SO 4 ) 3 . 24 H 2 O
Sodium alum Na 2 SO 4 . Al 2 (SO 4 ) 3 . 24 H 2 O
Ammonium alum ( NH 4 ) 2 SO 4 . Al 2 (SO 4 ) 3 . 24 H 2 O
Chrome alum K 2 SO 4 .Cr2 (SO 4 ) 3 . 24 H 2 O