The boron family

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BORON FAMILY(GROUP 13)BORON FAMILY(GROUP 13)
PART V.PART V.

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GROUP 13 ELEMENTS-THE BORON FAMILY
ELEMENT SYMBOL ELECTR. CONF.
Boron
Aluminium
Gallium
Indium
Thallium
B
Al
Ga
In
Tl
[He]2s2
2p1
[Ne]3s2
3p1
Ar]3d10
4s2
4p1
[Kr]4d10
5s2
5p1
[Xe]4f14
5d10
6s2
6p1

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INTRODUCTION
 Boron is the only group 13 element that is a
non-metal.
 The remaining members of this group are
fairly reactive metals, and are called p-block
elements.
 Aluminium, Al, is the third most abundant
element in the Earth's crust
 All elements show a stable oxidation state of
+3, except for thallium. 3

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 The small sizes of the ions, their high charge and
large values for their sum of the 1st
ionization
energy suggest that the elements are largely
covalent.
 Boron is always covalent and many simple
compounds like AlCl3 and GaCl3 are covalent when
anhydrous.
 However, in solution, the large amount of
hydration energy evolved offsets the high I.E and
4

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 Unlike the s-block elements, some of the elements
of this group display lower valency.
 There is an increase in tendency to form
univalent compounds as you go down the group,
and univalent thallium compounds are the most
stable.
 This monovalency is occurs because s-electrons
in the outer shell remaining paired, and therefore
not participating in bond formation……because
5

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 This tendency occurs among heavy
elements in the p-block and is called the
INERT PAIR EFFECT.
 Inert Pair Effect is the resistance or
reluctance of s-electrons to get unpaired, or
take part in covalent bonding.
 It is only p orbital electrons, which are
involved in bond formation.
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 Group 13 metals have silver luster.
 Erratic variation in M.P is observed down the group.
B(2300 ºC), Al(660.4 ºC), Ga(29.78 ºC), In(152.6 ºC) and Tl(303 ºC)
• Low M.P of Ga is reflected in the unusual structure of
the metal, which contains Ga2.
• The general trend down Group from non-metallic to
metallic character.
– Boron has a covalent network structure.
– Other elements are more ionic and metallic in
character.

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– Aluminium is on the borderline between ionic and
covalent character in its compounds.
– The remainder of Group 13 elements are
generally considered to be metals, although some
compounds exhibit covalent characteristics.
Electropositive character /nature of the element in
this group increases from Boron to Aluminium and
then decreases from Aluminium to Thallium (why?)
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 REASON
 This increase of electropositivitity from Boron to Aluminium
is associated with increasing size.
 However, B and Al follows immediately after s block
elements, while Ga, In and Tl follows after d block elements.
 So the extra d-electrons in Ga, In and Tl do not shield the
nuclear very effectively, as a result their orbital electrons are
more tightly held and the metal are less electropositive.
 Evidenced by the increase of IE between Al and Ga even
though the large atom would be expected to have a lower
value.
9

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OCCURRENCE AND EXTRACTION
 Group 13 elements are not found free in nature, but are all present
in various minerals and ores.
 Aluminiun is the most abundant metal in the Earth's crust
making up 8% existing in igneous rocks.
– Feldspars -(KAlSi3O8, NaAlSi3O8and CaAl2Si2O8).
– Micas -given by general formula X2Y4-6Z8O20(OH, F)4 where X =
K, Na or Ca; Y = Al, Mg, Fe and Z = Si)
– Clays – occurs naturally, show plasticity through a variable range
of water content
e.g. Kaolinite, Al2Si2O5(OH) and Pyrophyllite, Al2Si4O10(OH)2 .
– Cryolite (Na AlF – sodium aluminium fluoride)
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– Spinel (MgAl2O4)
– Bauxite (Al2O3..H2O)
– Gemstone, which are impure form of the oxide
of Al2O3 containing small amount of transition
metals that give them colours. E.g.
 Ruby: Al2O3 + traces of Cr3+
 Blue Sapphire: Al2O3 + traces of Fe2+
, Fe3+
and Ti+4
 White Sapphire: The germ from
aluminium itself
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EXTRACTION OF ALUMINIUM
 Chief ore : Bauxite (Al2O3..nH2O)
 Impurities: Silica, Iron(III) Oxide,calcium Oxide And Titanium
Hydroxide, and other few oxides
 Additive: cryolite (Na3AlF6).
-Bauxite is dissolved in molten cryolite, Na3AlF6 (Sodium
hexafluoroalumiminate) so as to lower its M.P
 Method: Electrolytic reduction
-Since Aluminium is reactive it is not extracted by chemical reduction e.g. C
……..as bauxite forms carbide.
-Usually produced by the electrolysis of bauxite.
 Electrolytic process: achieved by the Bayer – Hall Herout processes (…
major industrial process for production of Al)

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Step 1:Digestion
 Ground metallurgical-grade bauxite is digested in caustic soda solution at
140 – 280°C in pressure tanks.
 Red mud is filtered off:
Al2O3 . 3H2O + 2NaOH →Na2O.Al2O3 + 4H2O + red mud
bauxite caustic soda Sodium Aluminate filtered
 NOTE: Except Alumina And Silica all other impurities (calcium oxide,
iron oxide, titanium oxide) DO NOT dissolve in the caustic soda liquor.
 The aluminate solution is filtered leaving behind the
impurities.
 Silica dissolved in the liquor is then precipitated from it by
slow heating. 15

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Step 2: Precipitation
 Caustic soda is added to precipitate pure Al(OH)3
from Sodium Aluminate solution by seeding.
Na2O.Al2O3 + 3NaOH→2Al(OH)3.nH2O(s) +liquor
 Seeding of aluminum hydroxide reverses the
reaction
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Step 3: Calcination
 "Hydrate", is calcined to form alumina .
 In the calcination process water is driven off to
form alumina, this takes place at 1050o
C:
2Al(OH)3.3H2O(s) →Al2O3 + nH2O
 The calcination process must be carefully
controlled since it dictates the properties of the
final product.
 A large amount of the alumina so produced is
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Step 4 : Smelting (Electrolytic Reduction)
 Pure Al2O3 is dissolved in a molten Cryolite, Na3AlF6, in an
electrochemical cell.
 Molten Cryolite lowers the M.P from above 2000°C to 950–
1000°C). to save energy operational cost.
 At the cathode,
- Al2O3 is reduced to molten Al.
 At the anode
-Oxygen from the alumina reacts with the C electrode to form
CO2(g).
NB:The overall cell reaction is written as:
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2Al2O3(l) + 3C → 4Al (l) + 3CO2 (g)

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BAYER – HALL HEROUT PROCESSES

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http://ibchem.com/IB/ibfiles/options/opt_E/ope_img/cell.jpg

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Uses Of Aluminum
Use Examples
Transport Superstructures of trains, ships and airplanes. Alloy
engines for cars.
Construction Window frames, doors roofing
Power
transmission
Overhead electricity cables, capacitor foil
Kitchen
utensils
Kettles, saucepans
Packaging Drink cans, foil wrapping
Chemical
industry
Al(OH)3 – flame retarder, paper making
Al2(SO4)3 – flocculant in sweage treatment and to
precipitate PO4
3-
Al2O3 – catalyst and catalytic support material, abrasive

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ENVIRONMENTAL IMPACT
 Smelting processes of aluminum requires enormous
amount of electricity.
 Also, the main process which is the electrolysis emits
carbon dioxide which is greenhouse gas.
 Recycling aluminum is an important method of
saving energy and minimizing the environmental
damage.
 Recycling aluminum requires only 5% of the energy

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 Boron is found in ores widely distributed in
Earth's crust.
 Chief ore: BORAX……..the hydrated borates,
Na2B4O7.10H2O and similarly for tri, tetra and
pentaborates of calcium and sodium.
 Additive: Na or Mg to as reducing agents
…..reduces the oxides (B2O3)
OR using H2 in the presence of BCl3 and Tungsten
(W) filament
24

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Extraction/Preparation of some boron
compound from Borax
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GALLIUM, INDIUM AND THALLIUM
The elements Gallium, Indium and
Thallium are only found as miner
components of various minerals.
 These elements are produced or extracted
by electrolytic reduction in aqueous
solution
 They are relatively soft and reactive,
which readily dissolve in acids.
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Property B Al Ga In Tl
Atomic number (Z) 5 13 31 49 81
Outer electron configuration 2s2
2p1
3s2
3p1
4s2
3d10
4p1
5s2
4d10
5p1
6s2
4f14
5d10
6p1
Atomic rradii (pm) 80-90 143 122 167 170
Ionic radii (pm) 20 54 62 80 89
Electronegativity 2.37 1.50 1.60 1.70 1.80
Melting point (°C) 2300 660 29.7 156 304
Boiling point (°C) 3650 2467 2403 2080 1357
Density (g/cm3
) 2.37 2.696 1.607 7.310 1.80
Ionization energies - 1st
Ionization energies - 2nd
Ionization energies - 3rd
M(s) → M3+(
aq) + 3e-
800.6
2427
3659
577.6
1816
2744
578.8
1979
2962
558
1820
2704
589.3
1970
2877
Standard Reduction
Potentials (V, at 25°C)
M2+(
aq) + 2 e- → M(s)
-0.87 -1.66 -0.53 -0.34 -0.72
Hardness - 2.75 1.5 1.2 1.25
Electroconductivity 59.7 9.1 19.0 8.82

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 The M.P of all the elements are high,
- but the melting point of boron is much
higher than that of beryllium in Group 2
- M.P of aluminium is similar to that of
magnesium in Group 2 (diagonal
relationship).
 The densities of all the Group 13
elements are higher than those of Group
2 elements.
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The chemical properties of Group 13 elements reflect
the increasingly metallic character down the group.
-Here only boron and aluminium will be considered.
Boron is chemically unreactive except at high
temperatures.
Finely divided boron burns in air to form oxide and
nitride:
4B(s) + 3O2(air) ® 2B2O3 (Oxide)
2B(s) + N2(air) ® 2BN (Nitride).
Accordingly in halogen Boron form trihalides
2B(s) + 2X3 (g) ® 2BX3.
31

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GROUP 13 COMPOUNDS
 Because of their electron-deficient nature,
M3+
compounds have a formally vacant npz
orbital and usually act as Lewis acids (electron
acceptors).
32

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OXIDES (M2O3) -SESQUIOXIDE
 SESQUIOXIDE is an oxide containing three
atoms of oxygen with two atoms (or radicals)
of another element.
 The M2O3 of all the elements can be made by
heating the elements in oxygen:
4M (s) + 3O2(g) → 2M2O3 (s)
 But B2O3 is more usually made by33

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B(OH)3
-H2O
+H2O
100 o
C
HBO2
-H2O
+H2O
Red hot
B2O3
Metaboric acid,
which exists in
three crystalline
forms of which
contains the cyclic
unit
Boron oxideOrthoboric acid
O
BO
B
O B
OH
OH
H
OH
H

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 H3BO3 is a weak acid……due to its electron deficient
tendency.
 The B(OH)3 accepts an OH-
ion from the self ionization of
water forming a complex ion.
B(OH)3 + 2H2O → [B(OH)4]-
+ H3O+
 The hydroxide boric acid B(OH)3 is formed by the
hydrolysis of many boron compounds.
 It has a layer structure made up of planar molecules linked
by hydrogen bonding (ref slide overleaf).
 It is a Lewis acid that acts as a Brønsted acid . 35

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Oxides of oxidation state +3 of the Group 13 Elements
Oxide Property
B2O3  Weak acid
 many metal oxides gives glasses
with B2O3 as in the borax bead test
Al2O3 and Ga2O3  Amphoteric
In2O and Tl2O3  Weak basic
 Tl2O3 gives O2 and Tl2O on heating
to 100 °C

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HALIDES OF GROUP 13 ELEMENTS
All elements form trihalides.
They are nonpolar with trigonal planar shape.
The halides of boron are BX3 are all volatile, highly
reactive, covalently bonded molecular compounds and
are gases
The Boron fluoride (BF3) form fluoroborates, WHILE
other Boron halides giving boric acids
BF3 + H2O → [BF3OH] H
BCl3 + H2O ¾→H3BO3 + 3HCl
37

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 BX3 are Lewis acids and the order of
their Lewis acidity strength is:
BF3 < BCl3 < BBr3
 In contrary to the order of
Electronegativity of the attached
halogens:
F > Cl > Br
Why???????
38

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 BX3 are trigonal planar and monomeric (not
dimerized in the way the BH3 does.)
E.g. the structure of BBr3.
39

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 BF3 is a useful organic catalyst for Friedel Craft
reaction such as:
 Alkylations
 Acylation
 Estirification
 Polymerization of olefines
 REASON:
-Boron (an electron deficient atom) in BX3 has 6
electrons in its outer shell and can readily accept a
40

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 The fluorides of: Al, Ga, In, and Tl are
ionic having high melting points.
 The other halides of these metals are
covalent when anhydrous.
 AlCl3 , AlBr3 and GaCl3 exist as dimers
thus attaining an octet of electrons 41

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Exist as dimeric molecules with
the formula M2X6 using two
halide atoms to bridge the
metals.
This dimeric formula is
retained when the halides
dissolves in non-polar solvent
(e.g.; Benzene).
But because of high heat of
hydration when halides
dissolves in water, the covalent
dimer is broken into [M.6H2O ]42
Figure. 1

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 Aluminium chloride, AlCl3, is a volatile
solid which sublimes at 458K.
 The vapour formed on sublimation
consists of an equilibrium mixture of
monomers (AlCl3) and dimers (Al2Cl6).
 It is used to prepare the powerful and
versatile reducing agent lithium43

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AlCl3 + C2H5 – O – C2H5
Lewis Acid dimethyl ether ( Lewis base )
44
 AlX3 (Aluminium halides) are very reactive lewis acids
– they accepts a pair of electrons forming an acid base
compound called adducts

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 AlX3 is used as a catalyst in a number of
organic reactions.
E.G. When benzene is treated with acyl halide in
the presence of anhydrous Alcl3 as catalyst ¾®
aromatic ketone
45

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 The B-X bond distances are shorter than might
be expected, and the B-X bond energies are
correspondingly higher.
E.g. B-F bond energy (646 KJmol-1
) is the highest
known for a single bond.
EXPLANATION:
 This suggest that some π-bonding may be
existing between the unhybridized 2p orbital of
the boron and the filled np orbitals of the
halides.
46

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HYDROXIDES
 Al(OH)3 is amphoteric and reacts with
acids in a manner as metal hydroxides do.
Al(OH)3 (s) + 3H3O+
→ [ Al( H2O )6]3+
(aq)
 Al(OH)3 also reacts with a base in the
reaction represented as the formation of a
hydro-complex
Al(OH)3 (s) + OH-
→ [ Al( OH )4]-
(aq)47

48
 When Al(OH)3 dissolves in a base, hydroxyl ion
and water bonds to Al ion forming a complex ion
[ Al( H2O )2 (OH)4]-
.
 The reaction is as follows
Al(OH)3(s) + OH-
(aq) + 2H2O(l) → [Al(H2O )2
(OH)4]-
 Al(OH)3 is used in the purification of water
because it carries down any suspended material48

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HYDRIDES
 Special compounds that start of predominantly
covalent and become more ionic as we go down the
group.
 Most of the group 13 elements react directly with
hydrogen, and large number of interesting hydrides
are known.
 Boron forms an extensive and interesting series of
hydrides, called BORANES.
 The simplest of these is not BH3 as expected, but its
dimer B H .
49

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 The 8 well characterized boranes which fall into two
series BnHn+4 and less stable series BnHn+6 are:
i. B2H6 Diborane
ii. B4H10 Tetraborane
iii.B5H9 Pentaborane (stable)
iv.B5H11 Pentaborane (unstable)
v. B6H110 Hexaborane
vi.B9H15 Nonaborane (enneaborane)
vii.B10H14 decaborane 50

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Cont…….
 The borane molecule (BH3) may exists as a reaction
intermediate.
 But no BH3 as it does not exist as separate molecules. Boranes
are highly unstable due to their extreme electron deficiency.
 Their highly exothermic reaction with oxygen lead to their
consideration as rocket fuels by the space program
 The B-atom in BH3 lacks the complete octet (i.e. it has only 6
electrons in the valence shell).
 The simplest boron hydride that have been isolated is
51

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THE STRUCTURE OF B2H6 (DIBORANE) – MULTICENTRE
BONDS
The question of interest is what
holds the diborane together?
EXPLANATION OF THE STRUCTURE OF
B2H6 .
There are 12 valence electrons at for
chemical bonding (B has 3, and H has
1, so 2 × B + 6 × H = 12)
•Each terminal B-H bond has two
electron bond, and there are four of
then, thus accounting for a total of 8
52

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 This leaves a total of four electrons to be
shared between the two bridging H
atoms and the two B-atoms.
 For this reason, two B  H  B
bridging bonds are formed, each
consisting two electrons forming the so
called three centre – two electron bond
(3C, 2e).
 Meaning that 3 atoms share 2 electrons
(This sometimes called banana bonds
because they non-linear but curved.
(Fig. below )
53

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Figure 4. The Structure of B2H6 (diborane) – multicentre bonds.
Contains a 3-centre-2-electron bond (called a banana bond)

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INDUSTRIAL INFORMATION / APPLICATION
 Boron is used in:
 flares to provide a highly visible green colour.
 Boron filaments are now used extensively in the
aerospace industry as a lightweight yet strong
material.
 Boric acic acid is used as a mild antiseptic.
 Borax as a water softener in washing powders.
 Borosilicate glass contains boron compounds. 55

The boron family

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