3. GROUP 13 ELEMENTS-THE BORON FAMILY
ELEMENT SYMBOL ELECTR. CONF.
Boron B [He]2s22p1
Aluminium Al [Ne]3s23p1
Gallium Ga Ar]3d104s2 4p1
Indium In [Kr]4d105s2 5p1
Tl [Xe]4f145d106s2 6p1
➢Boron is the only group 13 element that is a
▪ The remaining members of this group are
fairly reactive metals, and are called p-block
▪ 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
▪ 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
▪ However, in solution, the large amount of hydration
energy evolved offsets the high I.E and
hence all ions exists in hydrated states.
▪ 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
the energy to unpair them is too great.
▪ 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.
▪ 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
– Boron has a covalent network structure.
– Other elements are more ionic and metallic in
– 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?)
▪ 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 a9lower
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 -(KAlSi3
– 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 (Na3AlF6 – sodium aluminium fluoride)
– 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
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, Na3
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)
▪ 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
▪ Silica dissolved in the liquor is then precipitated from it by
slow heating. 15
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
Step 3: Calcination
▪ "Hydrate", is calcined to form alumina .
▪ In the calcination process water is driven off to
form alumina, this takes place at 1050oC:
2Al(OH)3.3H2O(s) →Al2O3 + nH2O
▪ The calcination process must be carefully
controlled since it dictates the properties of the final
▪ A large amount of the alumina so produced is
then subsequently smelted
Step 4 : Smelting (Electrolytic Reduction)
➢ Pure Al2O3 is dissolved in a molten Cryolite, Na3AlF6, in an
▪ 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
NB:The overall cell reaction is written as: 18
2Al2O3 (l) + 3C 4Al (l) + 3CO2 (g)
Uses Of Aluminum
Superstructures of trains, ships and airplanes. Alloy
engines for cars.
Window frames, doors roofing
Overhead electricity cables, capacitor foil
Packaging Drink cans, foil wrapping
Al(OH)3 – flame retarder, paper making
2 4 3
Al (SO ) – flocculant in sweage treatment and to
Al2O3 – catalyst and catalytic support material, abrasive
23. 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
▪ Recycling aluminum requires only 5% of the energy
➢Boron is found in ores widely distributed in
▪ 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
❑GALLIUM, INDIUM AND THALLIUM
➢The elements Gallium, Indium
Thallium are only found as
components of various minerals.
▪ These elements are produced or extracted
by electrolytic reduction in aqueous
▪ They are relatively soft and
which readily dissolve in acids.
▪ 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
▪ The densities of all the Group 13
elements are higher than those of Group
➢The chemical properties of Group 13 elements
reflect the increasingly metallic character down the
-Here only boron and aluminium will be considered.
▪Boron is chemically unreactive except at high
▪Finely divided boron burns in air to form oxide and
4B(s) + 3O2(air) ® 2B2O3 (Oxide)
2B(s) + N2(air) ® 2BN (Nitride).
▪ Accordingly in halogen Boron form trihalides
2B(s) + 2X3(g) ® 2BX3. 31
❑GROUP 13 COMPOUNDS
▪ Because of their electron-deficient nature,
M3+compounds have a formally vacant npz
orbital and usually act as Lewis acids (electron
OXIDES (M2O3) -SESQUIOXIDE
▪ SESQUIOXIDE is an oxide containing three
atoms of oxygen with two atoms (or radicals)
of another element.
▪ The M2
O3of all the elements can be made by
heating the elements in oxygen:
4M (s) + 3O2(g) → 2M2O3 (s)
▪ But B2O3 is more usually made 33
▪ H3BO3 is a weak acid……due to its electron deficient
▪ The B(OH)3 accepts an OH- ion from the self ionization of
water forming a complex ion.
▪ by the
B(OH)3 + 2H2O → [B(OH)4]- + H3O+
The hydroxide boric acid B(OH)3 is formed
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
HALIDES OF GROUP 13 ELEMENTS
BCl3 + H2O ¾→H3BO3 + 3HCl
▪ 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
▪The Boron fluoride (BF3) form fluoroborates, WHILE
other Boron halides giving boric acids
BF3 + H2O → [BF3OH] H
▪ BX3 are Lewis acids and the order of
their Lewis acidity strength is:
BF3 < BCl3 < BBr3
to the order of
of the attached
▪ In contrary
F > Cl > Br
▪ BX3 are trigonal planar and monomeric (not
dimerized in the way the BH3 does.)
E.g. the structure of BBr3.
electrons in its outer shell and can readily accept a
➢BF3 is a useful organic catalyst for Friedel Craft
reaction such as:
✓ Polymerization of olefines
-Boron (an electron deficient atom) in BX3 has 6
▪ 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
▪ Exist as dimeric molecules with
the formula M2X6 using two
atoms to bridge the
dimeric formula is
when the halides
dissolves in non-polar solvent
▪But because of high heat of
hydration when halides
dissolves in water, the covalent
dimer is broken into [M.6H2O42]
▪ 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
C2H5 – O – C2H5
dimethyl ether ( Lewis base )
▪ AlX3 (Aluminium halides) are very reactive lewis acids
– they accepts a pair of electrons forming an acid base
compound called adducts
▪ AlX3 is used as a catalyst in a number of
E.G. When benzene is treated with acyl halide in
the presence of anhydrous Alcl3 as catalyst ¾®
▪ The B-X bond distances are shorter than might
be expected, and the B-X bond energies are
E.g. B-F bond energy (646 KJmol-1) is the highest
known for a single bond.
This suggest that some π-bonding may be
existing between the unhybridized 2p orbital of
the boron and the filled np orbitals of the
▪ Al(OH)3 is amphoteric and reacts with
acids in a manner as metal hydroxides do.
Al(OH)3 (s) + 3H3
O+ → [ Al( H2
▪ Al(OH)3 also reacts with a base in the
reaction represented as the formation of a
Al(OH)3 (s) + OH- [ Al( OH )4
▪ When Al(OH)3 dissolves in a base, hydroxyl ion
and water bonds to Al ion forming a complex ion
[ Al( H2
O )2 (OH)4]-.
▪ The reaction is as follows
(s) + OH-(aq) + 2H2
O(l) → [Al(H2
▪ Al(OH)3 is used in the purification of water
because it carries down any suspended materi4a8l
in the water including most of the bacteria.
▪ Special compounds that start of predominantly
covalent and become more ionic as we go down the
▪ Most of the group 13 elements react directly with
hydrogen, and large number of interesting hydrides
▪ Boron forms an extensive and interesting series of
hydrides, called BORANES.
▪ The simplest of these is not BH3as expected, but its
dimer B H .
▪ The 8 well characterized boranes which fall into two
series BnHn+4 and less stable series BnHn+6 are:
vi.B9H15 Nonaborane (enneaborane)
vii.B10H14 decaborane 50
▪ The borane molecule (BH3) may exists as a reaction
▪ But no BH3as 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 simplest boron hydride that have been isolated is
▪ The B-atom in BH3 lacks the complete octet (i.e. it has only 6
electrons in the valence shell).
THE STRUCTURE OF B2H6 (DIBORANE) – MULTICENTRE
▪The question of interest is what
holds the diborane together?
EXPLANATION OF THE STRUCTURE OF
▪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 fou5
then, thus accounting for a total of 8
▪ 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
bridging bonds are
B H B
consisting two electrons forming the so
called three centre – two electron bond
▪ Meaning that 3 atoms share 2 electrons
(This sometimes called banana bonds
because they non-linear but curved.
(Fig. below )
Figure 4. The Structure of B2H6 (diborane) – multicentre bonds.
Contains a 3-centre-2-electron bond (called a banana bond)
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
✓Boric acic acid is used as a mild antiseptic.
✓Borax as a water softener in washing powders.
✓Borosilicate glass contains boron compounds. 55