The occurrence of oxidation states two unit less than the group oxidation states are
sometime attributed to the ‘inert pair effect’.
non-metals and metalloids exist only in the p-block of the periodic table.
In general, non-metals have higher ionisation enthalpies and higher electro-negativities than
the metals.
The non-metal oxides are acidic or neutral whereas metal oxides are basic in nature.
The first member of a group differs from the heavier members in its ability to form pπ - pπ
multiple bonds to itself ( e.g., C=C, C≡C, N≡N). This type of π – bonding is not particularly
strong for the heavier p-block elements.
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GROUP 13 ELEMENTS: THE BORON FAMILY
Boron is a typical non-metal, aluminium is a metal but shows many chemical similarities to boron,
and gallium, indium, thallium and nihonium (Nh,113, 286 g mol-1, 5f14 6d10 7s2 7p2, Half Life 20
seconds, a synthetically prepared radioactive element) are almost exclusively metallic in character.
Boron is a fairly rare element, mainly occurs as
Orthoboric acid, (H3BO3),
Borax, Na2B4O7·10H2O, and
Kernite, Na2B4O7·4H2O.
In India borax occurs in Puga Valley (Ladakh) and Sambhar Lake (Rajasthan).
There are two isotopic forms of boron 10B (19%) and 11B (81%).
Aluminium is the most abundant metal and the third most abundant element in the earth’s
crust (8.3% by mass) after oxygen (45.5%) and Si (27.7%).
Bauxite, Al2O3. 2H2O and cryolite,Na3AlF6 are the important minerals of aluminium.
1. Atomic Radii:
Atomic radius of Ga is less than that of Al. This can be understood from the variation in the inner
core of the electronic configuration. The presence of additional 10 d-electrons offer only poor
screening effect (Unit 2) for the outer electrons from the increased nuclear charge in gallium.
Consequently, the atomic radius of gallium (135 pm) is less than that of aluminium (143 pm).
2. Ionization Enthalpy:
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The observed discontinuity in the ionisation enthalpy values between Al and Ga, and between In and
Tl are due to inability of d- and f-electrons ,which have low screening effect, to compensate the
increase in nuclear charge.
Physical Properties
Boron is non-metallic in nature. It is extremely hard and black coloured solid. Due to very strong
crystalline lattice, boron has unusually high melting point. Gallium with unusually low melting point
(303K), could exist in liquid state during summer. Its high boiling point (2676K) makes it a useful
material for measuring high temperatures.
Chemical Properties of Group 13 Elements:
Why Boron form only covalent compounds?
Due to small size of boron, the sum of its first three ionization enthalpies is very high. This
prevents it to form +3 ions and forces it to form only covalent compounds.
In Ga, In and Tl, both +1 and +3 oxidation states are observed. The relative stability of +1
oxidation state progressively increases for heavier elements: Al<Ga<In<Tl.
In thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly
oxidising in character.
The compounds in +1 oxidation state are more ionic than those in +3 oxidation state.
BCl3 easily accepts a lone pair of electrons from ammonia to form BCl3⋅NH3.
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Aluminium chloride in acidified aqueous solution forms octahedral ion.
In this complex ion, the 3d orbitals of Al ar involved and the hybridisation
state of Al is sp3d2.
For example, the trichlorides on hyrolysis in water form
tetrahedral species.
(i) Reactivity towards air
Boron is unreactive in crystalline form and Aluminium forms a very thin oxide layer on the
surface which protects the metal from further attack.
Amorphous boron and aluminium metal on heating in air form B2O3 and Al2O3 respectively.
Boron trioxide is acidic and reacts with basic (metallic) oxides forming metal borates.
Aluminium and gallium oxides are amphoteric and those of indium and thallium are basic in
their properties.
(ii) Reactivity towards acids and alkalies
Boron does not react with acids and alkalies even at moderate temperature. But Al shows
amphoteric behaviour .
(iii) Reactivity towards halogens
These elements react with halogens to form trihalides (except TlI3).
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IMPORTANT TRENDS AND ANOMALOUS PROPERTIES OF BORON
The tri-chlorides, bromides and iodides of all these elements being covalent in nature are
hydrolysed in water. Species like tetrahedral [M(OH)4]– and octahedral [M(H2O)6]3+, except in
boron, exist in aqueous medium.
The maximum covalence of B is 4.It is due to the absence of d orbitals.
Most of the other metal halides (e.g., alcl3) are dimerised through halogen bridging (e.g.,
Al2Cl6).
SOME IMPORTANT COMPOUNDS OF BORON:
1. Borax
It is a white crystalline solid of formula Na2B4O7⋅10H2O.
In fact it contains the tetranuclear units [B4O5(OH)4]2-and correct formula; therefore, is
Na2[B4O5 (OH)4].8H2O.
Borax dissolves in water to give an alkaline solution.
On heating, borax first loses water molecules and swells up. On further heating it turns into
a transparent liquid, which solidifies into glass like material known as borax bead.
2NaBO2+B2O3= is called Borax Bead.
o For example, when borax is heated in a Bunsen burner flame with CoO on a loop of
platinum wire, a blue coloured Co(BO2)2 bead is formed.
2. Orthoboric acid (Boric acid)
Orthoboric acid, H3BO3 is a white crystalline solid, with soapy touch.
It is sparingly soluble in water but highly soluble in hot water.
Preparation:
It is also formed by the hydrolysis (reaction with water or dilute acid) of most boron
compounds (halides, hydrides, etc.).
It has a layer structure in which planar BO3 units are joined by hydrogen bonds.
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Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by
accepting electrons from a hydroxyl ion:
On heating, orthoboric acid above 370K forms metaboric acid, HBO2 which on further
heating yields boric oxide, B2O3.
Diborane, B2H6
The simplest boron hydride known, is diborane.
Method of Preparation:
1. It is prepared by treating boron trifluoride with LiAlH4 in diethyl ether.
2. Diborane is produced on an industrial scale by the reaction of BF3 with sodium hydride.
3. A convenient laboratory method for the preparation of diborane involves the oxidation of
sodium borohydride with iodine.
Properties of Diborane
Colourless Gas. (B.P. is 180K)
Highly toxic.
Diborane catches fire spontaneously upon exposure to air and releases enormous amount of
energy.
Boranes are readily hydrolysed by water to give boric acid.
Diborane undergoes cleavage reactions with Lewis bases(L) to give borane adducts, BH3⋅L
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Reaction of ammonia with diborane
Structure of diborane:
The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and
below this plane, there are two bridging hydrogen atoms.
Boron also forms a series of hydridoborates (borohydrides,); the most important one is the
tetrahedral [BH4]–ion.
Both LiBH4 and NaBH4 are used as reducing agents in organic synthesis.
They are useful starting materials for preparing other metal borohydrides.
USES OF BORON AND ALUMINIUM AND THEIR COMPOUNDS
Boron being extremely hard refractory solid of
o High melting point,
o Low density and
o Very low electrical conductivity
Boron fibres are used in making
o Bullet-proof vest and
o Light composite material for aircraft.
The boron-10 (10B) isotope has high ability to absorb neutrons and, therefore, metal borides
are used in nuclear industry as protective shields and control rods.
The main industrial application of borax and boric acid is in the manufacture of heat
resistant glasses (e.g., Pyrex), glass-wool and fibreglass.
Borax is also used as a flux for soldering metals, for heat, scratch and stain resistant glazed
coating to earthenwares and as constituent of medicinal soaps.
Orthoboric acid is generally used as a mild antiseptic.
Aluminum:
On a weight-to-weight basis, the electrical conductivity of aluminium is twice that of
copper.
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The use of aluminium and its compounds for domestic purposes is now reduced
considerably because of their toxic nature. (Overdose of aluminum provides oxidative stress
in the brain, liver, and kidney.)
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