1. Study Notes on p-Block Elements: Boron Family
OCCURRENCE (Boron Family)
Boron is a rare element and occurs upto 0.0001% by the mass of earth crust. It occurs as boric acid and borates
such as borax, kernite and colemanite. Boron has two isotopes having atm mass 10 and 11 respectively.
Aluminium is most abundant metal and third most abundant element after oxygen and silicon . It is present 8.13%
in earth crust . The most important ores of aluminium are bauxite (mixture of aluminium oxide and hydroxide),
cryolite and corundum.
Gallium ,Indium and Thallium are less abundant and occurs as sulphide ores. The ore of gallium is germanite
(mixed sulphide of Zn+Cu+Ga+Arsenic). Indium and thallium are present in traces (sulphide ores of zinc and lead
Outer Electronic Configuration:-ns2
group members: boron (B), aluminum (Al), gallium (Ga), indium (In)& thallium (Tl) . All, except boron, are
Boron show diagonal relationship with Silicon; both are semiconductors metalloids & forms covalent compounds.
Atomic radii are less than s-block elements .The atomic size does not increase in regular manner, as expected, on
descending the group. From left to right in the period, the magnitude of nuclear charge increases but the electrons
are added to, the same shell. These electrons do not screen each other, therefore, the electrons experience greater
nuclear charge. In other words, effective nuclear charge increases and thus, size decreases. Therefore, the elements
of this group have smaller size than the corresponding elements of second group.On moving down the group both
atomic and ionic radii are expected to increase due to the addition of new shells. However, the observed atomic
radius of Al (143 pm) is slightly more than that of Ga (l35 pm).
The first ionization energies of group 13 elements are less than the corresponding members of the alkaline
earths. The sharp decrease in I.E. from B to Al is due to increase in size. In case of Ga, there are Ten d-electrons in
its inner electronic configuration. The very high value of 3rd I. E. of Thallium indicates that +3 O.N. state is not
stable, rather +1 is more stable for thallium .
Electropositive (or metallic) character the elements of group 13 are less electropositive as compared to elements
of group 2. On moving down the group the electropositive (metallic) character increases because ionization energy
decreases. For e.g., Boron is a non-metal white the other elements are typical metals.
The common oxidation states of group 13 elements are +3 and + l .The stability of the + 1 oxidation state
increases in the sequence Al <Ga< In <Tl, Due to Inert pair effect.
Electronegativity first decreases from B to Al and then increases to some extent. This is due to difference in the
atomic structure of elements.
Melting points do not vary regularly and decrease from B to Ga and then increase.
Due to similar atomic and ionic radii, the elements of gp 13 have higher density as compared to group 2 elements.
On moving down the group density increases due to increase in atomic Mass.
All metals of group 13 react with dioxygen at high temp to form trioxides of the formula M2O3.
4M(s) + 3O2(g) at High temp→2M2O3(s)
All metals of group 13 react with dinitrogen at high temp to form nitrides of the formula MN.
2M(s) + N2(g) at High temp→ 2MN(s)
The remaining elements i,e. Ga, In, Tl do not react with N2 to form the corresponding nitrides.
Note: The trioxide of group 13 reacts with water to form their corresponding hydroxides.
M2O3 + 3H2O ---» 2M(OH)3
Basic character increases down the group.
The elements of group 13 react with halogen at high temperature forming trihalides of general formula MX3.
2. 2M(s) + 3X2 on Heating -------» 2MX3 (X=F,Cl,Br,I)
ANOMALOUS PROPERTIES OF BORON
Boron is non mental.
It always forms covalent compounds
Boron show diagonal relationship with Si.
High Ionisation enthalpy.
Boron being small is harder than the other elements of its group.
Boron does not displace hydrogen from acids
Oxide of boron B2O3 is acidic oxide
Hydroxide of boron is acidic ( B (OH) 3 orH3 BO3 )
Maximum covalency of boron is 4
Boron form stable covalent hydrides which are known as bornanes.
Boron never appears as a cation
Among halides of boron only BF3 undergoes complete hydrolysis.
PREPARATION OF BORON :
In the laboratory, it is prepared by reducing B2O3with metallic magnesium.
95-98% pure boron is obtained on reducing B2O3 with Mg or Na metal at high temperature. It is known as
"Moissan Boron". This is amorphous Boron.
B2 O3 + Mg ----Δ(Bright red hot) ----→ 2B + 2MgO
B2O3 is obtained from borax
Boron exists in two allotropic form
a) Amorphous B b) Crystalline B.
Amorphous B dissolves in molten Al at 15300C which on cooling gives yellow colored crystalline solid. Al is
eliminated by dissolving in HCl. This results in very hard crystalline B.
B is a non conductor of electricity
B sublimes at ordinary pressure
B is a good neutron absorber. B and boron carbide are used in the shielding of atomic piles and in the control rods
used for controlling of chain reactions.
High purity B is used as a semi conductor in the place of Ge and Si
Boron is essential minor element for the healthy growth of plants.
It is used in steel industry for increasing hardness of steel.
In making light and composite material for air crafts.
Compounds of Boron:
Orthoboric acid (H3BO3)
Preparation of Orthoboric acid
From borax : Na2B4O7 + H2SO4 + 5H2O → Na2SO4 + 4H3BO3
From colemanite : Ca2B6O11 + 2SO2 + 11H2O → 2Ca(HSO3)2 + 6H3BO3
Properties of Orthoboric acid
Action of Heat:
3. Weak monobasic acidic behavior:
B(OH)3 ↔ H3BO3 ↔ H+
+ H2O +
Thus on titration with NaOH, it gives sodium metaborate salt
H3BO3 + NaOH ↔ NaBO2 + 2H2O
Reaction with Metaloxide:
Reaction with Ammonium boro fluoride:
Borax(sodium tetraborate) Na2B4O7. 10H2O
Preparation from Boric Acid
4H3BO3 + Na2CO3 --> Na2B4O7 + 6H2O + CO2
Properties of Borax
Aqueous solution of borax is alkaline in nature due to its hydrolysis
a) Na2B4O7 + 3H2O → NaBO2 + 3H3BO3 b) NaBO2 + 2H2O → NaOH + H3BO3
Action of heat:
Preparation of Diborane:
Reduction of Boron Trifluoride:
BF3 + 3LiAlH4 → 2B2H6 + 3 LiAl F4
2NaBH4 + H2SO4 → B2H6 + 2H2 + Na2SO4
2NaBH4 + H3PO4 → B2H6 + 2H2 + NaH2PO4
Properties of Diborane:
Reaction with water: B2H6 + H2O -->2H3BO3 + 6H2
Combustion: B2H6 +2O2 --? B2O3 + 3H2O ΔH = -2615 kJ/mol
Compounds of Aluminium:
Aluminium Oxide or Alumina (Al2O3)
a) 2Al(OH)3 +Heat → Al2O3 + 2H2O b) 2Al(SO4)3 +Heat → Al2O3 + 2SO3
c) (NH4)2Al2(SO4)3·24H2O --> 2NH3 +Al2O3 + 4SO3 + 25 H2O
Aluminum Chloride AlCl3:
Properties of Aluminium Chloride
White, hygroscopic solid and sublimes at 183 0
Forms addition compounds with NH3, PH3, COCl2 etc.
Hydrolysis: AlCl3 + 3H2O --> Al(OH)3 + 3HCl + 3H2O
Action of Heat: 2AlCl3 .6H2O --> 2Al(OH)3 à Al2O3+ 6HCl + 3H2O
Chemical Properties Of Boron Family
4. Oxidation state
The elements of group 13 have to two electrons in the s- orbital and one electron in the p-orbital of the valence shell.
These elements are expected to show a uniform oxidation state of +3. Boron and aluminium which show an oxidation
state of +3 only but gallium, indium and thallium due to inert pair effect show oxidation state of both +1 and +3 .
As we move down the group, the stability of +3 oxidation state decreases while that of +1 oxidation state
increases. The order of stability of +1 oxidation states increases in the order : Al < Ga < In < Tl.
In gallium and indium +3 oxidation state is more stable than +1 oxidation state , therefore both Ga+
undergo disproportionation reaction in aqueous solution.
3 GaX (s) ——->2 Ga (s) + Ga3+
+ 3 X‾ (aq)
3 InX (s) ——->2 In (s) +In3+
+ 3 X‾ (aq)
Thallous compounds such as thallous hydroxide ( TlOH) and thallous perchlorate(TlClO4) are more stable than their
corresponding thallic compounds.
Due to lesser stability of , Tl3+
salts act as strong oxidising agent.
The stability of +1 oxidation State increases down the group.
Reason: As we move down the group, the tendency of s-electrons of the valence shell to participate in bond formation
decreases. This reluctance of the s-electrons to participate in bond formation is called inert pair effect. The
electron pair in gallium ,indium and thallium tends to remain paired. This is due to poor or ineffective shielding of
electrons of the valence shell by intervening d and f electrons.
As the size of the atom increases from aluminium to thallium, the energy required to unpair the ns2
electrons is not
compensated by the energy released in forming the two additional bonds.
The inert pair effect becomes more predominant as we go down the group because of increased nuclear charge
which outweighs the effect of the corresponding increase in atomic size. The s-electrons thus become more tightly
held ,and ,therefore ,becomes more reluctant to participate in bond formation. Thus down the group , +1 oxidation
State become more and more stable as compared to +3 oxidation state.
Trends in chemical reactivity
In +3 oxidation state , elements of this group are expected to form covalent bonds because of the following three
1) According to Fajan’s rule, the small size of the ions and their high charge of +3 , favour the formation of covalent
2) The sum of first three ionization enthalpies is very large. This also suggest that bonds will be largely covalent .
3) The electronegativity values of group 13 elements are higher than those of group 1 and 2 .When these elements
react with other elements, the electronegativity difference will be small. This also favours the formation of covalent
1) Boron because of its small size and high sum of first three ionization enthalpies, does not lose its 3 Valence
electrons to form B3+
ions. It does not form ionic compounds. Instead , boron always form covalent compounds by
sharing is valence .
2) As we move from Boron to aluminium, the sum of first three ionization enthalpies decreases and thus aluminium
has only little tendency to form ionic compounds. It has a strong tendency to form covalent compounds.
For Ex: AlF3 and Al2(SO4)3 are ionic while AlCl3 , AlBr3 and AlI3 are covalent.
GaCl3 , InCl3 are covalent when anhydrous. Al , Ga , In and Tl all for metal ions in solution. The change from
covalent to ionic nature occurs due to the reason that in aqueous solution , these ions are hydrated and the amount of
hydration enthalpy released exceeds the ionization enthalpy.
Gallium, indium and thallium show two oxidation states of +1 and +3 due to inert pair effect . Since the ions in the +1
oxidation state are much larger than the ions in the +3 oxidation state, therefore ,these compounds in +1 oxidation
states are more ionic than in +3 oxidation state.
In the trivalent state , most of compounds being covalent are hydrolysed by water to form either tetrahedral species,
[M(OH)4]‾ in which the element is sp3
hybridized or octahedral species, [M(H2O)6 ]3+
in which the element is
BCl3 on hydrolysis forms tetrahedral [B(OH)4]‾ species because boron due to the absence of d – orbital cannot
expand its covalency beyond 4.
5. Al2Cl6 on hydrolysis give octahedral [Al (H2O) ]3+
species because Al due to presence of 3d orbital can expand its
covalency from 4 to 6.
In trivalent state , the number of electrons around the central atom in a molecule of these elements will be only 6 and
thus behave as electron deficient molecules.They have a strong tendency to accept a pair of electrons to achieve the
stable inert gas configuration and thus behave as Lewis acid.
BCl3 easily accepts a lone pair of electrons from ammonia to form the adduct BCl3.NH3
The Lewis character decreases down the group as the size of the element increases i.e. BX3 > AlX3 > GaX3 >
Boron and aluminium halides are Lewis acid but only aluminium halides exist as dimer whereas boron halides exist
only as monomers.
Reason : Boron atom is so small that it cannot accommodate 4 large size halogen atoms around it.
Reactivity towards dioxygen or air
All the metals of group 13 react with dioxygen at high temperature to form trioxide, M2O3.
4 M + 3 O2 ——> 2 M2O3
The reactivity of these elements towards dioxygen, however, increases down the group.
Boron is unreactive in the crystalline form. Aluminium does not react with dry air. In moist air, its surface get
tarnished due to the formation of 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 boron trioxide.
4 B + 3 O2 ———> 2 B2O3
4 Al + 3 O2 ——> 2 Al2O3
With dinitrogen at high temperature , this from nitrides.
2B + N2 ———> 2 BN
2 Al + N2 ——-> 2 AlN
The remaining elements i.e. Ga, In , Tl do not react with the nitrogen to form the corresponding nitrites.
Boron nitride is a white slippery solid. One boron and one nitrogen atom together have the same number of valence
electrons as 2 carbon atoms. Boron nitride has almost the same structure of graphite consisting of sheets made up of
hexagonal rings of alternate Boron and nitrogen atoms joined together.
These sheets are stacked one on top of the other, giving a layered structure similar to that of graphite. It is because of
the similarity of structure of boron nitride and graphite that boron nitride is also called inorganic graphite.
Acid-base character of oxides and hydroxides
The trioxides of group 13 react with water to form their corresponding hydroxides.
M2O3 + 3 H2O —–> 2 M(OH)3
The nature of these oxides and hydroxides varies down the group. B2O3 and B(OH)3 are weakly acidic. They dissolve
in alkalis forming metal borates.
B2O3 + 2 NaOH —-> 2 NaBO2 + H2O
On moving down the group ,the acidic character decreases and the basic character increases.
6. Because down the group, ionisation enthalpy decreases. Consequently , M-O bond weakens and is easily broken
resulting in increased basic strength down the group. The oxides and hydroxides of Al and Ga are amphoteric while
those of indium and thallium or basic.
Thallium forms two hydroxide : thallic hydroxide Tl(OH)3 and thallous hydroxide TlOH.
Tl(OH)3 is insoluble in water but TlOH is soluble and is a strong base like alkali metal hydroxide.
Being amphoteric ,alumina and aluminium hydroxide dissolve both in acids as well as in alkalies forming salts.
Al2O3 + 2 NaOH———> 2 NaAlO2 + H2O
Al(OH)3 + NaOH ——-> Na+
2 Al(OH)3 + 3 H2SO4 —–> Al2(SO4)3 + 6 H2O
Reactivity towards acids and bases
Action of acids
Boron does not react with non- oxidising acid such as hydrochloric acid. It is attacked at high temperature by strong
oxidising acid such as mixture of hot conc H2SO4 forming boric.
B + 3 HNO3 ——-> H3BO3 + 3 NO2
All other elements react with both non – oxidising and oxidising acids. Aluminium reacts with dilute acids liberating
2 Al + 6 HCl ——–> 2 Al3+
+ 6 Cl‾ + 3 H2
With conc HNO3 ,aluminium becomes passive. This passivity is due to the formation of thin protective layer of its
oxides on the surface of the metal which prevents it from further actions.
2 Al + 6 HNO3 ———> Al2O3 + 6 NO2 + 3 H2O
Action of alkalis
Boron resists the action of alkalies upto 773 K but above this temperature, it reacts forming borates and liberating
2 B + 6 KOH ——–> 2 K3BO3 + 3 H2
It also dissolves in fused Na2CO3 / NaNO3 mixture at 1123 K.
2 B + 3 Na2CO3 + 3 NaNO3 —–> 2 Na3BO3 + 3 NaNO2 + 3 CO2
Al and Ga being amphoteric also react with aqueous alkalies with the evolution of dihydrogen gas.
2 Al + 2 NaOH + 6 H2O ——-> 2 Na+
[ Al (OH) 4]‾ + 3 H2
2 Ga + 2 NaOH + 6 H2O ——-> 2 Na+
[ Ga(OH) 4]‾ + 3 H2
In and Tl , however, do not react with alkalies.
Reactivity towards halogens
The elements of group 13 react with halogens at high temperature forming trihalides, MX3.
2 M + 3 X2 —–> 2 MX3 (X= F, Cl, Br , I)
Trihalides of boron
Due to small size and high ionization enthalpy ,boron forms covalent trihalides. BF3 is a gas, BCl3 and BBr3 are
liquids while BI3 is a solid.
All these trihalide are planar molecules in which Boron is sp2
hybridised. The three half filled p-orbitals ,one on each
halogen ,overlap along their internuclear axis with 3 sp2
– orbitals of Boron to form 3 sp2
-p , C-X σ bond.The
unhybridised p-orbital is, however,empty.
Due to the presence of only 6 electrons in their respective valence shells ,all these trihalides are Lewis acids. Their
Lewis acid character, however, decreases in the order : BF3 > BCl3 > BBr3 > BI3
Order of acidic strength can be easily explained on the basis of the tendency of the halogen atom to back donate
its lone pair of electron to the boron anatomy through pπ-pπ bonding.
7. Since the size of the vacant 2p-orbital of boron and the 2p orbital of F containing a lone pair of electrons are almost
identical ,therefore ,the Lone pair of electrons on F is donated towards the Boron atom. Due to back donation by three
F atoms, BF3 can be represented as a resonance hybrid of the following three structures:
The back bonding in BF3 molecule is supported by the fact that the observed B-F bond length in BF3 molecule is
much less than the sum of their covalent radii.
As a result of pπ-pπ back donation and resonance, the electron deficiency of Boron decreases and thus BF3 is the
weakest Lewis acid. As the size of the halogen atom halogen atom increases from Cl to Br to I ,the extent of overlap
between 2p orbital of Boron and a bigger p-orbital of halogen decreases and consequently the electron deficiency of
Boron increases and thus the Lewis acid character increases accordingly from BF3 to BCl3 to BBr3 to BI3