11. THE p-BLOCK ELEMENTS
These are elements in which the last electron enters in the outer most p-subshell. They include
elements of groups 13 to 18. Their general outer electronic configuration is ns2np1-6 (except for He). They
include metals, non-metals and metalloids.
Their maximum oxidation state = the total no. of valence electrons (i.e., the sum of the s- and p-
electrons). But their common oxidation state may differ from the maximum oxidation state or group
oxidation state. Generally, in the p-block elements, down the group, the common oxidation state is 2 less
than the maximum oxidation state. This is due to the Inert pair effect. It is the reluctance of s-electrons to
participate in chemical bonding. It is commonly seen in the elements of groups 13, 14 & 15. [Down the group,
due to the poor shielding effect of inner d and f orbitals, the effective nuclear charge is greater, which holds
the s-electrons tightly. So they cannot participate in bonding].
Due to the above reason, TlCl is more stable than TlCl3. Similarly lead mainly form PbCl2 than PbCl4.
Or, Tl is stabler than Tl3+ and Pb2+ is stabler than Pb4+).
+
Group 13 Elements (Boron Family)
Group 13 include Boron (B(, Aluminium (Al), Gallium (Ga), Indium (In), Thalium (Tl) and Nihonium
(Nh). Among these elements, Boron is a typical non-metal and the other elements are metals.
Atomic radii: Down the group, atomic radius increases. But atomic radius of gallium is less than that of
aluminium. This is due to the presence of completely filled d-orbitals in Ga (Ga - [Ar] 3d10 4s2 4p1). The
presence of 10 d-electrons offer only poor shielding effect for the outer electrons, from the increased nuclear
charge.
Ionisation Enthalpy: The ionisation enthalpy values do not decrease smoothly down the group. This is due to
the poor shielding effect of the completely filled inner d and f electrons.
Oxidation state: The common oxidation state of 13th group elements is +3. Due to high ionisation enthalpy,
boron does not form +3 ions and it only forms covalent compounds. The stability of +3 oxidation state
decreases and that of +1 oxidation state increases down the group. This is due to inert pair effect.
The trivalent compounds formed by 13th group elements are called electron deficient compounds. In
these compounds, the number of electrons around the central atom of the molecule is only 6. In order to
attain stable octet configuration, they accept a pair of electrons and so they behave as Lewis acids. e.g. BF3,
BCl3, AlCl3, B2H6 etc.
Chemical properties
Reactivity towards air: Boron is unreactive in crystalline form. Due to the presence of an oxide layer on the
surface, Al does not react with air at normal temperature. But at high temperatures, they form oxide and
nitride.
2E + 3O2 2 E2O3
2E + N2 2 EN [E = Any 13th group element]
The oxide of boron (B2O3) is acidic, the oxides of Aluminium and Gallium are amphoteric and that of Indium
and Thalium are basic.
The p-Block Elements Page 1
, Reactivity towards acids and alkalies: Boron does not react with acids and alkalies even at moderate
temperatures. But aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric
nature.
2Al(s) + 6 HCl(aq) 2 AlCl3(aq) + 3H2(g)
2Al(s) + 2NaOH(aq) + 6H2O(l) 2Na[Al(OH)4](aq) + 3H2(g)
But it does not react with nitric acid due to the presence of the oxide layer on the surface.
Reactivity towards halogens: They react with halogens and form trihalides.
2E(s) + 3X2(g) 2EX3(s)
2Al(s) + 3Cl2(g) 2AlCl3(s)
AlCl3 exists as dimer to attain stability.
Anhydrous aluminium chloride is partially hydrolysed with moisture to liberate fumes of HCl gas. So white fumes
appears around the bottle of anhydrous AlCl3.
Some important compounds of Boron
1. Borax [Na2B4O7.10H2O]: It is a white crystalline solid with formula Na2B4O7⋅10H2O (Sodium
tetraborate decahydrate). It contains the tetranuclear units [B4O5(OH)4]2- and hence its correct formula is
Na2[B4O5 (OH)4].8H2O.
It dissolves in water to give NaOH and orthoboric acid. Since NaOH is a strong alkali and orthoboric acid is
weak acid, the solution is basic in nature.
Na2B4O7 + 7H2O → 2NaOH + 4H3BO3
(Orthoboric acid)
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.
Na2B4O7.10H2O ⎯⎯Δ → Na2B4O7 ⎯⎯Δ → 2NaBO2 + B2O3
(Sodium (Boric anhydride)
metaborate)
The metaborates of many transition metals have characteristic colours and, therefore, borax bead
test can be used to identify them in the laboratory.
2. Ortho boric acid [H3BO3 or B(OH)3]: It is a white crystalline solid with soapy touch. It is prepared by
acidifying an aqueous solution of borax.
Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4H3BO3
It is also obtained by the hydrolysis of boron halides or hydrides.
It is a weak monobasic non-protic acid. It acts as a Lewis acid by accepting electrons from a hydroxyl
ion.
B(OH)3 + 2H2O → [B(OH)4]- + H3O+
On heating above 370K, it forms metaboric acid (HBO2) which on further heating gives boric oxide (B2O3).
H3BO3 ∆ HBO2 ∆ B2O3
The p-Block Elements Page 2
These are elements in which the last electron enters in the outer most p-subshell. They include
elements of groups 13 to 18. Their general outer electronic configuration is ns2np1-6 (except for He). They
include metals, non-metals and metalloids.
Their maximum oxidation state = the total no. of valence electrons (i.e., the sum of the s- and p-
electrons). But their common oxidation state may differ from the maximum oxidation state or group
oxidation state. Generally, in the p-block elements, down the group, the common oxidation state is 2 less
than the maximum oxidation state. This is due to the Inert pair effect. It is the reluctance of s-electrons to
participate in chemical bonding. It is commonly seen in the elements of groups 13, 14 & 15. [Down the group,
due to the poor shielding effect of inner d and f orbitals, the effective nuclear charge is greater, which holds
the s-electrons tightly. So they cannot participate in bonding].
Due to the above reason, TlCl is more stable than TlCl3. Similarly lead mainly form PbCl2 than PbCl4.
Or, Tl is stabler than Tl3+ and Pb2+ is stabler than Pb4+).
+
Group 13 Elements (Boron Family)
Group 13 include Boron (B(, Aluminium (Al), Gallium (Ga), Indium (In), Thalium (Tl) and Nihonium
(Nh). Among these elements, Boron is a typical non-metal and the other elements are metals.
Atomic radii: Down the group, atomic radius increases. But atomic radius of gallium is less than that of
aluminium. This is due to the presence of completely filled d-orbitals in Ga (Ga - [Ar] 3d10 4s2 4p1). The
presence of 10 d-electrons offer only poor shielding effect for the outer electrons, from the increased nuclear
charge.
Ionisation Enthalpy: The ionisation enthalpy values do not decrease smoothly down the group. This is due to
the poor shielding effect of the completely filled inner d and f electrons.
Oxidation state: The common oxidation state of 13th group elements is +3. Due to high ionisation enthalpy,
boron does not form +3 ions and it only forms covalent compounds. The stability of +3 oxidation state
decreases and that of +1 oxidation state increases down the group. This is due to inert pair effect.
The trivalent compounds formed by 13th group elements are called electron deficient compounds. In
these compounds, the number of electrons around the central atom of the molecule is only 6. In order to
attain stable octet configuration, they accept a pair of electrons and so they behave as Lewis acids. e.g. BF3,
BCl3, AlCl3, B2H6 etc.
Chemical properties
Reactivity towards air: Boron is unreactive in crystalline form. Due to the presence of an oxide layer on the
surface, Al does not react with air at normal temperature. But at high temperatures, they form oxide and
nitride.
2E + 3O2 2 E2O3
2E + N2 2 EN [E = Any 13th group element]
The oxide of boron (B2O3) is acidic, the oxides of Aluminium and Gallium are amphoteric and that of Indium
and Thalium are basic.
The p-Block Elements Page 1
, Reactivity towards acids and alkalies: Boron does not react with acids and alkalies even at moderate
temperatures. But aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric
nature.
2Al(s) + 6 HCl(aq) 2 AlCl3(aq) + 3H2(g)
2Al(s) + 2NaOH(aq) + 6H2O(l) 2Na[Al(OH)4](aq) + 3H2(g)
But it does not react with nitric acid due to the presence of the oxide layer on the surface.
Reactivity towards halogens: They react with halogens and form trihalides.
2E(s) + 3X2(g) 2EX3(s)
2Al(s) + 3Cl2(g) 2AlCl3(s)
AlCl3 exists as dimer to attain stability.
Anhydrous aluminium chloride is partially hydrolysed with moisture to liberate fumes of HCl gas. So white fumes
appears around the bottle of anhydrous AlCl3.
Some important compounds of Boron
1. Borax [Na2B4O7.10H2O]: It is a white crystalline solid with formula Na2B4O7⋅10H2O (Sodium
tetraborate decahydrate). It contains the tetranuclear units [B4O5(OH)4]2- and hence its correct formula is
Na2[B4O5 (OH)4].8H2O.
It dissolves in water to give NaOH and orthoboric acid. Since NaOH is a strong alkali and orthoboric acid is
weak acid, the solution is basic in nature.
Na2B4O7 + 7H2O → 2NaOH + 4H3BO3
(Orthoboric acid)
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.
Na2B4O7.10H2O ⎯⎯Δ → Na2B4O7 ⎯⎯Δ → 2NaBO2 + B2O3
(Sodium (Boric anhydride)
metaborate)
The metaborates of many transition metals have characteristic colours and, therefore, borax bead
test can be used to identify them in the laboratory.
2. Ortho boric acid [H3BO3 or B(OH)3]: It is a white crystalline solid with soapy touch. It is prepared by
acidifying an aqueous solution of borax.
Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4H3BO3
It is also obtained by the hydrolysis of boron halides or hydrides.
It is a weak monobasic non-protic acid. It acts as a Lewis acid by accepting electrons from a hydroxyl
ion.
B(OH)3 + 2H2O → [B(OH)4]- + H3O+
On heating above 370K, it forms metaboric acid (HBO2) which on further heating gives boric oxide (B2O3).
H3BO3 ∆ HBO2 ∆ B2O3
The p-Block Elements Page 2
