NCERT Textbook - The p block elements | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET PDF Download

 Page 1


In Class XI, you have learnt that the p-block elements
are placed in groups 13 to 18 of the periodic table.
Their valence shell electronic configuration is ns
2
np
1–6
(except He which has 1s
2
 configuration). The properties
of p-block elements like that of others are greatly
influenced by atomic sizes, ionisation enthalpy, electron
gain enthalpy and electronegativity. The absence of d-
orbitals in second period and presence of d or d and f
orbitals in heavier elements (starting from third period
onwards) have significant effects on the properties of
elements. In addition, the presence of all the three types
of elements; metals, metalloids and non-metals bring
diversification in chemistry of these elements.
Having learnt the chemistry of elements of Groups
13 and 14 of the p-block of periodic table in Class XI,
you will learn the chemistry of the elements of
subsequent groups in this Unit.
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements s s s s
7
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements ss ss
After studying this Unit, you will be
able to
• appreciate general trends in the
chemistry of elements of groups
15,16,17 and 18;
• learn the preparation, properties
and uses of dinitrogen and
phosphorus and some of their
important compounds;
• describe the preparation,
properties and uses of dioxygen
and ozone and chemistry of some
simple oxides;
• know allotropic forms of sulphur,
chemistry of its important
compounds and the structures of
its oxoacids;
• describe the preparation,
properties and uses of chlorine
and hydrochloric acid;
• know the chemistry of
interhalogens and structures of
oxoacids of halogens;
• enumerate the uses of noble
gases;
• appreciate the importance of
these elements and their
compounds in our day to day life.
Objectives
Diversity in chemistry is the hallmark of p–block elements manifested
in their ability to react with the elements of s–, d– and f–blocks as
well as with their own.
Group 15 includes nitrogen, phosphorus, arsenic, antimony and
bismuth. As we go down the group, there is a shift from non-metallic
to metallic through metalloidic character. Nitrogen and phosphorus
are non-metals, arsenic and antimony metalloids and bismuth is a
typical metal.
Molecular nitrogen comprises 78% by volume of the atmosphere.
In the earth’s crust, it occurs as sodium nitrate, NaNO
3
 (called Chile
saltpetre) and potassium nitrate (Indian saltpetre). It is found in the
form of proteins in plants and animals. Phosphorus occurs in minerals
7.1 7.1 7.1 7.1 7.1 Group 15 Group 15 Group 15 Group 15 Group 15
Elements Elements Elements Elements Elements
7.1.1 Occurrence
Unit Unit Unit Unit Unit
7
© NCERT 
not to be republished
Page 2


In Class XI, you have learnt that the p-block elements
are placed in groups 13 to 18 of the periodic table.
Their valence shell electronic configuration is ns
2
np
1–6
(except He which has 1s
2
 configuration). The properties
of p-block elements like that of others are greatly
influenced by atomic sizes, ionisation enthalpy, electron
gain enthalpy and electronegativity. The absence of d-
orbitals in second period and presence of d or d and f
orbitals in heavier elements (starting from third period
onwards) have significant effects on the properties of
elements. In addition, the presence of all the three types
of elements; metals, metalloids and non-metals bring
diversification in chemistry of these elements.
Having learnt the chemistry of elements of Groups
13 and 14 of the p-block of periodic table in Class XI,
you will learn the chemistry of the elements of
subsequent groups in this Unit.
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements s s s s
7
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements ss ss
After studying this Unit, you will be
able to
• appreciate general trends in the
chemistry of elements of groups
15,16,17 and 18;
• learn the preparation, properties
and uses of dinitrogen and
phosphorus and some of their
important compounds;
• describe the preparation,
properties and uses of dioxygen
and ozone and chemistry of some
simple oxides;
• know allotropic forms of sulphur,
chemistry of its important
compounds and the structures of
its oxoacids;
• describe the preparation,
properties and uses of chlorine
and hydrochloric acid;
• know the chemistry of
interhalogens and structures of
oxoacids of halogens;
• enumerate the uses of noble
gases;
• appreciate the importance of
these elements and their
compounds in our day to day life.
Objectives
Diversity in chemistry is the hallmark of p–block elements manifested
in their ability to react with the elements of s–, d– and f–blocks as
well as with their own.
Group 15 includes nitrogen, phosphorus, arsenic, antimony and
bismuth. As we go down the group, there is a shift from non-metallic
to metallic through metalloidic character. Nitrogen and phosphorus
are non-metals, arsenic and antimony metalloids and bismuth is a
typical metal.
Molecular nitrogen comprises 78% by volume of the atmosphere.
In the earth’s crust, it occurs as sodium nitrate, NaNO
3
 (called Chile
saltpetre) and potassium nitrate (Indian saltpetre). It is found in the
form of proteins in plants and animals. Phosphorus occurs in minerals
7.1 7.1 7.1 7.1 7.1 Group 15 Group 15 Group 15 Group 15 Group 15
Elements Elements Elements Elements Elements
7.1.1 Occurrence
Unit Unit Unit Unit Unit
7
© NCERT 
not to be republished
166 Chemistry
of the apatite family, Ca
9
(PO
4
)
6
. CaX
2
 (X = F, Cl or OH) (e.g., fluorapatite
Ca
9 
(PO
4
)
6
. CaF
2
) which are the main components of phosphate rocks.
Phosphorus is an essential constituent of animal and plant matter. It
is present in bones as well as in living cells. Phosphoproteins are present
in milk and eggs. Arsenic, antimony and bismuth are found mainly as
sulphide minerals.
The important atomic and physical properties of this group elements
along with their electronic configurations are given in Table 7.1.
Property N P As Sb Bi
Atomic number 7 15 33 51 83
Atomic mass/g mol
–1
14.01 30.97 74.92 121.75 208.98
Electronic configuration [He]2s
2
2p
3
[Ne]3s
2
3p
3
[Ar]3d
10
4s
2
4p
3
[Kr]4d
10
5s
2
5p
3
[Xe]4f
14
5d
10
6s
2
6p
3
Ionisation enthalpy I 1402 1012 947 834 703
(?
i
H/(kJ mol
–1
) II 2856 1903 1798 1595 1610
III 4577 2910 2736 2443 2466
Electronegativity 3.0 2.1 2.0 1.9 1.9
Covalent radius/pm
a
70 110 121 141 148
Ionic radius/pm 171
b
212
b
222
b
76
c
103
c
Melting point/K 63* 317
d
1089
e
904 544
Boiling point/K 77.2* 554
d
888
f
1860 1837
Density/[g cm
–3
(298 K)] 0.879
g
1.823 5.778
h
6.697 9.808
Table 7.1: Atomic and Physical Properties of Group 15 Elements
a
 E
III
 single bond (E = element); 
b
 E
3–
; 
c
 E
3+
; 
d
 White phosphorus; 
e
 Grey a-form at 38.6 atm; 
f 
Sublimation temperature;
g 
At 63 K; 
h
Grey a-form; * Molecular N
2
.
Trends of some of the atomic, physical and chemical properties of the
group are discussed below.
The valence shell electronic configuration of these elements is ns
2
np
3
.
The s orbital in these elements is completely filled and p orbitals are
half-filled, making their electronic configuration extra stable.
Covalent and ionic (in a particular state) radii increase in size
down the group. There is a considerable increase in covalent radius
from N to P. However, from As to Bi only a small increase in
covalent radius is observed. This is due to the presence of
completely filled d and/or f orbitals in heavier members.
Ionisation enthalpy decreases down the group due to gradual increase
in atomic size. Because of the extra stable half-filled p orbitals electronic
configuration and smaller size, the ionisation enthalpy of the group 15
elements is much greater than that of group 14 elements in the
corresponding periods. The order of successive ionisation enthalpies,
as expected is ?
i
H
1 
< ?
i
H
2 
< ?
i
H
3 
(Table 7.1).
7.1.2 Electronic
Configuration
7.1.3 Atomic and
Ionic Radii
7.1.4 Ionisation
Enthalpy
© NCERT 
not to be republished
Page 3


In Class XI, you have learnt that the p-block elements
are placed in groups 13 to 18 of the periodic table.
Their valence shell electronic configuration is ns
2
np
1–6
(except He which has 1s
2
 configuration). The properties
of p-block elements like that of others are greatly
influenced by atomic sizes, ionisation enthalpy, electron
gain enthalpy and electronegativity. The absence of d-
orbitals in second period and presence of d or d and f
orbitals in heavier elements (starting from third period
onwards) have significant effects on the properties of
elements. In addition, the presence of all the three types
of elements; metals, metalloids and non-metals bring
diversification in chemistry of these elements.
Having learnt the chemistry of elements of Groups
13 and 14 of the p-block of periodic table in Class XI,
you will learn the chemistry of the elements of
subsequent groups in this Unit.
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements s s s s
7
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements ss ss
After studying this Unit, you will be
able to
• appreciate general trends in the
chemistry of elements of groups
15,16,17 and 18;
• learn the preparation, properties
and uses of dinitrogen and
phosphorus and some of their
important compounds;
• describe the preparation,
properties and uses of dioxygen
and ozone and chemistry of some
simple oxides;
• know allotropic forms of sulphur,
chemistry of its important
compounds and the structures of
its oxoacids;
• describe the preparation,
properties and uses of chlorine
and hydrochloric acid;
• know the chemistry of
interhalogens and structures of
oxoacids of halogens;
• enumerate the uses of noble
gases;
• appreciate the importance of
these elements and their
compounds in our day to day life.
Objectives
Diversity in chemistry is the hallmark of p–block elements manifested
in their ability to react with the elements of s–, d– and f–blocks as
well as with their own.
Group 15 includes nitrogen, phosphorus, arsenic, antimony and
bismuth. As we go down the group, there is a shift from non-metallic
to metallic through metalloidic character. Nitrogen and phosphorus
are non-metals, arsenic and antimony metalloids and bismuth is a
typical metal.
Molecular nitrogen comprises 78% by volume of the atmosphere.
In the earth’s crust, it occurs as sodium nitrate, NaNO
3
 (called Chile
saltpetre) and potassium nitrate (Indian saltpetre). It is found in the
form of proteins in plants and animals. Phosphorus occurs in minerals
7.1 7.1 7.1 7.1 7.1 Group 15 Group 15 Group 15 Group 15 Group 15
Elements Elements Elements Elements Elements
7.1.1 Occurrence
Unit Unit Unit Unit Unit
7
© NCERT 
not to be republished
166 Chemistry
of the apatite family, Ca
9
(PO
4
)
6
. CaX
2
 (X = F, Cl or OH) (e.g., fluorapatite
Ca
9 
(PO
4
)
6
. CaF
2
) which are the main components of phosphate rocks.
Phosphorus is an essential constituent of animal and plant matter. It
is present in bones as well as in living cells. Phosphoproteins are present
in milk and eggs. Arsenic, antimony and bismuth are found mainly as
sulphide minerals.
The important atomic and physical properties of this group elements
along with their electronic configurations are given in Table 7.1.
Property N P As Sb Bi
Atomic number 7 15 33 51 83
Atomic mass/g mol
–1
14.01 30.97 74.92 121.75 208.98
Electronic configuration [He]2s
2
2p
3
[Ne]3s
2
3p
3
[Ar]3d
10
4s
2
4p
3
[Kr]4d
10
5s
2
5p
3
[Xe]4f
14
5d
10
6s
2
6p
3
Ionisation enthalpy I 1402 1012 947 834 703
(?
i
H/(kJ mol
–1
) II 2856 1903 1798 1595 1610
III 4577 2910 2736 2443 2466
Electronegativity 3.0 2.1 2.0 1.9 1.9
Covalent radius/pm
a
70 110 121 141 148
Ionic radius/pm 171
b
212
b
222
b
76
c
103
c
Melting point/K 63* 317
d
1089
e
904 544
Boiling point/K 77.2* 554
d
888
f
1860 1837
Density/[g cm
–3
(298 K)] 0.879
g
1.823 5.778
h
6.697 9.808
Table 7.1: Atomic and Physical Properties of Group 15 Elements
a
 E
III
 single bond (E = element); 
b
 E
3–
; 
c
 E
3+
; 
d
 White phosphorus; 
e
 Grey a-form at 38.6 atm; 
f 
Sublimation temperature;
g 
At 63 K; 
h
Grey a-form; * Molecular N
2
.
Trends of some of the atomic, physical and chemical properties of the
group are discussed below.
The valence shell electronic configuration of these elements is ns
2
np
3
.
The s orbital in these elements is completely filled and p orbitals are
half-filled, making their electronic configuration extra stable.
Covalent and ionic (in a particular state) radii increase in size
down the group. There is a considerable increase in covalent radius
from N to P. However, from As to Bi only a small increase in
covalent radius is observed. This is due to the presence of
completely filled d and/or f orbitals in heavier members.
Ionisation enthalpy decreases down the group due to gradual increase
in atomic size. Because of the extra stable half-filled p orbitals electronic
configuration and smaller size, the ionisation enthalpy of the group 15
elements is much greater than that of group 14 elements in the
corresponding periods. The order of successive ionisation enthalpies,
as expected is ?
i
H
1 
< ?
i
H
2 
< ?
i
H
3 
(Table 7.1).
7.1.2 Electronic
Configuration
7.1.3 Atomic and
Ionic Radii
7.1.4 Ionisation
Enthalpy
© NCERT 
not to be republished
167 The p-Block Elements
The electronegativity value, in general, decreases down the group with
increasing atomic size. However, amongst the heavier elements, the
difference is not that much pronounced.
All the elements of this group are polyatomic. Dinitrogen is a diatomic gas
while all others are solids. Metallic character increases down the group.
Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids
and bismuth is a metal. This is due to decrease in ionisation enthalpy and
increase in atomic size. The boiling points, in general, increase from top to
bottom in the group but the melting point increases upto arsenic and then
decreases upto bismuth. Except nitrogen, all the elements show allotropy.
Oxidation states and trends in chemical reactivity
The common oxidation states of these elements are –3, +3 and +5.
The tendency to exhibit –3 oxidation state decreases down the group due
to increase in size and metallic character. In fact last member of the group,
bismuth hardly forms any compound in –3 oxidation state. The stability
of +5 oxidation state decreases down the group. The only well characterised
Bi (V) compound is BiF
5
. The stability of +5 oxidation state decreases and
that of +3 state increases (due to inert pair effect) down the group. Nitrogen
exhibits + 1, + 2, + 4 oxidation states also when it reacts with oxygen.
Phosphorus also shows +1 and +4 oxidation states in some oxoacids.
In the case of nitrogen, all oxidation states from +1 to +4 tend to
disproportionate in acid solution. For example,
3HNO
2 
? HNO
3
 + H
2
O + 2NO
Similarly, in case of phosphorus nearly all intermediate oxidation
states disproportionate into +5 and –3 both in alkali and acid. However
+3 oxidation state in case of arsenic, antimony and bismuth becomes
increasingly stable with respect to disproportionation.
Nitrogen is restricted to a maximum covalency of 4 since only four
(one s and three p) orbitals are available for bonding. The heavier elements
have vacant d orbitals in the outermost shell which can be used for
bonding (covalency) and hence, expand their covalence as in PF
–
6
.
Anomalous properties of nitrogen
Nitrogen differs from the rest of the members of this group due to
its small size, high electronegativity, high ionisation enthalpy and
non-availability of d orbitals. Nitrogen has unique ability to form
pp p p p p-pp p p p p multiple bonds with itself and with other elements having
small size and high electronegativity (e.g., C, O). Heavier elements of
this group do not form pp-pp bonds as their atomic orbitals are so
large and diffuse that they cannot have effective overlapping.
Thus, nitrogen exists as a diatomic molecule with a triple bond (one
s and two p) between the two atoms. Consequently, its bond enthalpy
(941.4 kJ mol
–1
) is very high. On the contrary, phosphorus, arsenic
and antimony form single bonds as P–P, As–As and Sb–Sb while
bismuth forms metallic bonds in elemental state. However, the single
N–N bond is weaker than the single P–P bond because of high
interelectronic repulsion of the non-bonding electrons, owing to the
small bond length. As a result the catenation tendency is weaker in
7.1.5
Electronegativity
7.1.6 Physical
Properties
7.1.7 Chemical
Properties
© NCERT 
not to be republished
Page 4


In Class XI, you have learnt that the p-block elements
are placed in groups 13 to 18 of the periodic table.
Their valence shell electronic configuration is ns
2
np
1–6
(except He which has 1s
2
 configuration). The properties
of p-block elements like that of others are greatly
influenced by atomic sizes, ionisation enthalpy, electron
gain enthalpy and electronegativity. The absence of d-
orbitals in second period and presence of d or d and f
orbitals in heavier elements (starting from third period
onwards) have significant effects on the properties of
elements. In addition, the presence of all the three types
of elements; metals, metalloids and non-metals bring
diversification in chemistry of these elements.
Having learnt the chemistry of elements of Groups
13 and 14 of the p-block of periodic table in Class XI,
you will learn the chemistry of the elements of
subsequent groups in this Unit.
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements s s s s
7
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements ss ss
After studying this Unit, you will be
able to
• appreciate general trends in the
chemistry of elements of groups
15,16,17 and 18;
• learn the preparation, properties
and uses of dinitrogen and
phosphorus and some of their
important compounds;
• describe the preparation,
properties and uses of dioxygen
and ozone and chemistry of some
simple oxides;
• know allotropic forms of sulphur,
chemistry of its important
compounds and the structures of
its oxoacids;
• describe the preparation,
properties and uses of chlorine
and hydrochloric acid;
• know the chemistry of
interhalogens and structures of
oxoacids of halogens;
• enumerate the uses of noble
gases;
• appreciate the importance of
these elements and their
compounds in our day to day life.
Objectives
Diversity in chemistry is the hallmark of p–block elements manifested
in their ability to react with the elements of s–, d– and f–blocks as
well as with their own.
Group 15 includes nitrogen, phosphorus, arsenic, antimony and
bismuth. As we go down the group, there is a shift from non-metallic
to metallic through metalloidic character. Nitrogen and phosphorus
are non-metals, arsenic and antimony metalloids and bismuth is a
typical metal.
Molecular nitrogen comprises 78% by volume of the atmosphere.
In the earth’s crust, it occurs as sodium nitrate, NaNO
3
 (called Chile
saltpetre) and potassium nitrate (Indian saltpetre). It is found in the
form of proteins in plants and animals. Phosphorus occurs in minerals
7.1 7.1 7.1 7.1 7.1 Group 15 Group 15 Group 15 Group 15 Group 15
Elements Elements Elements Elements Elements
7.1.1 Occurrence
Unit Unit Unit Unit Unit
7
© NCERT 
not to be republished
166 Chemistry
of the apatite family, Ca
9
(PO
4
)
6
. CaX
2
 (X = F, Cl or OH) (e.g., fluorapatite
Ca
9 
(PO
4
)
6
. CaF
2
) which are the main components of phosphate rocks.
Phosphorus is an essential constituent of animal and plant matter. It
is present in bones as well as in living cells. Phosphoproteins are present
in milk and eggs. Arsenic, antimony and bismuth are found mainly as
sulphide minerals.
The important atomic and physical properties of this group elements
along with their electronic configurations are given in Table 7.1.
Property N P As Sb Bi
Atomic number 7 15 33 51 83
Atomic mass/g mol
–1
14.01 30.97 74.92 121.75 208.98
Electronic configuration [He]2s
2
2p
3
[Ne]3s
2
3p
3
[Ar]3d
10
4s
2
4p
3
[Kr]4d
10
5s
2
5p
3
[Xe]4f
14
5d
10
6s
2
6p
3
Ionisation enthalpy I 1402 1012 947 834 703
(?
i
H/(kJ mol
–1
) II 2856 1903 1798 1595 1610
III 4577 2910 2736 2443 2466
Electronegativity 3.0 2.1 2.0 1.9 1.9
Covalent radius/pm
a
70 110 121 141 148
Ionic radius/pm 171
b
212
b
222
b
76
c
103
c
Melting point/K 63* 317
d
1089
e
904 544
Boiling point/K 77.2* 554
d
888
f
1860 1837
Density/[g cm
–3
(298 K)] 0.879
g
1.823 5.778
h
6.697 9.808
Table 7.1: Atomic and Physical Properties of Group 15 Elements
a
 E
III
 single bond (E = element); 
b
 E
3–
; 
c
 E
3+
; 
d
 White phosphorus; 
e
 Grey a-form at 38.6 atm; 
f 
Sublimation temperature;
g 
At 63 K; 
h
Grey a-form; * Molecular N
2
.
Trends of some of the atomic, physical and chemical properties of the
group are discussed below.
The valence shell electronic configuration of these elements is ns
2
np
3
.
The s orbital in these elements is completely filled and p orbitals are
half-filled, making their electronic configuration extra stable.
Covalent and ionic (in a particular state) radii increase in size
down the group. There is a considerable increase in covalent radius
from N to P. However, from As to Bi only a small increase in
covalent radius is observed. This is due to the presence of
completely filled d and/or f orbitals in heavier members.
Ionisation enthalpy decreases down the group due to gradual increase
in atomic size. Because of the extra stable half-filled p orbitals electronic
configuration and smaller size, the ionisation enthalpy of the group 15
elements is much greater than that of group 14 elements in the
corresponding periods. The order of successive ionisation enthalpies,
as expected is ?
i
H
1 
< ?
i
H
2 
< ?
i
H
3 
(Table 7.1).
7.1.2 Electronic
Configuration
7.1.3 Atomic and
Ionic Radii
7.1.4 Ionisation
Enthalpy
© NCERT 
not to be republished
167 The p-Block Elements
The electronegativity value, in general, decreases down the group with
increasing atomic size. However, amongst the heavier elements, the
difference is not that much pronounced.
All the elements of this group are polyatomic. Dinitrogen is a diatomic gas
while all others are solids. Metallic character increases down the group.
Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids
and bismuth is a metal. This is due to decrease in ionisation enthalpy and
increase in atomic size. The boiling points, in general, increase from top to
bottom in the group but the melting point increases upto arsenic and then
decreases upto bismuth. Except nitrogen, all the elements show allotropy.
Oxidation states and trends in chemical reactivity
The common oxidation states of these elements are –3, +3 and +5.
The tendency to exhibit –3 oxidation state decreases down the group due
to increase in size and metallic character. In fact last member of the group,
bismuth hardly forms any compound in –3 oxidation state. The stability
of +5 oxidation state decreases down the group. The only well characterised
Bi (V) compound is BiF
5
. The stability of +5 oxidation state decreases and
that of +3 state increases (due to inert pair effect) down the group. Nitrogen
exhibits + 1, + 2, + 4 oxidation states also when it reacts with oxygen.
Phosphorus also shows +1 and +4 oxidation states in some oxoacids.
In the case of nitrogen, all oxidation states from +1 to +4 tend to
disproportionate in acid solution. For example,
3HNO
2 
? HNO
3
 + H
2
O + 2NO
Similarly, in case of phosphorus nearly all intermediate oxidation
states disproportionate into +5 and –3 both in alkali and acid. However
+3 oxidation state in case of arsenic, antimony and bismuth becomes
increasingly stable with respect to disproportionation.
Nitrogen is restricted to a maximum covalency of 4 since only four
(one s and three p) orbitals are available for bonding. The heavier elements
have vacant d orbitals in the outermost shell which can be used for
bonding (covalency) and hence, expand their covalence as in PF
–
6
.
Anomalous properties of nitrogen
Nitrogen differs from the rest of the members of this group due to
its small size, high electronegativity, high ionisation enthalpy and
non-availability of d orbitals. Nitrogen has unique ability to form
pp p p p p-pp p p p p multiple bonds with itself and with other elements having
small size and high electronegativity (e.g., C, O). Heavier elements of
this group do not form pp-pp bonds as their atomic orbitals are so
large and diffuse that they cannot have effective overlapping.
Thus, nitrogen exists as a diatomic molecule with a triple bond (one
s and two p) between the two atoms. Consequently, its bond enthalpy
(941.4 kJ mol
–1
) is very high. On the contrary, phosphorus, arsenic
and antimony form single bonds as P–P, As–As and Sb–Sb while
bismuth forms metallic bonds in elemental state. However, the single
N–N bond is weaker than the single P–P bond because of high
interelectronic repulsion of the non-bonding electrons, owing to the
small bond length. As a result the catenation tendency is weaker in
7.1.5
Electronegativity
7.1.6 Physical
Properties
7.1.7 Chemical
Properties
© NCERT 
not to be republished
168 Chemistry
nitrogen. Another factor which affects the chemistry of nitrogen is
the absence of d orbitals in its valence shell. Besides restricting its
covalency to four, nitrogen cannot form dp p p p p –pp p p p p bond as the heavier
elements can e.g., R
3
P = O or R
3
P = CH
2 
(R = alkyl group). Phosphorus
and arsenic can form dp p p p p –dp p p p p bond also with transition metals when
their compounds like P(C
2
H
5
)
3
 and As(C
6
H
5
)
3
 act as ligands.
(i) Reactivity towards hydrogen: All the elements of Group 15
form hydrides of the type EH
3
 where E = N, P, As, Sb or Bi.
Some of the properties of these hydrides are shown in Table
7.2. The hydrides show regular gradation in their properties.
The stability of hydrides decreases from NH
3
 to BiH
3
 which can
be observed from their bond dissociation enthalpy.
Consequently, the reducing character of the hydrides increases.
Ammonia is only a mild reducing agent while BiH
3 
is the
strongest reducing agent amongst all the hydrides. Basicity also
decreases in the order NH
3
 > PH
3
 > AsH
3
 > SbH
3
 > BiH
3
.
Property NH
3
PH
3
AsH
3
SbH
3
BiH
3
Melting point/K 195.2 139.5 156.7 185 –
Boiling point/K 238.5 185.5 210.6 254.6 290
(E–H) Distance/pm 101.7 141.9 151.9 170.7 –
HEH angle (°) 107.8 93.6 91.8 91.3 –
?
f 
H
V
/kJ mol
–1
–46.1 13.4 66.4 145.1 278
?
diss
H
V
(E–H)/kJ mol
–1
389 322 297 255 –
Table 7.2: Properties of Hydrides of Group 15 Elements
(ii) Reactivity towards oxygen: All these elements form two types
of oxides: E
2
O
3
 and E
2
O
5
. The oxide in the higher oxidation state
of the element is more acidic than that of lower oxidation state.
Their acidic character decreases down the group. The oxides of
the type E
2
O
3 
of nitrogen and phosphorus are purely acidic,
that of arsenic and antimony amphoteric and those of bismuth
predominantly basic.
(iii) Reactivity towards halogens: These elements react to form two
series of halides: EX
3
 and EX
5
. Nitrogen does not form
pentahalide due to non-availability of the d orbitals in its valence
shell. Pentahalides are more covalent than trihalides. All the
trihalides of these elements except those of nitrogen are stable.
In case of nitrogen, only NF
3
 is known to be stable. Trihalides
except BiF
3
 are predominantly covalent in nature.
(iv) Reactivity towards metals: All these elements react with metals
to form their binary compounds exhibiting –3 oxidation state,
such as, Ca
3
N
2
 (calcium nitride) Ca
3
P
2 
(calcium phosphide),
Na
3
As
2
 (sodium arsenide), Zn
3
Sb
2 
(zinc antimonide) and Mg
3
Bi
2
(magnesium bismuthide).
© NCERT 
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Page 5


In Class XI, you have learnt that the p-block elements
are placed in groups 13 to 18 of the periodic table.
Their valence shell electronic configuration is ns
2
np
1–6
(except He which has 1s
2
 configuration). The properties
of p-block elements like that of others are greatly
influenced by atomic sizes, ionisation enthalpy, electron
gain enthalpy and electronegativity. The absence of d-
orbitals in second period and presence of d or d and f
orbitals in heavier elements (starting from third period
onwards) have significant effects on the properties of
elements. In addition, the presence of all the three types
of elements; metals, metalloids and non-metals bring
diversification in chemistry of these elements.
Having learnt the chemistry of elements of Groups
13 and 14 of the p-block of periodic table in Class XI,
you will learn the chemistry of the elements of
subsequent groups in this Unit.
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements s s s s
7
The The The The The p p p p p-Block -Block -Block -Block -Block
Element Element Element Element Elements ss ss
After studying this Unit, you will be
able to
• appreciate general trends in the
chemistry of elements of groups
15,16,17 and 18;
• learn the preparation, properties
and uses of dinitrogen and
phosphorus and some of their
important compounds;
• describe the preparation,
properties and uses of dioxygen
and ozone and chemistry of some
simple oxides;
• know allotropic forms of sulphur,
chemistry of its important
compounds and the structures of
its oxoacids;
• describe the preparation,
properties and uses of chlorine
and hydrochloric acid;
• know the chemistry of
interhalogens and structures of
oxoacids of halogens;
• enumerate the uses of noble
gases;
• appreciate the importance of
these elements and their
compounds in our day to day life.
Objectives
Diversity in chemistry is the hallmark of p–block elements manifested
in their ability to react with the elements of s–, d– and f–blocks as
well as with their own.
Group 15 includes nitrogen, phosphorus, arsenic, antimony and
bismuth. As we go down the group, there is a shift from non-metallic
to metallic through metalloidic character. Nitrogen and phosphorus
are non-metals, arsenic and antimony metalloids and bismuth is a
typical metal.
Molecular nitrogen comprises 78% by volume of the atmosphere.
In the earth’s crust, it occurs as sodium nitrate, NaNO
3
 (called Chile
saltpetre) and potassium nitrate (Indian saltpetre). It is found in the
form of proteins in plants and animals. Phosphorus occurs in minerals
7.1 7.1 7.1 7.1 7.1 Group 15 Group 15 Group 15 Group 15 Group 15
Elements Elements Elements Elements Elements
7.1.1 Occurrence
Unit Unit Unit Unit Unit
7
© NCERT 
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166 Chemistry
of the apatite family, Ca
9
(PO
4
)
6
. CaX
2
 (X = F, Cl or OH) (e.g., fluorapatite
Ca
9 
(PO
4
)
6
. CaF
2
) which are the main components of phosphate rocks.
Phosphorus is an essential constituent of animal and plant matter. It
is present in bones as well as in living cells. Phosphoproteins are present
in milk and eggs. Arsenic, antimony and bismuth are found mainly as
sulphide minerals.
The important atomic and physical properties of this group elements
along with their electronic configurations are given in Table 7.1.
Property N P As Sb Bi
Atomic number 7 15 33 51 83
Atomic mass/g mol
–1
14.01 30.97 74.92 121.75 208.98
Electronic configuration [He]2s
2
2p
3
[Ne]3s
2
3p
3
[Ar]3d
10
4s
2
4p
3
[Kr]4d
10
5s
2
5p
3
[Xe]4f
14
5d
10
6s
2
6p
3
Ionisation enthalpy I 1402 1012 947 834 703
(?
i
H/(kJ mol
–1
) II 2856 1903 1798 1595 1610
III 4577 2910 2736 2443 2466
Electronegativity 3.0 2.1 2.0 1.9 1.9
Covalent radius/pm
a
70 110 121 141 148
Ionic radius/pm 171
b
212
b
222
b
76
c
103
c
Melting point/K 63* 317
d
1089
e
904 544
Boiling point/K 77.2* 554
d
888
f
1860 1837
Density/[g cm
–3
(298 K)] 0.879
g
1.823 5.778
h
6.697 9.808
Table 7.1: Atomic and Physical Properties of Group 15 Elements
a
 E
III
 single bond (E = element); 
b
 E
3–
; 
c
 E
3+
; 
d
 White phosphorus; 
e
 Grey a-form at 38.6 atm; 
f 
Sublimation temperature;
g 
At 63 K; 
h
Grey a-form; * Molecular N
2
.
Trends of some of the atomic, physical and chemical properties of the
group are discussed below.
The valence shell electronic configuration of these elements is ns
2
np
3
.
The s orbital in these elements is completely filled and p orbitals are
half-filled, making their electronic configuration extra stable.
Covalent and ionic (in a particular state) radii increase in size
down the group. There is a considerable increase in covalent radius
from N to P. However, from As to Bi only a small increase in
covalent radius is observed. This is due to the presence of
completely filled d and/or f orbitals in heavier members.
Ionisation enthalpy decreases down the group due to gradual increase
in atomic size. Because of the extra stable half-filled p orbitals electronic
configuration and smaller size, the ionisation enthalpy of the group 15
elements is much greater than that of group 14 elements in the
corresponding periods. The order of successive ionisation enthalpies,
as expected is ?
i
H
1 
< ?
i
H
2 
< ?
i
H
3 
(Table 7.1).
7.1.2 Electronic
Configuration
7.1.3 Atomic and
Ionic Radii
7.1.4 Ionisation
Enthalpy
© NCERT 
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167 The p-Block Elements
The electronegativity value, in general, decreases down the group with
increasing atomic size. However, amongst the heavier elements, the
difference is not that much pronounced.
All the elements of this group are polyatomic. Dinitrogen is a diatomic gas
while all others are solids. Metallic character increases down the group.
Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids
and bismuth is a metal. This is due to decrease in ionisation enthalpy and
increase in atomic size. The boiling points, in general, increase from top to
bottom in the group but the melting point increases upto arsenic and then
decreases upto bismuth. Except nitrogen, all the elements show allotropy.
Oxidation states and trends in chemical reactivity
The common oxidation states of these elements are –3, +3 and +5.
The tendency to exhibit –3 oxidation state decreases down the group due
to increase in size and metallic character. In fact last member of the group,
bismuth hardly forms any compound in –3 oxidation state. The stability
of +5 oxidation state decreases down the group. The only well characterised
Bi (V) compound is BiF
5
. The stability of +5 oxidation state decreases and
that of +3 state increases (due to inert pair effect) down the group. Nitrogen
exhibits + 1, + 2, + 4 oxidation states also when it reacts with oxygen.
Phosphorus also shows +1 and +4 oxidation states in some oxoacids.
In the case of nitrogen, all oxidation states from +1 to +4 tend to
disproportionate in acid solution. For example,
3HNO
2 
? HNO
3
 + H
2
O + 2NO
Similarly, in case of phosphorus nearly all intermediate oxidation
states disproportionate into +5 and –3 both in alkali and acid. However
+3 oxidation state in case of arsenic, antimony and bismuth becomes
increasingly stable with respect to disproportionation.
Nitrogen is restricted to a maximum covalency of 4 since only four
(one s and three p) orbitals are available for bonding. The heavier elements
have vacant d orbitals in the outermost shell which can be used for
bonding (covalency) and hence, expand their covalence as in PF
–
6
.
Anomalous properties of nitrogen
Nitrogen differs from the rest of the members of this group due to
its small size, high electronegativity, high ionisation enthalpy and
non-availability of d orbitals. Nitrogen has unique ability to form
pp p p p p-pp p p p p multiple bonds with itself and with other elements having
small size and high electronegativity (e.g., C, O). Heavier elements of
this group do not form pp-pp bonds as their atomic orbitals are so
large and diffuse that they cannot have effective overlapping.
Thus, nitrogen exists as a diatomic molecule with a triple bond (one
s and two p) between the two atoms. Consequently, its bond enthalpy
(941.4 kJ mol
–1
) is very high. On the contrary, phosphorus, arsenic
and antimony form single bonds as P–P, As–As and Sb–Sb while
bismuth forms metallic bonds in elemental state. However, the single
N–N bond is weaker than the single P–P bond because of high
interelectronic repulsion of the non-bonding electrons, owing to the
small bond length. As a result the catenation tendency is weaker in
7.1.5
Electronegativity
7.1.6 Physical
Properties
7.1.7 Chemical
Properties
© NCERT 
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168 Chemistry
nitrogen. Another factor which affects the chemistry of nitrogen is
the absence of d orbitals in its valence shell. Besides restricting its
covalency to four, nitrogen cannot form dp p p p p –pp p p p p bond as the heavier
elements can e.g., R
3
P = O or R
3
P = CH
2 
(R = alkyl group). Phosphorus
and arsenic can form dp p p p p –dp p p p p bond also with transition metals when
their compounds like P(C
2
H
5
)
3
 and As(C
6
H
5
)
3
 act as ligands.
(i) Reactivity towards hydrogen: All the elements of Group 15
form hydrides of the type EH
3
 where E = N, P, As, Sb or Bi.
Some of the properties of these hydrides are shown in Table
7.2. The hydrides show regular gradation in their properties.
The stability of hydrides decreases from NH
3
 to BiH
3
 which can
be observed from their bond dissociation enthalpy.
Consequently, the reducing character of the hydrides increases.
Ammonia is only a mild reducing agent while BiH
3 
is the
strongest reducing agent amongst all the hydrides. Basicity also
decreases in the order NH
3
 > PH
3
 > AsH
3
 > SbH
3
 > BiH
3
.
Property NH
3
PH
3
AsH
3
SbH
3
BiH
3
Melting point/K 195.2 139.5 156.7 185 –
Boiling point/K 238.5 185.5 210.6 254.6 290
(E–H) Distance/pm 101.7 141.9 151.9 170.7 –
HEH angle (°) 107.8 93.6 91.8 91.3 –
?
f 
H
V
/kJ mol
–1
–46.1 13.4 66.4 145.1 278
?
diss
H
V
(E–H)/kJ mol
–1
389 322 297 255 –
Table 7.2: Properties of Hydrides of Group 15 Elements
(ii) Reactivity towards oxygen: All these elements form two types
of oxides: E
2
O
3
 and E
2
O
5
. The oxide in the higher oxidation state
of the element is more acidic than that of lower oxidation state.
Their acidic character decreases down the group. The oxides of
the type E
2
O
3 
of nitrogen and phosphorus are purely acidic,
that of arsenic and antimony amphoteric and those of bismuth
predominantly basic.
(iii) Reactivity towards halogens: These elements react to form two
series of halides: EX
3
 and EX
5
. Nitrogen does not form
pentahalide due to non-availability of the d orbitals in its valence
shell. Pentahalides are more covalent than trihalides. All the
trihalides of these elements except those of nitrogen are stable.
In case of nitrogen, only NF
3
 is known to be stable. Trihalides
except BiF
3
 are predominantly covalent in nature.
(iv) Reactivity towards metals: All these elements react with metals
to form their binary compounds exhibiting –3 oxidation state,
such as, Ca
3
N
2
 (calcium nitride) Ca
3
P
2 
(calcium phosphide),
Na
3
As
2
 (sodium arsenide), Zn
3
Sb
2 
(zinc antimonide) and Mg
3
Bi
2
(magnesium bismuthide).
© NCERT 
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169 The p-Block Elements
Though nitrogen exhibits +5 oxidation state, it does not form
pentahalide. Give reason.
Nitrogen with n = 2, has s and p orbitals only. It does not have d
orbitals to expand its covalence beyond four. That is why it does not
form pentahalide.
PH
3
 has lower boiling point than NH
3
. Why?
Unlike NH
3
, PH
3
 molecules are not associated through hydrogen bonding
in liquid state. That is why the boiling point of PH
3
 is lower than NH
3
.
Intext Questions Intext Questions Intext Questions Intext Questions Intext Questions
7.1 Why are pentahalides more covalent than trihalides ?
7.2 Why is BiH
3
 the strongest reducing agent amongst all the hydrides of
Group 15 elements ?
Preparation
Dinitrogen is produced commercially by the liquefaction and fractional
distillation of air. Liquid dinitrogen (b.p. 77.2 K) distils out first leaving
behind liquid oxygen (b.p. 90 K).
In the laboratory, dinitrogen is prepared by treating an aqueous
solution of ammonium chloride with sodium nitrite.
NH
4
CI(aq) + NaNO
2
(aq) ? N
2
(g) + 2H
2
O(l) + NaCl (aq)
Small amounts of NO and HNO
3
 are also formed in this reaction;
these impurities can be removed by passing the gas through aqueous
sulphuric acid containing potassium dichromate. It can also be obtained
by the thermal decomposition of ammonium dichromate.
(NH
4
)
2
Cr
2
O
7 
Heat
???? N
2
 + 4H
2
O + Cr
2
O
3
Very pure nitrogen can be obtained by the thermal decomposition
of sodium or barium azide.
Ba(N
3
)
2
?
 Ba + 3N
2
Properties
Dinitrogen is a colourless, odourless, tasteless and non-toxic gas.
Nitrogen atom has two stable isotopes: 
14
N and 
15
N. It has a very low
solubility in water (23.2 cm
3
 per litre of water at 273 K and 1 bar
pressure) and low freezing and boiling points (Table 7.1).
Dinitrogen is rather inert at room temperature because of the high
bond enthalpy of N=N bond. Reactivity, however, increases rapidly with
rise in temperature. At higher temperatures, it directly combines with
some metals to form predominantly ionic nitrides and with non-metals,
covalent nitrides. A few typical reactions are:
6Li + N
2 
Heat
???? 2Li
3
N
3Mg + N
2 
Heat
???? Mg
3
N
2
7.2 7.2 7.2 7.2 7.2 Dinitrogen Dinitrogen Dinitrogen Dinitrogen Dinitrogen
Example 7.1 Example 7.1 Example 7.1 Example 7.1 Example 7.1
Solution Solution Solution Solution Solution
Example 7.2 Example 7.2 Example 7.2 Example 7.2 Example 7.2
Solution Solution Solution Solution Solution
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