UNIT 3 HYDROGEN
Structure
3.1 Introduction
Objectives
3.2 Position of Hydrogen in the Periodic Table
3.3 Isotopes of Hydrogen
Deuterium Compounds
Tritium
3.4 Ortho and Para Hydrogen
3.5 Manufacture of Hydrogen
3.6 Properties of Hydrogen
3.7 Uses of Hydrogen
3.8, Hydrides
Ionic or Salt-like Hydrides
Covalent Hydrides
MetaIlic Hydrides
3.9 Hydrogen Bonding
intermolecular Hydrogen Bonding
Intramolecular Hydrogen Bonding
i
Effect of Hydrogen Bonding
3.10 Polarising Power of H+
3.1 1 Summary
3.12 Terminal Questions
3.13 Answers
3.1 INTRODUCTION
In Units I and 2, you studied the periodic table and periodicity in the properties of elements.
' You must have noticed that the very first element in the periodic table is hydrogen.
Hydrogen atom consists of only one proton and one electron. Hydrogen forms more
compounds than any other element. Besides this, you should know that hydrogen is the most
abundant of all the elements in the universe (73.9% by weight) and is the principal element
in the solar atmosphere. However, it is much less abundant (0.14% by weight) on earth,
where it exists only in the combined state. Keeping in view the importance of hydrogen, we
I will discuss some important aspects of the chemistry of hydrogen in this unit. Here, we will
discuss its position in the periodic table, isotopes, manufacture, uses and hydrides in general.
The specific hydrides, e.g., boranes, ammonia will be dealt with in their corresponding
groups in the following units.
Objectives
After studying this unit you should be able to:
justify the position of hydrogen in the periodic table,
describe isotopes of hydrogen,
differentiate between ortho and para forms of hydrogen molecule,
describe the various methods for the manufacture of hydrogen,
discuss the importance of hydrogen as a fuel,
classify the binary hydrides according to their structure, and
describe the conditions that are necessary for hydrogen bonding to occur and the
characteristic properties of hydrogen bonded systems.
3.2 POSITION OF HYDROGEN IN THE PERIODIC
TABLE
I
I
Position of hydrogen in the periodic table is of particular interest. Hydrogen is the first
element of the periodic table having electronic configuration Is'. On one hand this
configuration is similar to the outer electronic configuration (ns') of the alkali metals. On the
other hand, like halogens, it is one electron short of the corresponding inert gas helium.
Hydrogen, therefore, shows some properties similar to alkali metals, while some others are
similar to those of the halogens.
,Periodicity and s-Block Like alkali metals, hydrogen forms halides, oxides and sulphides. You know, the alkali
Elements metals have a high tendency of losing their outermost electron to,form Mt ions. Hydrogen
also forms H+ion, but it does not do so, under normal conditions, because the ionisation
energy of hydrogen (1312 kT mol-') is much higher than that of the alkali metals, e.g., Li,
520; Na, 495; K, 41 8 kJ mol-'.
In its high ionisation energy, hydrogen resembles more with halogens. The first ionisation
-
energies of fluorine, chlorine, bromine and iodine are 1618, 1255, 1142 and 1007 kJ mol-'.
respectively. Due to its high ionisation energy, hydrogen forme large number of covalent
compounds by sharing a pair of electrons. Hydrogen like halogens forms a diatomic
molecule by sharing a pair of electrons between the two atom!: By picking up an electron,
hydrogen forms the hydride ion (H-), just like the halogens form the halide ion (X).
From the above discussion, it is clear that hydrogen resembles both the alkali metals as well
as the halogens. So, hydrogen can be placed with either of them in the periodic table. However, .
conventionally, it is kept along with the alkali metals in group 1 in the periodic table.
SAQ 1
a) Why does hydrogen resemble the alkali metals?
b) Explain the formation of hydride ion;
3.3 ISOTOPES OF HYDROGEN
You have studied that atoms of an element which have the same atomic number but different
@ 1e
:H, hydrogen-1
or protium
mass numbers are called isotopes. Hydrogen has three different isotopes having mass
numbers, 1.2 and 3, called ordinary hydrogen or protium 'H, deuterium (D) or ZHand
tritium (T) or 3Hrespectively. These isotopes differ from one another in respect of the
nucleus
presence of neutrons. Ordinary hydrogen has no neutrons, deuterium has one and tritium
0: H, hydrogen-2
or deuterium ( D l
has two neutrons in the nucleus (Fig. 3.1).
Deuterium is also called heavy hydrogen. These isotopes have same electronic configuration
and therefore their chemical properties are almost the same. The only difference is in the
nucleus
rate of reactions. For example, hydrogen has a lower energy of activation than deuterium in
its reaction with halogens and therefore, reacts faster.
@ le
:H, hydrogen-3
or tritium (TI However, the physical properties of hydrogen, deuterium and tritium differ considerably due
to their large mass differences. Some of the important physical properties of hydrogen,
nucleus
deuterium and tritium are tabulated in Table 3.1.
Fig. 3.1: Isotopes of hydrogen.
3.3.1 Deuterium Compounds
Naturally occurring hydrogen contains 0.0156% deuterium. Like water. HzO, which IS the
oxide of hydrogen, deuterium also forms an oxide, D20,which is known as heavy water. It
can be obtained from ordinary water which contains 0.016% of deuterium oxide, either by
fractional distillation or by electrolys~s.Hydrogen is liberated more quickly than deuterium
at the cathode and the residual liquid continuously gets richer in deuterium content on
prolonged electrolysis. We can obtain virtually pure deuterium oxide. D,O. by continuouh
electrolysis of water. Deuterium oxide is used as a moderator in riuclcur reactor\ since il
slow>down neutrons quickly. Physical properties of H . 0 a n d D,O alho tlil'fcr fr-omcacti
other as in the case of H, and D,. These are given in Tnblc 3.2.
, Table 3.1: Properties of atomic and molecular hydrogen Hydrogen
Properties Hydrogen Deuterium Tritium
Natural abundance (%) 99.985 0.0156 10-l5
Atomic weight (C = 12) 1.008 2.014 3.016
Boiling point (K) 20.4 23.6 25.0
Melting point (K) 13.8 18.5 20.62
Density of liquid at
b.p.. H, (10' x kg m-') 0.071
*Ionisation energy, (kJ molkl) 1312
*Electron affinity, (kJ molki) -73
*Electronegativity(Pauling Scale) 2.1
*Bond length, H,(g) (pm) 74.9
*Bond energy (AHatom).H2(kJ mol-') 436
*Ionic radius, H- (pm) 208
'
*Covalent radius (pm) 37
*van der Waals radius (pm) 120
* As expected hydrogen, deuterium and tritium will have similar values.
Table 3.2: Properties of water and heavy water
Hz0 DZO
Density at 293K (10' x kg m-') 0.9982 1.1059
Melting point (K) 273.16 276.97 '
oiling point (K) 373.16 374.58
Temperature of maximum density (K) 277.04 284.39
Dielectric constant at 298 K 78.39 78.06
Hydrogen and deuterium are obtained by similar methods.
2Na + 2H20 +2NaOH + Hz
2Na + 2D20 2NaOD + D,
-
Many deuterium compounds, similar to those of hydrogen, are obtained from D20.
PCI,+ 3D20 -----+
Mg,N, + 6 ~
CaC, + 2D20 - ~
D,PO, + 3DCI
03Mg(OD),
Ca(OD),
+ 2ND3
+ C,D2
We can also employ exchange reactions like those given below for the preparation of
deuterium compounds;
NaOH + D20+ NaOD+HDO
NH4Cl+D20+ HDO+NH,DCl
& D2O
NH2D2Cl
& D2O
ND,cI.D'O
NHD3C1
3.3.2 Tritium
Tritium differs from the other two isotopes of hydrogen in being radioactive. Naturally
occurring hydrogen contains nearly 10-15 % tritium.The concentration of tritium increased by
over a hundredfold when thermonuclear weapon testing began in 1954 but is now subsiding
again as a result of the ban on atmospheric weapon testing.
Tritium was first obtained synthetically by the bombardment of deuterium compounds such
as (NDJSO, with fast deuterons.
2 2 3 1
ID+ 1 D+ I T + IH
Structure
3.1 Introduction
Objectives
3.2 Position of Hydrogen in the Periodic Table
3.3 Isotopes of Hydrogen
Deuterium Compounds
Tritium
3.4 Ortho and Para Hydrogen
3.5 Manufacture of Hydrogen
3.6 Properties of Hydrogen
3.7 Uses of Hydrogen
3.8, Hydrides
Ionic or Salt-like Hydrides
Covalent Hydrides
MetaIlic Hydrides
3.9 Hydrogen Bonding
intermolecular Hydrogen Bonding
Intramolecular Hydrogen Bonding
i
Effect of Hydrogen Bonding
3.10 Polarising Power of H+
3.1 1 Summary
3.12 Terminal Questions
3.13 Answers
3.1 INTRODUCTION
In Units I and 2, you studied the periodic table and periodicity in the properties of elements.
' You must have noticed that the very first element in the periodic table is hydrogen.
Hydrogen atom consists of only one proton and one electron. Hydrogen forms more
compounds than any other element. Besides this, you should know that hydrogen is the most
abundant of all the elements in the universe (73.9% by weight) and is the principal element
in the solar atmosphere. However, it is much less abundant (0.14% by weight) on earth,
where it exists only in the combined state. Keeping in view the importance of hydrogen, we
I will discuss some important aspects of the chemistry of hydrogen in this unit. Here, we will
discuss its position in the periodic table, isotopes, manufacture, uses and hydrides in general.
The specific hydrides, e.g., boranes, ammonia will be dealt with in their corresponding
groups in the following units.
Objectives
After studying this unit you should be able to:
justify the position of hydrogen in the periodic table,
describe isotopes of hydrogen,
differentiate between ortho and para forms of hydrogen molecule,
describe the various methods for the manufacture of hydrogen,
discuss the importance of hydrogen as a fuel,
classify the binary hydrides according to their structure, and
describe the conditions that are necessary for hydrogen bonding to occur and the
characteristic properties of hydrogen bonded systems.
3.2 POSITION OF HYDROGEN IN THE PERIODIC
TABLE
I
I
Position of hydrogen in the periodic table is of particular interest. Hydrogen is the first
element of the periodic table having electronic configuration Is'. On one hand this
configuration is similar to the outer electronic configuration (ns') of the alkali metals. On the
other hand, like halogens, it is one electron short of the corresponding inert gas helium.
Hydrogen, therefore, shows some properties similar to alkali metals, while some others are
similar to those of the halogens.
,Periodicity and s-Block Like alkali metals, hydrogen forms halides, oxides and sulphides. You know, the alkali
Elements metals have a high tendency of losing their outermost electron to,form Mt ions. Hydrogen
also forms H+ion, but it does not do so, under normal conditions, because the ionisation
energy of hydrogen (1312 kT mol-') is much higher than that of the alkali metals, e.g., Li,
520; Na, 495; K, 41 8 kJ mol-'.
In its high ionisation energy, hydrogen resembles more with halogens. The first ionisation
-
energies of fluorine, chlorine, bromine and iodine are 1618, 1255, 1142 and 1007 kJ mol-'.
respectively. Due to its high ionisation energy, hydrogen forme large number of covalent
compounds by sharing a pair of electrons. Hydrogen like halogens forms a diatomic
molecule by sharing a pair of electrons between the two atom!: By picking up an electron,
hydrogen forms the hydride ion (H-), just like the halogens form the halide ion (X).
From the above discussion, it is clear that hydrogen resembles both the alkali metals as well
as the halogens. So, hydrogen can be placed with either of them in the periodic table. However, .
conventionally, it is kept along with the alkali metals in group 1 in the periodic table.
SAQ 1
a) Why does hydrogen resemble the alkali metals?
b) Explain the formation of hydride ion;
3.3 ISOTOPES OF HYDROGEN
You have studied that atoms of an element which have the same atomic number but different
@ 1e
:H, hydrogen-1
or protium
mass numbers are called isotopes. Hydrogen has three different isotopes having mass
numbers, 1.2 and 3, called ordinary hydrogen or protium 'H, deuterium (D) or ZHand
tritium (T) or 3Hrespectively. These isotopes differ from one another in respect of the
nucleus
presence of neutrons. Ordinary hydrogen has no neutrons, deuterium has one and tritium
0: H, hydrogen-2
or deuterium ( D l
has two neutrons in the nucleus (Fig. 3.1).
Deuterium is also called heavy hydrogen. These isotopes have same electronic configuration
and therefore their chemical properties are almost the same. The only difference is in the
nucleus
rate of reactions. For example, hydrogen has a lower energy of activation than deuterium in
its reaction with halogens and therefore, reacts faster.
@ le
:H, hydrogen-3
or tritium (TI However, the physical properties of hydrogen, deuterium and tritium differ considerably due
to their large mass differences. Some of the important physical properties of hydrogen,
nucleus
deuterium and tritium are tabulated in Table 3.1.
Fig. 3.1: Isotopes of hydrogen.
3.3.1 Deuterium Compounds
Naturally occurring hydrogen contains 0.0156% deuterium. Like water. HzO, which IS the
oxide of hydrogen, deuterium also forms an oxide, D20,which is known as heavy water. It
can be obtained from ordinary water which contains 0.016% of deuterium oxide, either by
fractional distillation or by electrolys~s.Hydrogen is liberated more quickly than deuterium
at the cathode and the residual liquid continuously gets richer in deuterium content on
prolonged electrolysis. We can obtain virtually pure deuterium oxide. D,O. by continuouh
electrolysis of water. Deuterium oxide is used as a moderator in riuclcur reactor\ since il
slow>down neutrons quickly. Physical properties of H . 0 a n d D,O alho tlil'fcr fr-omcacti
other as in the case of H, and D,. These are given in Tnblc 3.2.
, Table 3.1: Properties of atomic and molecular hydrogen Hydrogen
Properties Hydrogen Deuterium Tritium
Natural abundance (%) 99.985 0.0156 10-l5
Atomic weight (C = 12) 1.008 2.014 3.016
Boiling point (K) 20.4 23.6 25.0
Melting point (K) 13.8 18.5 20.62
Density of liquid at
b.p.. H, (10' x kg m-') 0.071
*Ionisation energy, (kJ molkl) 1312
*Electron affinity, (kJ molki) -73
*Electronegativity(Pauling Scale) 2.1
*Bond length, H,(g) (pm) 74.9
*Bond energy (AHatom).H2(kJ mol-') 436
*Ionic radius, H- (pm) 208
'
*Covalent radius (pm) 37
*van der Waals radius (pm) 120
* As expected hydrogen, deuterium and tritium will have similar values.
Table 3.2: Properties of water and heavy water
Hz0 DZO
Density at 293K (10' x kg m-') 0.9982 1.1059
Melting point (K) 273.16 276.97 '
oiling point (K) 373.16 374.58
Temperature of maximum density (K) 277.04 284.39
Dielectric constant at 298 K 78.39 78.06
Hydrogen and deuterium are obtained by similar methods.
2Na + 2H20 +2NaOH + Hz
2Na + 2D20 2NaOD + D,
-
Many deuterium compounds, similar to those of hydrogen, are obtained from D20.
PCI,+ 3D20 -----+
Mg,N, + 6 ~
CaC, + 2D20 - ~
D,PO, + 3DCI
03Mg(OD),
Ca(OD),
+ 2ND3
+ C,D2
We can also employ exchange reactions like those given below for the preparation of
deuterium compounds;
NaOH + D20+ NaOD+HDO
NH4Cl+D20+ HDO+NH,DCl
& D2O
NH2D2Cl
& D2O
ND,cI.D'O
NHD3C1
3.3.2 Tritium
Tritium differs from the other two isotopes of hydrogen in being radioactive. Naturally
occurring hydrogen contains nearly 10-15 % tritium.The concentration of tritium increased by
over a hundredfold when thermonuclear weapon testing began in 1954 but is now subsiding
again as a result of the ban on atmospheric weapon testing.
Tritium was first obtained synthetically by the bombardment of deuterium compounds such
as (NDJSO, with fast deuterons.
2 2 3 1
ID+ 1 D+ I T + IH
