HYDROGEN

        1. Introduction

Hydrogen has the simplest atomic structure among all the elements around us in Nature. In atomic form it consists of only one proton and one electron. However, in elemental form it exists as a diatomic (H2) molecule and is called 

dihydrogen. It forms more compounds than any other element. The global concern related to energy can be overcome to a great extent by the use of hydrogen as a source of energy. In fact, hydrogen is of great industrial importance.

   2. POSITION OF HYDROGEN IN THE PERIODIC  TABLE

Hydrogen is the first element in the periodic table. However, its placement in the periodic table has been a subject of discussion in the past. The elements in the periodic table are arranged 

according to their electronic configurations.

Hydrogen has electronic configuration 1s ¹. On one hand, its electronic configuration is similar to the outer electronic configuration (n s¹) of alkali metals , which belong to the first group of the periodic table. On the other hand, like halogens (with n s² np 5 configuration belonging to the seventeenth group of the periodic table), it is short by one electron to the corresponding noble gas configuration, helium (1s²). Hydrogen, therefore, has resemblance to alkali metals, which lose one electron to form uni positive 

ions, as well as with halogens, which gain one electron to form uni negative ion. Like alkali metals, hydrogen forms oxides, halides and sulphides. However, unlike alkali metals, it has a very high ion i z at ion enthalpy and does not possess metallic characteristics under normal 

conditions. In fact, in terms of ionisation enthalpy, hydrogen resembles more with halogens, ∆i H of L i is 520 k J / mole, F is 

1680 k J / mole and that of H is 1312 k J / mole.

Like halogens, it forms a diatomic molecule,

combines with elements to form hydrides and

a large number of covalent compounds. However, in terms of reactivity, it is very low as compared to halogens. 

In spite of the fact that hydrogen, to a certain extent resembles both with alkali metals and halogens, it differs from them as well. Now the pertinent question arises as where should it be placed in the periodic table? Loss of the electron from hydrogen atom results in nucleus (H+ ) of ~1.5×10–3 pm size. This is extremely small as compared to normal atomic and ionic sizes of 50 to 200pm. As a consequence, H+ does not exist freely and is always associated with other atoms or molecules. Thus, it is unique in behaviour and 

is, therefore, best placed separately in the

periodic table. 

        3. DIHYDROGEN, H2

» O c c u r e n c e :

Dihydrogen is the most abundant element in

the universe (70% of the total mass of the

universe) and is the principal element in the

Property Hydrogen Deuterium Tritium

Relative abundance (%) 99.985 0.0156 10^–15

Relative atomic mass (g / mole) 1.008 2.014 3.016

Melting point / K 13.96 18.73 20.62

Boiling point/ K 20.39 23.67 25.0 

Density / g L–1 0.09 0.18 0.27

Enthalpy of fusion/k J / mole 0.117 0.197 -

Enthalpy of vapour is at ion/k J /mole 0.904 1.226 

Enthalpy of bond

dissociation/k J/ mole at 298.2K 435.88 443.35 -

Internuclear distance/pm 74.14 74.14 -

Ionisation enthalpy/k J /mole 1312 - -

Electron gain enthalpy/k J /mole –73 - -

Covalent radius/pm 37 - -

Ionic radius(H– )/pm 208 solar atmosphere. The giant planets Jupiter  and Saturn consist mostly of hydrogen. However, due to its light nature, it is much less abundant (0.15% by mass) in the earth’s atmosphere. Of course, in the combined form it constitutes 15.4% of the earth's crust and 

the oceans. In the combined form besides in

water, it occurs in plant and animal tissues,

carbohydrates, proteins, hydrides including

hydrocarbons and many other compounds.

» Isotopes of Hydrogen

Hydrogen has three isotopes: protium, 1

1 H, deuterium,  2 1H or D and tritium, 

3 1H or T. Can you guess how these isotopes differ from each other ? These isotopes differ from one another in respect of the presence of neutrons. Ordinary hydrogen, protium, has no neutrons, deuterium (also known as heavy hydrogen) has one and tritium has two neutrons in the nucleus. In the year 1934, an American 

scientist, Harold C. Urey, got Nobel Prize for

separating hydrogen isotope of mass number

2 by physical methods. The predominant form is protium. Terrestrial hydrogen contains 0.0156% of deuterium mostly in the form of HD. The 

tritium concentration is about one atom per

1018 atoms of protium. Of these isotopes, only

tritium is radioactive and emits low energy

β – particles (t½, 12.33 years).