INTRODUCTION

The hydrogen atom consists of only one proton or one electron. Inspite of that, hydrogen forms more compounds than any other element. In addition, it is the most abundant of all the elements in the universe (73.9% by weight). It is as well the principal element in the solar atmosphere. However although hydrogen is very much in abundance (0.14% by weight) on earth, it exists merely in the combined state.

We are sure seeing how important hydrogen is or its peculiarity of being the 1^st element in the periodic table, we would like to know more about hydrogen. In this chapter we will be learning some important aspects of the chemistry of hydrogen. We will as well be studying its position in the periodic table, its isotopes or other forms.

Position of hydrogen in the periodic table:

The position of hydrogen in the periodic table is of particular interest. Hydrogen is the first element of the periodic table, with an electronic configuration of Is^l. This electronic configuration is alike to the outer electronic configuration of the alkali metals (ns¹). On another hand, similar to halogens, it is one electron short of the analogous inert gas helium (Is²). Hydrogen, consequently show several properties like to alkali metals, while various others are alike to those of the halogens. Similar to alkali metals, hydrogen forms halides, oxides and sulphides. The alkali metals include tight tendency of losing their outer most electron to form m^+ions cons. Hydrogen as well forms H⁺ ion, but it does not do so, below normal conditions, since the ionization energy of hydrogen (1312 kg mol^-1) is much higher than that of the alkali metals, example Li 520; Na-495; k418 kg mol^-1 With a high ionization energy, hydrogen resembles halogens. (Their first ionization energies are fluorine (1618 kg mol^-1) chlorine (1255 kg mol^-1) bromine (1142kg mol^-1) or iodine (1007 kg mol^-1). Due to its high ionization energy, hydrogen forms large numbers of covalent compounds through sharing a pair of electrons. Hydrogen similar to halogens forms a diatomic molecule via sharing a pair of electrons between the 2 atoms. Through picking up an electron, hydrogen forms the hydride ion (H^-), just like the halogens form the halide ion (X^-). From the previous 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 period table. Though conventionally, it is kept along with the alkali metals in group 1 in the periodic table.

Isotopes of Hydrogen:

Atoms of an element that contain similar atomic number but different mass number are termed ISOTOPES. Hydrogen has 3 different Isotopes having mass numbers 1, 2, and 3 termed ordinary hydrogen or Protium 'H, deuterium (D) or ²H or Tritium (T) or ³H correspondingly. These Isotopes differ from one another in respect of the presence of neutrons. Ordinary hydrogen has no neutrons, deuterium has one or tritium has 2 neutrons in the nucleus.

Fig: Isotopes of hydrogen

Deuterium is also termed heavy hydrogen. These Isotopes have similar electronic configuration and hence their chemical properties are almost similar. The only difference is in the rate of reactions. For instance, protium has a lower energy of activation than deuterium in its reaction with halogens and thus, reacts faster. The physical properties of hydrogen, deuterium and tritium differ considerably due to their large mass differences. Several of the important physical properties of hydrogen, deuterium and tritium are tabulated in Table.

Deuterium Compounds:

Naturally occurring hydrogen contains 0.0156% deuterium. Similar to water, (H20) that is the oxide of hydrogen, deuterium as well forms an oxide, D20, which is known as HEAVY WATER. It can be gained from ordinary water that contains 0.016% of deuterium oxide. This can be done either by fractional distillation or via electrolysis. Hydrogen is liberated more quickly than deuterium at the cathode or the residual liquid continuously gets richer in deuterium content on prolonged electrolysis of water. Deuterium oxide is used as a moderator in nuclear reactions because it slows down neutrons quickly. Physical properties of H₂O and D₂O and as well differ from each and other as in the case of H₂D₂.

Hydrogen and deuterium are obtained via similar process.

2Na + 2H₂O → 2NaOH + H2

2Na + 2D₂O → 2NaOD + D₂

Many deuterium compounds, alike to those of hydrogen, are obtained from D₂O

PCl₃ + 3D₂O → D₃ P0₃ + 3DC1

Mg₃N₂   + 6D₂₀ → 3Mg (OD) ₂ + 2ND₂

CaC₂ 2D2O → Ca (OD) ₂   + C₂D₂

We can also employ exchange reactions like those given below for the preparation of deuterium compounds

NaOH + D₂O → NaOD + HDO

NH₄Cl + D₂O → ↓HDO + NH₃ DCl 

                             D₂0

                       NH₂D₂Cl

                            ↓

                 ND₄C1 ↔ NHD₃Cl

Tritium:

Tritium differs from the other 2 Isotopes of hydrogen in creature radioactive. Naturally occurring hydrogen have nearly 10-15% tritium. The concentration of tritium increased via over a hundred fold when thermonuclear weapon testing began in the year 1954 but is now subsiding again as a consequence of the ban on atmospheric weapon testing.

Tritium was 1^st obtained synthetically through the bombardment of deuterium compounds these as (ND₄) SO₄ with fast deuterons.

   ₁² D + ₁²D → ₁³T + ₁¹H.

It is now prepared on a large scale by irradiation of lithium-6 in the form of Li/Mg alloy of LiF with slow neutrons in a reactor.

  ₁³Li + ₀¹n →   ₁³T + 42He

The following reaction occurs in nature.

₇ ¹⁴ N+ ₀¹n →1 T + ₆¹²C

Tritium is radioactive and decays via emission of a beta - particle. Its half life period is 12.3 years

     ₁³T→ ₂³He + β - partical

Tritium can be easily incorporated into biological molecules since it performs chemically, just similar to ordinary hydrogen. The radiation that tritium provides off within an organism, as a consequence of its decay, can cause many diseases, including cancer.

Ortho and para hydrogen:

Ortho or Para are 2 different forms of hydrogen molecule. Such different forms arise as a consequence of differences in the direction of nuclear spin. When 2 hydrogen atoms join to form a molecule, there are 2 possibilities. The two nuclei will also spin in the similar direction (parallel spins) to provide the form termed ortho hydrogen, and they would spin in opposite ways to give para hydrogen. This phenomenon is recognized as spin isomerism.

Fig. Ortho and Para forms of hydrogen

Due to spin Isomerism, differences in the interior energy of the 2 forms of hydrogen arises, causing differences in the physical properties similar to boiling point, specific heat or thermal conductivity. Para hydrogen has a lower interior energy than which of ortho hydrogen. Hydrogen gas is an equilibrium mixture of Ortho or Para hydrogen. The ration of Ortho to Para hydrogen varies with temperature as shown in fig.

Fig. Ortho-Para equillibria for H₂, D₂ and T₂

Evidently, this ratio amplifies with the rise in temperature up to a temperature of about 300k (27°C) and remains constant thereafter. The percentage of hydrogen at 300k or above is 75%. This means it is not possible to get 100% ortho hydrogen at any temperature. The equilibrium mixture of Para and Ortho hydrogen transforms to approximately 100% Para hydrogen when cooled to nearby complete zero. Para hydrogen is stable for weeks in the absence of catalysts similar to activated charcoal, Fe, Ni Pt, O₂, NO₂ etc. these catalyse the conversion of Para to Ortho hydrogen, Deuterium or tritium as well exhibit spin Isomerism and exist in .Ortho and Para forms. However the ratio of Ortho to Para forms in deuterium or tritium is dissimilar from that in hydrogen. The deviation of Ortho/Para ratio at different temperatures is as well different if we look carefully in fig we will see that tritium resembles hydrogen more closely than deuterium in this respect.

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