Hydrogen in Periodic Table of Elements
Hydrogen, symbol H, molecular formula H2 is a colorless, odorless, tasteless, flammable gaseous chemical substance among the periodic table family. In chemistry or chemical science, the hydrogen atom is the only member of the chemical element in which the valence electron under the direct influence of the nucleus, bearing one unit positive and negative electrical charge. Under ordinary conditions, hydrogen gas consists of a pair of atoms or a diatomic molecule with a wide range of bonding. The most important chemical compound water (H2O) is obtained by burning it with oxygen molecules. Due to its unique characteristics properties, hydrogen (H) forms chemical compounds or molecules with nearly every periodic table active element.
The normal oxidation number or state of hydrogen in chemical compounds is +1 but highly electropositive metals (alkaline and alkaline earth), show -1 oxidation state. The periodic table position of the unique member describes by its characteristics properties and electron configuration. The ns1 electron configuration justifies its position in group-1 with the alkali metals. Consider other facts, the electronic configuration of hydrogen is one electron short of the next noble gas helium (He), thus it is placed in group-17 in the periodic table with the halogen. Due to the presence of a half-filled valence shell, it also placed in group-16 with the carbon family, forming covalent bonds with a wide range of polarities in chemistry.
Properties of Hydrogen Gas
Due to the presence of three isotopes, the gas molecules are represented as H2 (dihydrogen), D2 (di-deuterium), T2 (di-tritium). The chemical properties of all these gas molecules are basically the same but hydrogen isotopes differ by their physical properties. At standard temperature and pressure, hydrogen gas exists as a colorless, orderless, tasteless, highly combustible, non-toxic, nonmetallic chemical compound. The chemistry of hydrogen has late gained, it uses as the alternative source of energy in near future (fuel cells) due to huge stock of H2 in the earth’s surface water molecule.
The molecule is a very stable species, at 2000°K and 1-atmosphere pressure, only one percent of atomic hydrogen present in the gas molecules. Therefore, these facts suggested the lack of reactivity at ordinary temperature and pressure. Among atomic forms, it forms various unstable ionized species like a proton (H+), a hydride ion (H–), a molecular ion (H2+).
Hydrogen is prepared by the action of metals on water and acids, by the electrolysis of water, or by the reforming hydrocarbon by the stream. It is also obtained as a byproduct in the electrolytic manufacture of caustic potash from brine.
Small Scale Preparations
The alkali and alkaline earth metals like sodium (Na), calcium (Ca), magnesium (Mg), aluminum (Al) react with water in cold conditions evolving hydrogen gas. In hot conditions, metals like Zn, Fe, Co, Ni, and Mn decompose water.
Large Scale Production
Very pure hydrogen and oxygen obtained from the electrolysis of 20 percent caustic soda or potash solution in steel cells using iron electrodes. The anode is nickel plated to prevent oxidation. The anode and cathode are separated by a porous diaphragm to prevent diffusion and mixing of the gases.
Large amounts of H2 molecule are now obtained from the reforming of natural gas (hydrocarbon) by steam in presence of a nickel catalyst at 900°C. CnH2n+2 + nH2O → nCO + (2n+1)H2. Some carbon dioxide and methane are also formed by this process. The gas mixture cooled and removed carbon dioxide (CO2) and methane(CH4) for the fresh preparation of H2 gas molecules.
Uses of Hydrogen Gas
Hydrogen is used for the manufacture of chemicals like ammonia, hydrochloric acid, alcohols (methanol), aldehyde (formaldehyde), etc. The atomic, molecular, and liquid form is also used in fuel cells (hydrogen-oxygen fuel), hydrogenation of oils, welding atomic torches, reduction of metal ores, propulsion of rocket (in Saturn-V), in bubble chambers for the study of high energy particles. In near future, liquid hydrogen is used as an alternative source of energy when our reserves of fossil fuel (coal, oils, and natural gas) will get exhausted.
The great advantage of liquid hydrogen used as fuel is that it is free from the pollution hazard of conventional fuels. The only problems lie with the storage and transmission of large quantities of gases. Therefore, the vacuum isolated cryogenic tans used to stage huge quantities of liquid hydrogen in the United States, Canada, Germany, United kingdom space programs.