Helium in Periodic Table
Helium, chemical symbol He, atomic number 2, the second most abundant chemical element in the earth universe after hydrogen (H 76 percent; He 23 percent) is a colorless, orderless, and tasteless gas and liquid at -268 °C. The noble gases or group-18 element of the periodic table, helium formed in the thermonuclear reactions on stars but it present to a very negligible extent in the earth atmosphere due to the lightness of gas. The boiling and freezing point of helium is lower than any known chemical substances of the universe and cannot be solidified by sufficient cooling at normal atmospheric pressure. Helium gas consists of two stable isotopes with chemical formula 4He and 3He, He-4 is formed on earth by alpha particle decay of radioactive elements, but He-3 is formed by nuclear reactions induced by cosmic radiations.
Prior to 1960 helium, neon, argon, krypton, xenon, and radon were belong to the family of inert or rare gases, now these are named noble gases. The noble gases were not known at Mendeleev’s time. Today all the noble gases like helium, neon, argon, xenon, and radon are discovered and characterize and extreme inertness distinct from any other chemical elements of the periodic table. Helium atom (atomic number = 2 and mass number = 4) has two protons and neutrons in the nucleus and two extranuclear electrons surrounded the nucleus of the atom.
Discovery and History of Helium Gas
A new yellow line close to Sodium-D lines was observed by English astronomer Joseph Norman Lockyer and chemist Edward Frankland in the electromagnetic spectrum of the sun chromophore recorded during the total solar eclipse on 18 August. Lockyer and Frankland observed the facts to discovered a new chemical element, named helium (Greek halos means sun), the only elements found before the earth. The same line was observed by L. Palmieri in the spectrum of volcanic gases from Mt. Vesuvius. Lord Rayleigh observed helium when isolated nitrogen gas from atmospheric gases and the British chemist Sir William Ramsay in 1895 identified the gas in uranium minerals.
Abundance and Isotopes
The noble gas helium contains 23 percent of the mass of our universe and formed by thermonuclear reaction (nuclear fusion) in stars. Helium occurs 5.24 ppm by volume in dry air and 3 × 10-3 ppm by weight in radioactive minerals, meteoric iron, and mineral springs. However, the most economical source of helium is natural gas which contains 7 percent of He in some samples of the United States (Texas, New Mexico, Kansas, Oklahoma, Arizona, and Utah) but usually the gas present in the extent of 0.1 to 2 percent. About 3400 tonnes of helium are recovered annually in the United States alone from natural gas, while in eastern Europe, Algeria, Australia, Poland, Qatar, and Russia are the good producer of the noble gas.
The thorium mineral (monazite sand) contains 1 ml of helium per gram substances recovered by heating the sand in a vacuum at 1000 °C or digesting with sulfuric acid. He-3 and He-4 are the most common and stable isotopes of helium obtained by alpha ray decay and cosmic radiation of radioactive isotopes.
Properties of Helium Gas
Helium (atomic number = 2) and other noble gases family are colorless, orderless monoatomic gases having Cp/Cv ratios close to 1.67, and closed-shell electron configuration developed little intermolecular attraction posses very low boiling and melting point. Due to weak Van der Waals or London forces between the molecule of noble gases, electric polarization increasing with the atomic number (He to Xe). The fact reflects that the trends boiling point, melting point, enthalpy of vaporization, and solubility in water increases with the increasing atomic number.
The forces of attraction between helium gas atoms (normal phase) are so small, it suggests that the zero-point vibrational energy not sufficient for liquid and solid formation. In solid-state, neon (Ne), argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn) form a face-centered cubic crystal lattice. The atoms with a small mass like helium, the zero-point energy influence crystal structure of an atom, hence at 1K and 25 atmosphere He-4 has a hexagonal closed packed crystal structure but the lighter He-3 adopts body-centered cubic crystalline solid.
The facts of high ionization energy, we do not expect the lighter noble gases like helium, neon, and argon form any compounds with a positive oxidation number of states. It has been suggested that these elements may act as strong electron pair dinner to strong Lewis acids. Some support to the idea comes through the characterization of several XHe+ species in interstellar spaces, where x= an electron-deficient radical. From the calculation, it has been suggested that a number of acetylene derivatives of helium may have sufficient stability.
Cations like He+ and He2+ are producing in the mass spectrometers. At first, He+ formed when a He atom collides with an energetic electron in a high vacuum (He + e → He+ + 2e). Such ions are kept away from the walls of the vessel by magnetic and electric fields and allowed to reach the collector plates of detectors. If the pressure inside the spectrometer is suitably adjusted (10-5 mm Hg), the He+ ion will combine with a neutral helium atom to form an He2+ ion. Such species, stable only in a vacuum, have the chemical bond order of 0.5 and unusually high bond energy.
Superfluidity of Liquid Helium
Liquid helium (4He), when cooled below the normal boing point, it undergoes a transition from He-I to He-II at 2.2K temperature, but when the evocation continues without turbulence, the thermal conductivity raises by the factor of 106, the specific heat increases by a factor of 10 and the viscosity becomes effectively zero. These physical properties of helium describe the characteristics of a superfluid which may be explained by quantum chemistry or physics.
Production And Uses
After hydrogen, helium is the second most abundant chemical element in the universe produced by thermonuclear reactions, alpha decay of radioactive minerals, and natural gas. It is primarily obtained from natural gas, the hydrocarbon is liquefied at about -150 °C, and the helium pumped out.
Helium is used in the laboratory as a refrigerant or coolest substances in low-temperature physics, provide an inert atmosphere in welding metals, propulsion in rocket liquid fuel tanks, in metrological balloons, in cryogenics (as a coolant because the liquid form is the coldest substance), used as a substituent for nitrogen in bathing gas (mixed with oxygen) for deep-sea divers. Helium is also used as a coolant in high-temperature nuclear reactors, for analyzing the chemical properties and composition of rocks in learning chemistry.