Helium Element Properties
Helium chemical symbol He, atomic number 2 is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas or chemical element of Group 18 of the periodic table. It is the second lightest element after hydrogen which can not be solidified at normal temperature and pressure.
During the solar eclipse, a new yellow line of the electromagnetic spectrum was first observed in the sun chromophore. The scientist Lockyer and Frankland assigned it for the new element named helium from the Greek word helios meaning sun. The same line was observed by L. Palmieri in the spectrum of volcanic gases. Ramsay suggested that helium and argon belong to a new group like zero groups in the periodic table with the other three gases to complete the table.
|Properties of Helium|
|Melting point||−272.20 °C, −457.96 °F|
|Boiling Point||−268.928 °C, −452.070 °F|
|Density||At gas 0.1786 g/L
At liquid 0.145 g/cm3
|Ionization energies||1st – 2372.3 kJ/mol
2nd – 5250.5 kJ/mo
Abundance and Occurrence
Helium (23 percent) is the second most abundant element after hydrogen (76 percent). Due to its lightweight, it is present in the earth’s atmosphere very negligible amount to the extent of one percent. Helium consists of two stable isotopes like 4He and 3He. He-4 is formed by alpha-ray decay of radioactive elements and He-3 is formed by the nuclear reaction of cosmic radiation. It occurs to the extent of 5.24 ppm by volume in dry air and 3 × 10-3 ppm by weight of igneous rocks. The most economical source of helium is natural gas which contains 7 percent of the noble gas.
Chemistry of Helium Gas
Helium is the second least reactive noble gas after neon and the second least reactive element of the periodic table. The lack of chemical reactivity of noble gases like helium is the reason to call it inert gas. All the elements of the noble gas family, except helium, have an s2p6 configuration but He has a closed-shell 1s2 configuration. The electronic configuration of the elements suggests that it contains completely filled s-orbitals with a very stable configuration. The energy required for the promotion of an electron to the next available orbitals is quite large. For this reason, covalent bonding by hybridization in helium atoms is highly unfavorable.
Due to very high ionization energy, ionic bonding is similarly difficult for the said element. The ionization energy of elements along the group decreases for higher members of the family.
Cations like He+ and He2+ are formed in the mass spectrometer. First, He+ is formed when a He atom collides with an energetic electron in a high vacuum. If the pressure inside the spectrometer is suitably adjusted, the He+ ion combines with the natural He atom to form a He2+ ion. Such species are stable in vacuum only with bond order 0.5 and very high bond energy.
Uses of Helium
Liquid helium is used in the laboratory as a refrigerant in low-temperature physics, for the formation of the inert atmosphere in welding metals, in various scientific experiments, and in meteorological balloons. It is used as a substituent for nitrogen in the breathing gas for deep-sea rivers. In a high-temperature nuclear reactor, helium is used as a coolant for the reactor in nuclear power generation.