Helium is a chemical element of group 18 (noble gases) that belongs to the first period of the periodic table. The name “noble gases” suggests that they tend to be non-reactive, analogous to noble metals, which hardly react.
Helium has two electrons, which complete the 1s2 shell, unlike the other noble gases, which have a complete octet of electrons, ns2np6, in the outermost shell. This electronic configuration is very stable and is related to the low chemical reactivity of this element. Helium has an electron affinity equal to zero (or slightly negative), and a very high ionization energy. Under normal conditions, it has little tendency to lose or gain electrons. Therefore, the chances of this element forming bonds are very small, and it usually occurs in the form of an isolated atom.
Because it has a lower boiling point than any other known liquid, helium is used in cryogenics to achieve extremely low temperatures, necessary for the operation of lasers and devices based on superconducting materials. It is also used as a cooling gas in one type of gas-cooled nuclear reactor and as a carrier gas in gas-liquid chromatography. It is still used in weather balloons and airships. Although H2 is cheaper, more easily available, and has a lower density, for safety reasons, helium is preferably used in airships.
Helium is less dense than air. For reference, 1 m3 of He, at atmospheric pressure, can lift up to 1 kg. Helium is widely used to dilute the oxygen in diver’s air tanks, although nitrogen can also be used for this purpose. Helium is preferred because nitrogen is highly soluble in blood, and a sudden pressure change can cause N2 bubbles to form in the bloodstream. This painful (and sometimes fatal) accident is known as “decompression sickness” or “the bends.” Helium is much less soluble in blood, and with its use in air tanks, the risk of this accident is greatly reduced.
As for its physical presentation, helium is a monoatomic, colorless, and odorless gas. The enthalpy of vaporization is a measure of the forces that hold atoms together. In helium, this value is very low because the only forces between atoms are very weak van der Waals forces. Due to these weak intermolecular forces, the melting and boiling points of the element are very low.
Helium is able to diffuse through glass, rubber, plastic materials, and some metals. Because of this, handling it in the laboratory is difficult, especially since glass Dewar flasks cannot be used experimentally at low temperatures.