Helium
He
Atomic Number: 2
Atomic Weight: 4.002602
Melting Point: 0.95 K (-272.2°C or -458.0°F)
Boiling Point: 4.22 K (-268.93°C or -452.07°F)
Density: 0.0001785 grams per cubic centimeter
Phase at Room Temperature: Gas
Element Classification: Non-metal
Period Number: 1
Group Number: 18
Group Name: Noble Gas
History
Helium, the second most abundant element in the universe, was discovered on the sun before it was found on the earth. Pierre-Jules-César Janssen, a French astronomer, noticed a yellow line in the sun's spectrum while studying a total solar eclipse in 1868. Sir Norman Lockyer, an English astronomer, realized that this line, with a wavelength of 587.49 nanometers, could not be produced by any element known at the time. It was hypothesized that a new element on the sun was responsible for this mysterious yellow emission. This unknown element was named helium by Lockyer.
Uses of Helium
The uses of helium depend either on its small atomic mass, or on its chemical inertness. Many applications involve both properties. The uses of argon depend on its inertness. Both helium and argon are available in industrial quantities, and are relatively cheap. The rarer inert gases Ne, Kr and Xe are used in very small amounts in electrical discharges, together with He and A. Xe, as we have remarked, is useful as an anaesthetic.
Helium and argon are used in welding to shield the hot metal from the atmosphere, especially in the case of reactive metals. Classic arc welding uses consumable electrodes coated in a flux. In the heat of the arc, the flux evolves protective gases and forms a liquid slag to protect the metal. In gas metal arc welding (GMAW), helium or argon is blown on the weld, protecting it without producing a slag and excluding oxygen or hydrogen. This seems to have been developed in World War II for welding magnesium, but has now been applied much more generally. It is generally considered necessary for good welds in titanium, as well as for aluminium and stainess steel, where it eliminates weld porosity. In gas tungsten arc welding (GTAW), non-consumable tungsten electrodes are used. Helium is often used as an inert atmosphere in growing semiconductor crystals and for similar processes.
The major use of argon is to fill incandescent lamp bulbs. Although the gas cools the filament somewhat, evaporation of tungsten is discouraged and the life of the lamps is significantly extended.
The small atomic mass leads to large thermal velocities, and this implies rapid diffusion and easy heat transfer. Hydrogen leads in these properties, of course, but helium is not far behind. Helium is used for leak detection in vacuum systems. The gas is blown around likely leak sources, and will diffuse much more rapidly than air through even the smallest opening. The helium is easily detected by an ionization gauge or other means, signalling the presence of a leak. The rapid diffusion makes helium a good carrier gas for gas chromatography. Helium is also used as a driver gas in hypersonic wind tunnels. The high thermal conductivity, and its zero neutron capture cross section, make helium a good coolant in gas-cooled nuclear reactors, though its low density works against it.
Helium gained fame as a lifting gas, and we have seen that this spurred the large-scale production and conservation of the gas, in spite of its inherent rarity. We have already remarked that it is nearly as effective as hydrogen in this respect, as 25 is to 27. The net lift of a balloon is the weight of the air displaced, less the weight of the balloon, lifting gas and ballast. All balloons using hydrogen or helium are strongly affected by the rapid diffusion of these gases, which soon leads to a decrease in lift, which is compensated by throwing ballast overboard. Incidentally, the lifting gas is contained in separate bags like the compartments of a ship, so that the compromise of one is not the loss of all. The use of helium in small balloons instead of hydrogen seems to be a frivolous waste of a valuable substance. Of course, the reason it is used is the elimination of the hazards of hydrogen combustion, perhaps more in the gas sources than in the actual balloon. If the predictions of a "hydrogen economy" do indeed come true, we will hear more about the dangers of hydrogen. All the problems of working with helium arise with hydrogen as well, with the added excitement of explosion.
The inert gases are excellent as filling gases for electrical discharges, since they do not react with the electrodes, and tubes containing them have long service lives. Neon and argon are used in glow discharge lamps, where the light comes principally from the negative glow near the cathode. Neon has its typical orange-red color, while argon produces a violet. The electrodes are specially treated to encourage electron emission, so that the striking voltage of the lamps is low. "Neon lights" use the positive column in a long tube as the light source. Helium gives a yellow light, and neon red-orange. Mercury with argon to aid starting gives blue. Colored glass tubing is used to create other colors. The pressure is a few mm Hg, and the voltage drop around 100 V per metre. These lamps are supplied with a high-voltage transformer with large leakage reactance so that the voltage is high for striking, and decreases when current flows. The current may be 25 to 50 mA.
A discharge through a mixture of helium and neon creates a population inversion in the neon that can be used in a laser. The helium is ionized in the discharge, and when the electrons recombine and cascade down in energy, they pile up in the two metastable states 2s3S (19.82 eV) and 2s1 (20.62 eV) of the electron configuration 1s2s. Fast transitions to the ground state are discouraged by the selection rules, in one case forbidding triplet-singlet transitions and in the other S-S transitions. By good luck, the energies of each of these metastable states is about equal to the energies of two upper levels of laser transitions in neon. The transfer of energy is quite likely, populating these states. One state makes a transition of wavelength 632.8 nm, the familiar He-Ne visible red laser line, to a lower state, while the other makes a transition of 1152.3 nm to the same lower state. This lower state is kept empty by allowed transitions of wavelength 594.5 nm to a still lower state that is depopulated by collisions with the walls of the discharge tube.
All of the inert gases have a large energy gap between the ground state and the first excited state, corresponding to a transition in the far ultraviolet. The visible lines in the spectra are due to transitions between higher states. This explains why the inert gases are transparent, since ordinary visible frequencies are too low to excite them when they are in the ground state. The helium spectrum in a discharge tube shows strong lines at 447.1 nm (blue), 501.6 nm (green), 587.56 nm (yellow) and 667.8 nm (red). This spread of lines creates a whitish light. Neon has a cluster of lines in the red and orange, while argon has groups of lines in the blue and in the red.