We continue to review some of the most important materials in heat treatment and metallurgy.
Argon (chemical symbol: Ar)
Argon is an odorless, colorless, nonflammable gas that is almost completely inert and does not combine with other elements except under extreme conditions. This is due to the fact that its electron shells are full. Argon has three electron shells. The third shell is filled with eight electrons (Fig. 1), which is the maximum allowable, so none are available to bond with other elements.
Argon is one of the seven noble gases, the others being helium (He), neon (Ne), krypton (Kr), xenon (Xe), radon (Rn) and oganesson (Og). The term noble gas was coined in 1898 by German chemist Hugo Erdmann and is translated from the German noun Edelgas. The term was a takeoff of “noble metals,” which was already used to describe particularly nonreactive metals. The reason for the inertness of the noble gases can be seen in their electron configurations. The maximum number of electrons possible in the first (innermost) electron shell is two. The second shell can hold eight (for a total of 10), and the third holds eight more (for a total of 18), as reflected by each gas’s atomic number.
The first four noble gases in the periodic table and their atomic numbers are: helium (2), neon (10), argon (18) and krypton (38).
Note that each of the noble gases has a full electron shell. Helium, with an atomic number of two, and therefore two electrons, has a full shell represented in red in Figure 1. Neon, with 10 electrons, has the first two shells filled, represented by the color blue. Argon, with 18 electrons, has a full third shell, represented in green and so on.
Argon is the third-most abundant gas (at 0.934%) in the Earth's atmosphere after nitrogen and oxygen. It was first identified in 1894 by Lord Rayleigh and Sir William Ramsay, who won Nobel prizes as a result. Rayleigh and Ramsey removed oxygen, carbon dioxide, water and nitrogen from a sample of clean air and noticed the remaining gas was heavier than nitrogen. Continued investigation revealed it was a new element, and it was named argon, which is Greek for “lazy” or “inactive.”
Argon’s most common use in industry capitalizes on its inertness. It is common as an inert gas in vacuum furnaces for the aerospace industry, as a shield gas in arc welding and cutting, and to form a nonreactive cover gas in the manufacture of titanium and other reactive metals. It is also used to provide a protective atmosphere during the growth of silicon and germanium crystals (Fig. 2).
Here are some interesting facts about argon.
- Melting point: 83.81 K (−189.34 °C, −308.81 °F)
- Boiling point: 87.302 K (−185.848 °C, −302.526 °F)
- Density (at STP): 1.784 g/L
- Density as a liquid (at b.p.): 1.3954 g/cm3
- Triple point: 83.8058 K, 68.89 kPa
- Critical point: 150.687 K, 4.863 MPa
- Heat of fusion: 1.18 kJ/mole
- Heat of vaporization: 6.53 kJ/mole
- Molar heat capacity: 20.85 J/(mole·K)