Short answers, pulled from the story.
Argon is a chemical element with symbol Ar and atomic number 18. It is a noble gas in group 18 of the periodic table. The name comes from the Ancient Greek word meaning lazy or inactive, a reference to the element's near-total resistance to chemical reactions.
Argon was first isolated from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London. They removed oxygen, carbon dioxide, water, and nitrogen from a clean air sample and found an unreactive residue. H. F. Newall and W. N. Hartley had independently observed unidentified spectral lines in air in 1882, though neither isolated the gas.
Argon constitutes 0.934 percent by volume of Earth's atmosphere, making it the third most abundant atmospheric gas. It is more than twice as abundant as water vapor on average, 23 times more abundant than carbon dioxide, and more than 500 times more abundant than neon.
Argon can form a small number of compounds under extreme conditions, despite its reputation for chemical inertness. Argon fluorohydride (HArF) was formed in August 2000 by researchers at the University of Helsinki and is stable only below 17 kelvins. Argonium ions have also been detected in interstellar space associated with the Crab Nebula, the first noble-gas molecule found in outer space.
Argon is widely used as a shielding gas in arc welding processes such as gas metal arc welding and gas tungsten arc welding. It is also used in graphite electric furnaces to prevent burning, for growing silicon and germanium crystals, in incandescent light bulbs to protect filaments, and in gas-discharge lamps and argon-ion lasers.
Since 2002, the American National Archives has stored the Declaration of Independence and the Constitution in argon-filled cases to inhibit their degradation. Argon replaced helium, which had been used for the preceding five decades but escapes through the intermolecular pores of most containers and requires regular replacement.