Argon (atomic number 18; abbreviation Ar) is a colourless, oudorless gas which is classified as one of the noble gases (also known as inert gases). It is commonly found in certain industrial activities and in certain types of electric lighting, and is also used in an important form of archaeological rock dating.
Argon is actually present in the Earth's atmosphere at levels slightly above those of carbon dioxide. It was from the atmosphere that early Scottish chemists Rayleigh and William Ramsay first isolated it in the 1890s, confirming the presence of what they called argon after the Greek word for inert. (This description was later given to the class of inert gases, of which argon is a member.) Just as Henry Cavendish had theorized a century earlier, Rayleigh and Ramsay found that argon was even less reactive than nitrogen, the other - mostly but not entirely - inert component found in the atmosphere, and by far the most common.
Argon is in most respects a typical inert gas: It has no colour, no odour, is generally nontoxic, and fails to react with most other substances. A small number of argon compounds have been created under specific laboratory conditions by researchers, such as argon fluorohydride, but in general, argon is entirely non-reactive. (This non-reactiveness has to do with the chemical bonding process and is common to all of the noble gases.)
Argon is not found in large quantities in nature (at least on Earth; much higher quantities have been found on Mars), but large amounts of the gas are produced every year for industrial purposes by separating it from the air (air can be distilled to yield separate samples of nitrogen, argon, oxygen, and carbon dioxide). Most of what is produced is used by industry in areas where gases are needed that will accept high temperatures and not react with other substances, such as certain graphite furnaces and welding systems. In most cases where it used, argon is not the only substance which would be suitable; it is simply the cheapest to acquire of the inert gases, because of its presence in the atmosphere around us.
In everyday life, however, the two most common places people (unknowingly) encounter argon are in windows and lights. Incandescent lighting makes use of argon, which allows wiring inside the bulb to heat to high temperatures without risk of fire. In addition, argon is used in windows that have been rated for energy efficiency: between double panes of glass, argon is a particularly useful gas for providing a slightly added degree of insulation.
Argon has also emerged as one of several useful techniques for dating rocks to periods beyond the range of carbon dating - typically in archaeological and paleontological fieldwork. Argon-40 is an isotope with a degree of radioactivity, and a half-life of about 270 years. Archaeologists and paleontologists currently make use of this in a form of chemical dating of samples. Realizing that once embedded in rock small amounts of argon-40 slowly radioactively decay into argon-39, and small amounts of potassium (another element) slowly decay into potassium, scientists can estimate how long a piece of rock has been buried on the basis of how much of this decay has taken place. Argon dating was used to fix the estimated extinction of the dinosaurs (and end of the Triassic Period) at 66 million years ago.