Argon

Argon is a chemical element with the symbol Ar and the atomic number 18. It belongs to group 18 on the periodic table, which is the category of noble gases.

Noble gases are unique because they are very stable. Argon is the third most abundant gas in Earth's atmosphere. It makes up 0.934% of our air by volume. This means it is much more common than water vapor. It is also 23 times more abundant than carbon dioxide. Compared to neon, argon is more than 500 times more common.

Argon is chemically inert, which means it does not react easily with other things. This happens because argon has a complete octet of electrons. An octet is a full outer shell of eight electrons. This full shell makes the atom very stable. It resists bonding with other elements under most conditions. Because of this inactivity, the name "argon" comes from an Ancient Greek word meaning "lazy" or "inactive."

While it is mostly unreactive, scientists have found it can form compounds under extreme conditions. For example, argon fluorohydride (HArF) is stable below 17 kelvins. Argon can also form clathrates. These are structures where argon atoms are trapped inside a lattice of water molecules.

There are different versions of argon called isotopes. On Earth, argon-40 is the most common isotope at 99.6%. This specific isotope is radiogenic. It is created by the decay of potassium-40 in the Earth's crust. Scientists use the ratio of argon isotopes to determine the age of rocks. This process is called K–Ar dating. In the wider universe, argon-36 is the most common isotope. This is because argon-36 is produced during stellar nucleosynthesis in supernovas.

The isotopic makeup of argon varies greatly across our solar system. For instance, the atmospheres of Mars and Mercury are dominated by argon-40.

History shows that argon was a puzzle for early chemists. In 1785, Henry Cavendish suspected an unreactive gas existed in the air. Later, in 1894, Lord Rayleigh and Sir William Ramsay isolated it. They worked at University College London. They removed oxygen, carbon dioxide, water, and nitrogen from clean air. They used an electric arc and an alkali solution to help the process.

Before they isolated it, they noticed nitrogen from compounds was slightly lighter than atmospheric nitrogen. This led them to realize another gas was mixed in. Before 1957, the symbol for argon was simply "A." The International Union of Pure and Applied Chemistry changed it to "Ar" later that year.

Industrially, argon is produced through fractional distillation. This process happens in a cryogenic air separation unit. Scientists cool air until it becomes liquid. Then, they separate the liquids based on their boiling points. Nitrogen boils at 77.3 K, argon at 87.3 K, and oxygen at 90.2 K.

This method makes argon very inexpensive to produce. About 700,000 tonnes of argon are produced worldwide every year. Because it is a byproduct of making oxygen and nitrogen, it is very widely available. This makes it a cheaper alternative to other noble gases for many jobs.

Argon has many important uses in industry and science. Because it is inert, it is used as a shielding gas in welding. It protects metals like titanium during high-temperature processes. In graphite electric furnaces, an argon atmosphere prevents the graphite from burning. It is also used in incandescent and fluorescent lighting. Argon can even create a distinctive blue-green gas laser.

In the food industry, argon is used to replace oxygen. This helps to enhance the shelf life of products. It is also used to extinguish fires. This is helpful for protecting expensive server equipment that water might damage.

In advanced scientific research, liquid argon is very useful. It is used as a target in experiments searching for neutrinos. It is also used in searches for dark matter. Scientists look for light produced when particles hit an argon nucleus.

Argon's discovery also helped fix the periodic table. Early scientists like Mendeleev struggled with argon's weight. Because it was so unreactive, they weren't sure where to put it. Henry Moseley eventually showed that the table should be arranged by atomic number. This solved the mystery of argon's placement and helped organize all known elements.