Argon is a noble gas, a colorless, odorless, non-toxic gas that is an important constituent of air. It was first detected in 1894 by Wilhelmine Mohn and Paul Harteck, two chemists at the University of Berlin. Argon is also the second-lightest of all elements.
Argon has a density of 0.7750 g/cm3. Argon is also third among all elements in terms of atomic mass (atomic number 28) and weight (atomic weight 19). Argon has a boiling point of -183 °C at 101 kPa (1 atm). Argon has no known compounds.
The term “argon” is derived from the Greek word for “heavy” or “grave”.
It is the first element in its group and is also the lightest noble gas. Argon is lighter than neon, helium, krypton, and xenon yet heavier than fluorine. Argon has an electrically neutral charge of +0, which is shared with its four lighter congeners.
Argon has a triatomic molecular structure consisting of three atoms in its most common allotrope. It has been assigned the symbol Ar or Ar-3. The isotopes Ar-38, Ar-39, and Ar-40 have been synthesized, but only from tritium by neutron irradiation. The electron configuration of argon is 1s22s22p2. Ar-39 has a half-life of 5.1 × 106 years and Ar-40 has a half-life of 2.4 × 106 years. Ar-38 has a half-life of 8.75 days, so it is radioactive and decays into He3+ (helium 3+) with a half-life of 1 hour. The other two isotopes have been artificially produced from argon gas in a nuclear reaction by bombarding the gas with protons, neutrons, and electrons (a process known as ionization).
Argon comprises three stable isotopes: Argon-38, Argon-39, and Argon-40. Each of these isotopes has a mass number of 28 and a nuclear isomeric mass number (NIM) of 194. Of these, Argon-38 has the longest half-life at 8.75 days, followed by Argon-39 at 10.3 days, and Argon-40 with a half-life of 2.4 × 106 years. Argon-38 is the most abundant isotope at a natural abundance of about 0.711%. Argon-41 with a half-life of less than one second has been artificially synthesized by bombarding argon gas in a nuclear reaction with neutrons (a process known as ionization).
An interesting point to note here is that argon occurs naturally in the atmosphere as Ar+ ion. This is due to the fact that when argon gas is ionized, each atom of argon gains one extra electron due to the separation of an additional electron from a neighboring atom. As a result, there is an excess of electrons in the ionic form and this extra charge gives it a positive charge. This results in Ar+ being in its most stable form, which is then found naturally in the atmosphere.
Consequently, argon is also referred to as a noble gas. This is because noble gases are those that have a very low electron affinity and thus do not readily react with other elements. The common names for the three isotopes of argon are argon-38, argon-39, and argon-40.
Due to its extremely low reactivity, Ar+ and its ions are found in the atmosphere as a result of its ionization. Ar+ ions have a mass of 17 (argon-38) to 21 (argon-40) amu or atomic mass units (amu). The atomic mass unit is the basic unit of atomic masses used in physics. It is defined as the mass of one atom of argon. The atomic number is the number of protons in the atomic nucleus. The ionic radius of Ar+ (18 pm) is also about 3 times that of oxygen-17, which is 13 pm. Ar + ion, on the other hand, has an ionic radius of 18 pm. Argon-38 has a density of 0.7750 g/cm3 and its melting point is -183 °C at 101 kPa (1 atm).
Argon is a nonpolar gas because it has no dipole moment and its covalent bonds are not polarizable due to its nonpolarity (Ar+ has no charge). Argon gas has a boiling point of -183 °C at 101 kPa (1 atm). Argon has a critical point of -64.2 °C at 101 kPa (1 atm).
As a noble gas, argon is inert and chemically unreactive. It does not react with other elements or lose its identity in reactions with compounds. It is a nonpolar gas which means that it does not have any dipole moment and has no electronegativity.
Argon is a non-metal and non-halogen gas. It has no definite chemical properties, as it is highly unreactive. Argon is an inert gas that has no chemical properties.