Noble gas compound

Noble gas compounds are chemical substances that include an element from group 18 of the periodic table. For a long time, scientists believed these elements were completely inert, meaning they could not react with anything. This belief was based on the fact that noble gases have full valence shells of electrons. A valence shell is the outermost layer of electrons around an atom. Having a full shell makes an atom very stable and resistant to chemical changes. However, researchers eventually discovered that many compounds could indeed be formed, particularly using heavier noble gases.

The ability of a noble gas to form a compound depends heavily on its atomic structure. Heavier noble gases like krypton, xenon, and radon have more electron shells than lighter ones like helium or neon. Because they have more shells, the outermost electrons are much further from the positively charged nucleus. These outer electrons experience a shielding effect from the inner electrons. This shielding reduces the electrostatic attraction between the nucleus and the outer electrons. As a result, these gases have lower ionization energies, which is the energy required to remove an electron. Because it is easier to move these electrons, they can form stable bonds with highly electronegative elements like fluorine and oxygen.

The history of these discoveries is a story of breaking scientific assumptions. In 1933, the chemist Linus Pauling predicted that heavy noble gases could form compounds with fluorine and oxygen. He specifically suggested the existence of krypton hexafluoride and xenon hexafluoride. Despite his accurate predictions, no one had successfully synthesized a noble gas compound by 1960. The breakthrough occurred in June 1962 when Neil Bartlett performed a famous experiment. Bartlett noticed that platinum hexafluoride could ionize. He observed that the ionization energy of platinum hexafluoride was 1165 kJ/mol, which was very close to the 1170 kJ/mol required to ionize xenon. He reacted xenon with platinum hexafluoride and produced a crystalline product called xenon hexafluoroplatinate. This was the first confirmed noble gas compound.

Following Bartlett's success, many different types of xenon compounds were identified. Scientists have synthesized various xenon fluorides, such as xenon tetrafluoride, and xenon oxides. Xenon can even bond with less common elements like nitrogen, carbon, boron, and gold. The variety of xenon compounds is vast, with some research suggesting thousands of possible combinations. One notable example is the compound Xe₂, which contains a Xe–Xe bond. This is actually the longest known element-element bond, measuring 308.71 pm. Xenon is also used in excimer lasers, which utilize short-lived molecules called excimers.

Other noble gases also form unique structures, though they often require extreme conditions. Krypton can react with fluorine to form krypton difluoride, but this requires intense forcing. Argon is much harder to bond, yet scientists have discovered argon monohydride and the argon hydride ion. Interestingly, the argon hydride ion has been identified in space within the Crab Nebula. Even the lightest gas, helium, has seen recent breakthroughs. In 2016, researchers created disodium helide. This was the first helium compound ever discovered. In this substance, the helium atoms help stabilize the solid lattice rather than forming traditional chemical bonds.

Sometimes, noble gases are not bonded chemically but are instead trapped physically. One method is through the creation of clathrates, also known as cage compounds. In a clathrate, noble gas atoms are trapped inside the cavities of a crystal lattice made of other substances. For example, argon, krypton, and xenon can be trapped within crystalline hydroquinone. Another method involves endohedral fullerenes. A fullerene is a tiny cage made of carbon atoms. Scientists can trap helium, neon, or xenon inside these carbon cages. In 1993, it was found that exposing C60 to high pressure could trap helium or neon atoms inside the cages.

Noble gas compounds have several important scientific and industrial uses. Many of these compounds act as powerful oxidizing agents, which are substances that can cause other materials to lose electrons. Xenic acid is a valuable oxidizing agent because it does not introduce impurities into a reaction; when it reacts, the xenon is simply released as a gas. Perxenates are even more powerful in this role. Additionally, noble gases are often stored in dense forms using these chemical processes. Understanding these compounds helps scientists explore the limits of chemical bonding and the behavior of matter under extreme pressure and temperature.