Neon compounds

Neon is a chemical element that belongs to the noble gas group on the periodic table. For a long time, scientists believed that neon could not form any chemical compounds. A compound is a substance formed when different atoms or molecules join together. Today, we know that neon can indeed form various types of connections. These include molecular ions, which are atoms with an electric charge, and excimers. Excimers are temporary molecules that exist only when they are in an excited state. While neon is the least reactive element, its ability to interact with other substances is a fascinating area of study.

To understand why neon is so quiet, we must look at its energy levels. Neon has a very high first ionization potential of 21.564 eV. This is the amount of energy required to pull an electron away from the atom. Only helium has a higher ionization potential at 24.587 eV. Because it takes so much energy to move electrons, stable ionic compounds are very difficult to create. Additionally, neon has a very low polarisability of 0.395 Å3. Polarisability describes how easily an atom's electron cloud can be distorted. Because neon's cloud is hard to distort, it has a very low tendency to link to other atoms.

Despite this low reactivity, neon can form Van der Waals molecules. In these structures, neon is held to other components by London dispersion forces. These are very weak forces that can be easily broken by molecular vibration. If the temperature rises above that of solid neon, these bonds will disrupt. Scientists use expanding supersonic jets of neon gas to study these molecules. They have identified neon clusters like the dimer, trimer, and tetramer. The neon dimer has an average distance of 3.3 Å between atoms. The neon trimer is shaped like an equilateral triangle with 3.3 Å sides, though it is often a floppy isosceles triangle. The neon tetramer forms a tetrahedron with sides of about 3.2 Å.

Neon can also act as a ligand, which is a molecule that attaches to a central metal atom. It forms very weak bonds with transition metals such as chromium, molybdenum, and tungsten. For example, the molecule Cr(CO)5Ne contains neon. In some cases, neon is attracted to atoms with a positive charge. In the cyclic molecule Be2O2, neon atoms are attracted to the beryllium atoms. When neon is added to this molecule, the O-Be bond lengths increase and the ∠O-Be-O angle decreases. Researchers have even detected NeBeCO3 using infrared spectroscopy in a solid neon matrix. This was created using beryllium gas, dioxygen, and carbon monoxide.

High pressure allows neon to be trapped in unique ways. It can form clathrates, which are cage-like structures, in ice II at 480 MPa. Neon atoms can also be trapped inside fullerenes, which are carbon cages like C60 and C70. To put neon inside buckminsterfullerene, scientists heat it to 600 °C under three atmospheres of pressure for one hour. This process is rare, with only about 1 in 8,500,000 molecules capturing a neon atom. This trapped neon will escape if the temperature reaches 900 °C. Neon can also intercalate, or pack into the spaces, of fullerene crystals. This is only stable at low temperatures, as it degasses in under 24 hours at standard conditions.

In the world of ions, neon can form complex clusters with metal ions. The shape of these clusters depends on the repulsion between neon atoms and the metal's d-orbital electrons. For instance, the copper ion Cu+ can hold up to 24 neon atoms. The Cu+Ne12 cluster takes an icosahedral shape. Other ions, like the helium neonide cation (HeNe+), show relatively strong covalent bonds. Scientists have also observed neon-containing molecular anions. In 2020, the anion [B12(CN)11Ne]− was reported. This molecule remained stable up to 50 K, which is much higher than the neon condensation temperature of 25 K.

Finally, neon plays a role in specialized technology and space science. Excimers are used in excimer lamps to emit vacuum ultraviolet light. These lamps peak at 83 nm, but because no window material can transmit such short wavelengths, they must be used in a vacuum. In space, the isotope 22Ne is found in carbonaceous chondrite meteorites at levels 1,000 times higher than on Earth. This happens because carbon cages formed after a supernova trapped sodium atoms. These atoms, like 22Na, later decayed into 22Ne. This connection between neon, meteorites, and the history of the universe shows how even the most "inert" elements tell a story.