Helium compounds

Helium is a very light gas. It does not like to join with other things. Most people think it stays alone.

But helium can join others if you squeeze it. This needs very high pressure. You might find this deep inside big planets.

Helium can also hide inside tiny cages. These cages can be made of ice. It can even hide inside carbon balls.

Sometimes, helium and sodium make a solid. This happens when they are squeezed hard. It looks like a clear crystal.

Scientists also make a snow-like gel. This gel uses helium and other bits. It can have a lot of power.

Helium is a very light gas. It is one of the most unreactive elements. This means it does not like to join with other things. Most people thought helium could not make compounds at all. This is because its shell of electrons is already full.

However, helium can join with others if you squeeze it. You must use very high pressure to do this. You might find these combinations deep inside big planets. For example, helium can make a solid with sodium. This is called disodium helide. It is a clear crystal that stays stable under high pressure.

Helium can also hide inside tiny cages. These are called clathrates. Some cages are made of ice. Helium can even hide inside carbon balls called fullerenes. In these balls, the helium is trapped inside a small space.

Scientists can also make a snow-like gel. This is called an impurity helium condensate. It forms when other atoms are put into liquid helium. This gel can hold a lot of power. It can have as much power as common explosives. If the gel gets too warm, it can explode.

Helium is a very light and tiny gas. It is a noble gas, which means it is very unreactive. Most people used to think helium could not make compounds at all. This is because its outer shell of electrons is already full. A full shell means the atom does not want to take extra electrons. It also does not want to join with other atoms to make covalent bonds.

To make helium react, you have to use extreme methods. One way is to use very high pressure to squeeze it. You can also turn helium into an ion, which is a charged atom. An ion like He II is very high energy and can pull electrons from other atoms. Another way is to excite an electron to a higher level. This creates something called an excimer. These excimers do not last long because they quickly decay back to a state where the atoms repel each other.

Scientists have discovered many ways that helium behaves under pressure. It can form a crystalline compound with sodium called disodium helide. This was first predicted using a computer tool called USPEX code. It was then actually made in 2016. This compound is stable at pressures above 160 GPa. It has a cubic crystal structure that looks like a mineral called fluorite. In this solid, the helium atoms do not actually bond with the sodium.

Helium can also hide inside tiny cages in other materials. These structures are called clathrates. For example, helium can fit inside cages made of ice. These ice structures might exist deep inside planets like Neptune or Uranus. Helium can also enter carbon balls called fullerenes. It can move into the spaces between these balls in two stages. The first stage happens quickly over a couple of days. The second stage takes thousands of hours to finish.

There is also a strange, snow-like substance called an impurity helium condensate. This forms when other atoms are absorbed into superfluid helium. It looks like a crumbly gel or an aerogel. These gels can hold a huge amount of stored energy. Some versions can have as much power as common explosives. If the temperature rises above 2.19 K, the substance can explode. Researchers first looked at these as a possible kind of rocket fuel in 1974.

Helium is the smallest and lightest noble gas in the periodic table. Because it is a noble gas, it is famously unreactive with other elements. This stability comes from its electron configuration. Helium has a complete outer shell of electrons, which makes it very difficult for the atom to accept more electrons. It also has a very high first ionization energy of 24.57 eV, the highest of any element. This means it requires a massive amount of energy to remove an electron. Because of these factors, scientists once believed that helium compounds could not exist under normal conditions.

To force helium to interact with other atoms, scientists must use extreme methods. One method is applying immense pressure to overcome the repulsive forces between atoms. Another way is to change the state of the helium atom itself. If you turn helium into an ion, known as He II, it becomes a high-energy material. This ion has an electron configuration similar to hydrogen and can form covalent bonds. You can also excite a helium electron to a higher energy level, such as moving a 1s electron to a 2s level. This creates an excimer, a short-lived molecule that eventually decays back to a repulsive state. These excited states can be achieved through electric discharge or electron impact.

One of the most significant discoveries in this field is disodium helide, or Na2He. This crystalline compound was first predicted using the USPEX computer code. It was successfully synthesized in 2016. Disodium helide is stable at high pressures above 160 GPa. It possesses a cubic crystal structure that resembles a mineral called fluorite. In this structure, helium atoms do not actually participate in chemical bonding. Instead, the compound acts as an electride because of isolated electron pairs. These electron pairs are positioned on the edges and the center of the unit cell.

Helium also forms unique structures called clathrates by hiding inside the gaps of other materials. A clathrate is a molecular cage that can trap guest atoms. Under high pressure, helium can enter the crystal structures of silicates. For example, the mineral melanophlogite can form a new clathrate when compressed with helium. Helium can also penetrate the mineral arsenolite. This process is quite slow and can take several days at 3 GPa. Interestingly, if the pressure reaches 13 GPa, the gaps become too small for helium to enter. These clathrates might even exist naturally inside giant planets like Neptune or Uranus.

Another fascinating area of study involves fullerenes, which are carbon molecules shaped like balls. Helium can move into the spaces between these C60 or C70 balls through a process called intercalation. This happens in two distinct stages. The first stage is rapid and takes a few days to fill the larger octahedral sites. The second stage is much slower, taking thousands of hours to fill the smaller tetrahedral sites. Scientists have also created endohedral fullerenes, where helium atoms are trapped directly inside the carbon cage. This is often done by breaking and reforming the carbon structure to achieve higher concentrations.

At extremely low temperatures, helium can form impurity helium condensates, or IHCs. These appear as a snow-like gel when other atoms are absorbed into superfluid helium. The impurities form tiny clusters coated in helium by van der Waals forces. These substances can store a massive amount of energy. For instance, some nitrogen-helium mixtures can have an energy density of up to 5 kJ/g. This makes them as powerful as conventional explosives. In 1974, researchers investigated these materials as a potential fuel for rockets. If the temperature rises above the lambda point of 2.19 K, the substance can explode.

Helium's ability to enter different structures allows it to change the properties of materials. When helium enters certain silicates or zeolites, it reduces their compressibility. In the case of arsenolite, the presence of helium makes the mineral stronger and harder. It also prevents the mineral from becoming amorphous under pressure. These interactions show that while helium is unreactive, it is not truly inert when pushed to its limits. By studying how helium fits into crystals, scientists learn more about how matter behaves under the extreme pressures found deep within planets.