chemistry

Helium compounds

Helium compounds is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Helium compounds rather than just read about it. In short: Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly assumed that helium compounds could not exist at all, or at least not under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element.

Key takeaways

Helium compounds belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Helium compounds to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Helium compounds from memory before moving on to harder problems.

Reference excerpt

Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly assumed that helium compounds could not exist at all, or at least not under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero. The helium atom is small, with the radius of the outer electron shell at 0.29 Å. Helium is a very hard atom with a Pearson hardness of 12.3 eV. It has the lowest polarizability of any kind of atom; however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point (4.2 K) of any known substance. Repulsive forces between helium and other atoms may be overcome by high pressures. Helium has been shown to form a crystalline compound with sodium under pressure. Suitable pressures to force helium into solid combinations could be found inside planets. Clathrates are also possible with helium under pressure in ice, and other small molecules such as nitrogen. Other ways to make helium reactive are: to convert it into an ion, or to excite an electron to a higher level, allowing it to form excimers. Ionised helium (He+), also known as He II, is a very high energy material able to extract an electron from any other atom. He+ has an electron configuration like hydrogen, so as well as being ionic it can form covalent bonds. Excimers do not last for long, as the molecule containing the higher energy level helium atom can rapidly decay back to a repulsive ground state, where the two atoms making up the bond repel. However, in some locations such as helium white dwarfs, conditions may be suitable to rapidly form excited helium atoms. The excited helium atom has a 1s electron promoted to 2s. This requires 1,900 kilojoules (450 kcal) per gram of helium, which can be supplied by electron impact, or electric discharge. The 2s excited electron state resembles that of the lithium atom.

Known solid phases Most solid combinations of helium with other substances require high pressure. Helium does not bond with the other atoms, but the substances can have a well defined crystal structure.

Disodium helide

Disodium helide (Na2He) is a compound of helium and sodium that is stable at high pressures above 113 gigapascals (1,130,000 bar). Disodium helide was first predicted using USPEX code and was first synthesised in 2016. It was predicted to be thermodynamically stable over 160 GPa and dynamically stable over 100 GPa. Na2He has a cubic crystal structure, resembling fluorite. At 300 GPa the edge of a unit cell of the crystal has a = 3.95 Å. Each unit cell contains four helium atoms on the centre of the cube faces and corners, and eight sodium atoms at coordinates a quarter cell in from each face. Double electrons (2e−) are positioned on each edge and the centre of the unit cell. Each pair of electrons is spin paired. The presence of these isolated electrons makes this an electride. The helium atoms do not participate in any bonding. However the electron pairs can be considered as an eight-centre two-electron bond. Disodium helide is predicted to be an insulator and transparent.

Silicates Helium was first observed to enter into a silicate in 2007. The mineral melanophlogite is a natural silica clathrate (clathrasil) that normally would contain carbon dioxide, methane or nitrogen. When compressed with helium, a new clathrate forms. This has a much higher bulk modulus, and resists amorphization. Helium was taken up around 17 GPa, enlarging the unit cell, and given off again when pressure dropped to 11 GPa. Cristobalite He II (SiO2He) is stable between 1.7 and 6.4 GPa. It has a rhombohedral space group R-3c with unit cell dimensions a = 9.080 Å, α = 31.809° and V = 184.77 Å3 at 4 GPa. Cristobalite He I (SiO2He) can be formed under higher helium pressures over 6.4 GPa. It has a monoclinic space group P21/C with unit cell dimensions a = 8.062 Å, b = 4.797 Å, c = 9.491 Å, β = 120.43° and V = 316.47 Å3 at 10 GPa. Helium penetrates into fused silica at high pressure, reducing its compressibility. Chibaite, another natural silica clathrate has its structure penetrated by helium under pressures higher than 2.5 GPa. The presence of guest hydrocarbons does not prevent this happening. Neon requires a higher pressure, 4.5 GPa to penetrate, and unlike helium shows hysteresis. Linde-type A zeolites are also rendered less compressible when penetrated by helium between 2 and 7 GPa.

Arsenolite helium inclusion compound

Arsenolite helium inclusion compound As4O6·2He is stable from pressures over 3 GPa and up to at least 30 GPa. Arsenolite is one of the softest and most compressible minerals. Helium prevents amorphization that would otherwise occur in arsenolite under pressure. The solid containing helium is stronger and harder, with a higher sound velocity than plain arsenolite. The helium that is included into the crystal causes a more uniform stress on the As4O6 molecules. No actual bond is formed from arsenic to helium despite the lone pairs of electrons available. The diffusion of helium into arsenolite is a slow process taking days at a pressure around 3 GPa. However, if the pressure on the crystal is too high (13 GPa) helium penetration does not take place, as the gaps between arsenolite molecules become too small. Neon does not diffuse into arsenolite.

Perovskites Helium can be inserted into the A sites of negative thermal expansion perovskites that otherwise have defects at the A site. At room temperature and 350 MPa helium is included into CaZrF6 to expand its unit cell yielding HeCaZrF6. About half of the A sites are filled by helium atoms. This substance loses helium over several minutes on depressurisation at ambient temperature, but below 130 K it retains helium when depressurised. At 1 GPa all the A sites are filled by helium, yielding He2CaZrF6.

Formates Under pressure helium penetrates dimethylammonium iron formate (CH3)2NH2Fe(HCOO)3. It affects this by causing a change to a monoclinic ordered state at a lower pressure (around 4 GPa) than if helium were absent.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Helium compounds

Start with the simplest possible case. Write down what Helium compounds claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Helium compounds before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Helium compounds ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Helium compounds

In research: Helium compounds appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Helium compounds in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Helium compounds is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical compounds by element, Helium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Helium compounds outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Helium compounds in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Helium compounds means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Helium compounds out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Helium compounds in simple terms?

Why does Helium compounds matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Helium compounds?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Helium compounds.