chemistry

Helium trimer

Helium trimer 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 trimer rather than just read about it. In short: The helium trimer (or trihelium) is a weakly bound molecule consisting of three helium atoms. Van der Waals forces link the atoms together.

Key takeaways

Helium trimer 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 trimer to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Helium trimer from memory before moving on to harder problems.

Reference excerpt

The helium trimer (or trihelium) is a weakly bound molecule consisting of three helium atoms. Van der Waals forces link the atoms together. The combination of three atoms is much more stable than the two-atom helium dimer. The three-atom combination of helium-4 atoms is an Efimov state. Helium-3 is predicted to form a trimer, although ground state dimers containing helium-3 are completely unstable. Helium trimer molecules have been produced by expanding cold helium gas from a nozzle into a vacuum chamber. Such a set up also produces the helium dimer and other helium atom clusters. The existence of the molecule was proven by matter wave diffraction through a diffraction grating. Properties of the molecules can be discovered by Coulomb explosion imaging. In this process, a laser ionizes all three atoms simultaneously, which then fly away from each other due to electrostatic repulsion and are detected. The helium trimer is large, being more than 100 Å, which is even larger than the helium dimer. The atoms are not arranged in an equilateral triangle, but instead form random shaped triangles. Interatomic Coulombic decay can occur when one atom is ionised and excited. It can transfer energy to another atom in the trimer, even though they are separated. However this is much more likely to occur when the atoms are close together, and so the interatomic distances measured by this vary with half full height from 3.3 to 12 Å. The predicted mean distance for Interatomic Coulombic decay in 4He3 is 10.4 Å. For 3He4He2 this distance is even larger at 20.5 Å.

References

Extra reading Suno, Hiroya (14 January 2016). "Geometrical structure of helium triatomic systems: comparison with the neon trimer". Journal of Physics B: Atomic, Molecular and Optical Physics. 49 (1) 014003. Bibcode:2016JPhB...49a4003S. doi:10.1088/0953-4075/49/1/014003. S2CID 124846893.

Illustrations

Worked examples

Example 1 — a first encounter with Helium trimer

Start with the simplest possible case. Write down what Helium trimer 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 trimer 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 trimer 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 trimer

In research: Helium trimer 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 trimer 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 trimer is common in secondary-school and first-year university syllabi. It links to neighbouring topics Allotropes, Helium compounds, Homonuclear triatomic molecules, so understanding it makes those chapters shorter.
In everyday life: Look for Helium trimer 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 trimer in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Helium trimer 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 trimer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Helium trimer in simple terms?

The helium trimer (or trihelium) is a weakly bound molecule consisting of three helium atoms. Van der Waals forces link the atoms together.

Why does Helium trimer 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 trimer?

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 trimer.