chemistry

Isotopes of helium

Isotopes of helium 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 Isotopes of helium rather than just read about it. In short: Helium (2He) has nine known isotopes, but only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived; the only particle-bound ones are 6He and 8He with half-lives 806.9 and 119.5 milliseconds.

Key takeaways

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

Reference excerpt

Helium (2He) has nine known isotopes, but only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived; the only particle-bound ones are 6He and 8He with half-lives 806.9 and 119.5 milliseconds. In Earth's atmosphere, the ratio of 3He to 4He is 1.37×10−6. However, the isotopic abundance of helium varies greatly depending on its origin, though helium-4 is always in great preponderance. In the Local Interstellar Cloud, the proportion of 3He to 4He is 1.62(29)×10−4, which is about 120 times higher than in Earth's atmosphere. Rocks from Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to investigate the origin of rocks and the composition of the Earth's mantle. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. Equal mixtures of liquid 3He and 4He below 0.8 K separate into two immiscible phases due to differences in quantum statistics: 4He atoms are bosons while 3He atoms are fermions. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures as low as a few millikelvin. A mix of the two isotopes spontaneously separates into 3He-rich and 4He-rich regions. Phase separation also exists in ultracold gas systems. It has been shown experimentally in a two-component ultracold Fermi gas case. The phase separation can compete with other phenomena as vortex lattice formation or an exotic Fulde–Ferrell–Larkin–Ovchinnikov phase.

List of isotopes

Helium-2 (diproton)

Helium-2, 2He, is unbound. The only bound atom with a mass number of 2 is deuterium. The nucleus of 2He, a diproton, consists of two protons with no neutrons. Its instability is due to spin–spin interactions in the nuclear force and the Pauli exclusion principle, which states that within a given quantum system two or more identical particles with the same half-integer spins (fermions) cannot simultaneously occupy the same quantum state; so 2He's two protons have opposite-aligned spins and the diproton itself has negative binding energy. A rare form of radioactivity is diproton emission, where a nucleus emits two protons in a quasi-bound 1S0 configuration, which then separate. In 2000, Oak Ridge National Laboratory detected two-proton emission from 1810Ne, produced by a 179F ion beam onto a proton-rich target. But the experiment didn't have the sensitivity to distinguish if the emission was a decay by two separate protons, or by a diproton. In 2008, the Istituto Nazionale di Fisica Nucleare confirmed 18Ne decayed to a diproton with a 31% branching ratio. Several experiments have since detected diproton emission from other isotopes. The lack of a bound diproton has been used to argue for fine-tuning for the development of life due to its effect on Big Bang nucleosynthesis and stellar evolution. Hypothetical models suggest that if the strong force was 2% greater, then diprotons would be bound (but still β+ decay to deuterium). Recent studies have found that a universe with bound diprotons doesn’t preclude the development of stars and life. One impact of a hypothetical bound diproton is a change to the early steps of the proton-proton chain. In our universe, the first step of the proton-proton chain proceeds via the weak force,

p + p → 21D + e+ + νe + 0.42 MeV In the hypothetical, instead a diproton could form without the weak force,

p + p → 22He The diproton would then beta-plus decay into deuterium:

22He → 21D + e+ + νe. With the overall formula,

p + p → 21D + e+ + νe. Under the influence of electromagnetic interactions, the Jaffe-Low primitives may leave the unitary cut, creating narrow two-nucleon resonances, like a diproton resonance with a mass of 2000 MeV and a width of a few hundred keV. To search for this resonance, a beam of protons with kinetic energy 250 MeV and an energy spread below 100 keV is required, which is feasible considering the electron cooling of the beam.

Helium-3

3He is the only stable isotope other than 1H with more protons than neutrons. There are many such unstable isotopes, such as 7Be and 8B. There is only a trace (~2ppm) of 3He on Earth, mainly present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, 3He is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, has traces of 3He from solar wind bombardment. To become superfluid, 3He must be cooled to 2.5 millikelvin, ~900 times lower than 4He (2.17 K). This difference is explained by quantum statistics: 3He atoms are fermions, while 4He atoms are bosons, which condense to a superfluid more easily.

Helium-4

The most common isotope, 4He, is produced on Earth by alpha decay of heavier elements; the alpha particles that emerge are fully ionized 4He nuclei. 4He is an unusually stable nucleus because it is doubly magic. It was formed in enormous quantities in Big Bang nucleosynthesis. Terrestrial helium consists almost exclusively (all but ~2ppm) of 4He. 4He's boiling point of 4.2 K is the lowest of all known substances except 3He. When cooled further to 2.17 K, it becomes a unique superfluid with zero viscosity. It solidifies only at pressures above 25 atmospheres, where it melts at 0.95 K.

Helium-5

Helium-5 is extremely unstable, decaying to helium-4 with a half-life of 602 yoctoseconds. It is briefly produced in the favorable fusion reaction:

2 H +

3 H ⟶

5 H e ∗ ⟶

4 H e + n + 17.6 M e V {\displaystyle {}^{2}\mathrm {H} +{}^{3}\mathrm {H} \longrightarrow {}^{5}\mathrm {He} ^{*}\longrightarrow {}^{4}\mathrm {He} +n+17.6\ \mathrm {MeV} }

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Isotopes of helium

Start with the simplest possible case. Write down what Isotopes of helium 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 Isotopes of helium 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 Isotopes of helium 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 Isotopes of helium

In research: Isotopes of helium 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 Isotopes of helium 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: Isotopes of helium is common in secondary-school and first-year university syllabi. It links to neighbouring topics Helium, Isotopes of helium, Lists of isotopes by element, so understanding it makes those chapters shorter.
In everyday life: Look for Isotopes of helium 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 Isotopes of helium in 20 minutes

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

Frequently asked questions

What is Isotopes of helium in simple terms?

Why does Isotopes of helium 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 Isotopes of helium?

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 Isotopes of helium.