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helium-4
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phase diagrams of helium-3 and helium-4 The phase diagram of helium-4 shows which states of this isotope are stable.
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helium-4

chemical isotope
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phase diagrams of helium-3 and helium-4
phase diagrams of helium-3 and helium-4 The phase diagrams of (A) helium-3 and (B) helium-4 show which states of these isotopes are stable (see text).

Helium-4 (4He) is a stable isotope of helium, the second lightest element. It is the most abundant stable isotope of helium, outnumbering helium-3 by a significant margin. The nucleus of helium-4 contains two protons and two neutrons, giving it a mass number of four.1

Helium-4 is produced naturally through radioactive decay, specifically as alpha particles ejected from the nuclei of heavier radioactive elements. While helium is abundant in the universe, particularly in stars where it's formed by nuclear fusion, its presence in Earth's atmosphere is minimal due to its low density, which allows it to escape into space.1

Liquid helium-4 exhibits unique properties, including superfluidity below 2.18 K, a state where it flows with virtually no viscosity. This characteristic makes it invaluable in cryogenics for achieving extremely low temperatures. Helium-4 is also used in welding, rocket propulsion, and as a lifting gas for balloons.1

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