Helium (2 He) (standard atomic weight: 4.002 602(2)) has nine known isotopes, but only helium-3 (3 He) and helium-4 (4 He) are stable. [4] All radioisotopes are short-lived; the longest-lived is 6 He with half-life 806.92 (24) milliseconds.

氦 hài （英語： Helium；源于 希臘語： ἥλιος，轉寫为「helios」，直譯为「太陽」；舊譯 氜）是一種 化學元素， 化學符號 为 He， 原子序數 为2， 原子量 為 4.002 602 u。 是一種無色、無臭、無味、无毒的惰性 單原子氣體。 它也是 稀有气体 族的第一個元素。

This liquid form is called helium II to distinguish it from normal liquid helium I. Helium II exhibits the property called superfluidity: its viscosity, or resistance to flow, is so low that it has not been measured.

Helium (from Greek: ἥλιος, romanized: helios, lit. 'sun') is a chemical element; it has symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table.

氦-2是氦的同位素，同氢-1一样，只有质子没有中子。 但氦-2由于有两个质子，因此存在着很大的斥力。 由于没有”胶黏剂”作用的中子（中子为0电荷，但同样具有强核力，可以将质子“黏”在一起），所以还无人能制备出-氦-2原子。

2015年5月18日 · Helium is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table.

Helium is a chemical element with the He symbol and Helium is a colourless, tasteless, and odourless gas with an atomic number. Helium is the Universe’s second most common element (after hydrogen), accounting for around 24 per cent of its weight.

The second most abundant element in the universe, helium is a product of nuclear fusion and radioactive decay. It is the lightest noble gas, colorless, odorless, and inert with a low boiling point.

Helium-II is found to have an extraordinary high coefficient of thermal conductivity. The heat transported per unit temperature gradient is several 10 times as great as that in copper at …

Detailed decay information for the isotope helium-2 including decay chains and daughter products.