Figure 2 Balloons filled with helium rise rapidly if not tethered
Helium (Figure 1) is the second-most abundant element in the universe, making up about 24% of the mass of all matter. Hydrogen accounts for about 75%, with all the other elements contributing only about 1% of the total. But helium remained undiscovered until 1868, and was detected in the Sun before being found on Earth.
Helium does not react chemically and exists only as single atoms. At normal atmospheric pressure it is a gas even at temperatures close to absolute zero, and its low density (Figure 2) means that it easily escapes from Earth.
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