On the periodic table, the lightest noble gas is helium.
Helium has atomic number 2 and the symbol He. It is in group 18, the noble-gas column, and has an atomic mass of about 4.00 atomic mass units. Like other noble gases, helium is extremely unreactive because its outer electron shell is complete.
Helium was first detected in sunlight during the 1868 solar eclipse, before it was identified on Earth. Its name comes from Helios, the Greek personification of the Sun. This makes helium unusual among elements because it was discovered astronomically before a terrestrial sample was obtained.
Helium is lighter than air and does not burn, so it is used in balloons and airships. It is also important for cooling superconducting magnets, including those used in magnetic resonance imaging. Hydrogen is lighter, but it is not a noble gas and is highly flammable.