Which chemical element is the lightest noble gas?

Helium is the lightest noble gas.

Helium has atomic number 2 and the symbol He. It is a colorless, odorless gas whose atoms have a filled outer electron shell, making it exceptionally unreactive under ordinary conditions. Only hydrogen is lighter overall, but hydrogen is not a noble gas.

Helium was first identified through observations of sunlight during the 1868 solar eclipse. Its name comes from Helios, the Greek word for the Sun, because scientists detected its spectral signature in the Sun before finding it on Earth.

Helium is used in cryogenics, leak detection, controlled atmospheres, and lighter-than-air balloons. It has the lowest boiling point of any element, about −268.9°C. Although party balloons are a familiar use, helium is also important for cooling superconducting magnets in some medical imaging systems.