The noble gas with the lowest atomic number is helium.
Helium is element 2, following hydrogen in the periodic table. It belongs to group 18, the noble gases, whose atoms generally have filled outer electron shells and therefore low chemical reactivity. Helium has two protons and, in its common neutral form, two electrons filling its first electron shell.
Helium was first identified in the Sun’s spectrum in 1868, before it was found on Earth. Its name comes from Helios, the Greek word for the Sun. On Earth, helium is obtained mainly from natural-gas deposits, where it formed through radioactive decay and became trapped underground.
Helium is the second-lightest element and has the lowest boiling point of any element, about 4.2 kelvins at standard pressure. It is used in cryogenics, leak detection, shielding gas, and lighter-than-air balloons. Unlike hydrogen, helium is nonflammable.