The periodic table/Helium atom
Helium (He) is the second element of the periodic table. It exists almost solely as a gas, and is very unreactive. It belongs to a group of elements on the periodic table known as noble gases.
Pierre-Jules-César Janssen, a French astronomer, noticed a yellow line in the sun's spectrum while studying a total solar eclipse in 1868. Sir Norman Lockyer, an English astronomer, knew it could not be produced by any element known at the time. It was hypothesized that a new element on the sun was responsible for this yellow emission. This unknown element was named helium by Lockyer.
Helium was found on earth in 1895. Sir William Ramsay, a Scottish chemist, conducted an experiment with a mineral containing uranium called clevite. He exposed the clevite to mineral acids and collected the gases that were produced. He then sent a sample of these gases to two scientists, Lockyer and Sir William Crookes, who were able to identify the helium within it. Two Swedish chemists, Nils Langlet and Per Theodor Cleve, independently found helium in clevite at about the same time as Ramsay.
In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million. However, helium is found in large amounts in uranium, thorium, clevite, and pitchblende.
Atomic Mass: 4.002602
Classification: group 0 (noble gases)
Discovery in: 1895
Density: 0.1786 g/L
Crystal Structure: hexagonal
Melting Point: -272 °C (1 K)
Boiling Point: -268 °C (5 K)
Common Uses: artificial atmospheres, laser media, refrigerants, helium balloons, deep-sea diving, helium tanks, helium/neon lasers