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Discovery
Helium was discovered by the French astronomer, Pierre-Jules Janssen, while he was studying a solar eclipse in 1868. Janssen noticed a yellow line in his spectrum of the sun, which was later identified as a new element by Sir Norman Lockyer. It wasn't until 1895 that helium was discovered on earth, by a Scottish chemist named Sir William Ramsay.
Temperature
Helium is the only element that cannot be frozen to a solid at very low temperatures, although it can be cooled enough to liquefy it. It has the lowest melting point of any element and a boiling point that is close to absolute zero. These properties mean that helium is a gas at room temperature.
Abundance
Helium is the universe's second most abundant element after hydrogen. According to the Hyperphysics website, helium accounts for between 23 and 25 percent of all the nuclear matter in today's universe. Despite this, helium is relatively rare on earth. Earth's crust contains eight parts per billion by weight, notes the Chemicool website. The earth's atmosphere contains approximately 0.0005 percent helium.
Sources
The Chemicool website says that nearly all of earth's helium is created through radioactive decay. Helium is extracted by fractional distillation of natural gas, which can contain up to seven percent helium. Major sources include the natural gas deposits in wells in Kansas, Texas and Oklahoma. Extracted helium costs more than $5 per 100g, as of July 2011.
Structure
Helium's atomic number is 2, meaning each atom of helium has two protons in its nucleus. Helium also has two electrons surrounding the nucleus and two neutrons. The neutrons have no electrical charge and prevent the positively charged protons from repelling each other and causing instability. Helium is the second simplest atom in terms of structure, after hydrogen.
Isotopes
The Chemicool website reports that helium has eight isotopes whose half-lives are known. Isotopes are variants of the atoms of elements that have differing numbers of neutrons. Helium's isotopes have mass numbers between three and ten. Of its eight known isotopes, two are stable. The 4He helium isotope accounts for more than 99.9 percent of naturally occurring helium.
Weight
Helium is the second-lightest element. The Helium UK website states that helium weighs 0.1785 grams per liter. This lightness is made use of in filling balloons and airships. Helium-filled balloons rise because the weight of the helium plus the balloon is less than the air it displaces. The weight of air is approximately 1.25 grams per liter.
Reactivity
After neon, helium is the second-least reactive of the noble gases. It is odorless, tasteless and not known to be toxic. Because it is inert, liquefied helium is used in cryogenics to freeze biological materials for a long time without causing them to degrade. It is also used for cooling nuclear power plants.
Sun
Helium's name is from the Greek word "helios" which means "sun." This is apt as helium is formed in the center of the sun by nuclear fusion. The process of converting hydrogen nuclei into helium nuclei produces enormous amounts of energy. According to the Enchanted Learning website, the sun is made up of about 2 x 1030 kilograms of gas, 23 percent of which is helium.
Breathing
The air on earth contains mostly nitrogen and just 1 part in 200,000 of helium. Despite this, scuba divers use a mixture of 80 percent helium and 20 percent oxygen to fill their tanks. This is because after deep dives, helium leaves the body faster than nitrogen and allows faster decompression.