Helium, (Chemical Element, Symbol He, Atomic Number 2)

The 'Bournemouth Balloon' rises over the seaside resort of Bournemouth in Dorset, England, and is filled with Helium gas.

Helium is the second element of the Periodic Table.
Atoms of helium consist of two protons, four neutrons and two electrons.

Helium is the smallest element in the group of noble gases, also known as Group 0 of the Periodic Table. It is also the second most abundant element in the universe (after hydrogen), and is said to account for approx 24% of the elemental mass of our galaxy.

Discovery of Helium

Helium was discovered in the sun during the solar eclipse of 1868 when the French astronomer Jules Janssen noticed a previously unknown yellow spectral line signature in sunlight. Janssen shares the credit for discovery of the element helium with the English scientist and astronomer Sir Joseph Norman Lockyer (usually known as simply Norman Lockyer) who saw the same eclipse and was the first to suggest that the unknown yellow line was due to a new element. He named 'new element' helium after 'helios', the Greek word for sun.

Helium at standard temperature and pressure ('standard conditions')

Helium is a colourless, odourless, tasteless, non-toxic, inert gas at room temperature and pressure.

Helium gas consists of atoms rather than molecules, hence it may be described as a 'monatomic' gas.

Not only is helium a gas at room temperature and pressure but it exists only exists as a gas (except under extreme conditions / circumstances that are not usually considered in school-level chemistry).

Helium's boiling and melting points are the lowest of all the elements.

Reactivity of Helium

Helium is chemically unreactive under all normal conditions

There are some rare exceptions to the general rule of helium being an inert (unreactive) element but these are beyond the scope of school-level chemistry. For example, helium can form certain unstable compounds, known as excimers, with tungsten, iodine, fluorine, sulphur and phosphorus when it is subjected to an electric glow discharge, to electron bombardment, or is a plasma for some other reason. Recent work in theoretical chemistry includes suggested 'true compounds' of helium that may also be possible. If any of these are actually produced experimentally that would mean that helium could no-longer truely be said to be 'inert'.

Isotopes of Helium

There are several known isotopes of helium, of which only two, helium-3 and helium-4, are considered stable.

Helium-4 is by far the most abundant of these on earth, with only one helium-3 atom for every one million helium-4 atoms in the earth's atomsphere.

Helium-4 is formed on Earth by alpha decay of heavier radioactive elements. The alpha particles that emerge from the nuclei of large decaying radioactive elements are fully ionized helium-4 nuclei. Helium-4 is a very stable nucleus because its nucleons are arranged into complete shells. Trace amounts of helium-3 are produced by the beta decay of tritium (hydrogen-3) but most of the helium-3 present on earth is believed to have been in existance since the formation of the earth.

Isotopes of helium include:

Isotope:	Content of Nucleus:	Notes:
Helium-2	2 proton, 0 neutrons	Helium-2 is a radioisotope that decays by proton emission into protium (hydrogen-1), with a half-life of 3×10−27 s.
Helium-3	2 proton, 1 neutron	Helium-3 is present on earth only in trace amounts. Most of the helium-3 on earth is believed by scientists to have been present since the formation of the earth. The rest is thought to have arrived on earth trapped in cosmic dust. Trace amounts of helium-3 are also produced by the beta decay of tritium (hydrogen-3). Helium-3 is thought to be more abundant in stars, as a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon's surface contains helium-3 at concentrations on the order of 0.01 ppm, a lot higher than the ca. 5 ppt found in the Earth's atmosphere.
Helium-4	2 proton, 2 neutrons	The most common helium isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells.
Helium-5	2 proton, 3 neutrons	The shortest-lived heavy helium isotope is helium-5 with a half-life of 7.6×10−22 s.
Helium-6	2 proton, 4 neutrons	Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. It is also known to exhibit a nuclear halo.
Helium-7	2 proton, 5 neutrons	Helium-7 decays by emitting a beta particle and a gamma ray. This isotope of helium is created in certain nuclear reactions.
Helium-8	2 proton, 6 neutrons	This isotope of helium is created in certain nuclear reactions and is known to exhibit a nuclear halo.

Uses of Helium (Gas)

Uses of elemental helium include:

• Floating Balloons: Helium gas is perhaps best-known as the gas used to fill "party balloons" popular for use at home birthday parties and many other special events including weddings, anniversaries and commercial promotions. The balloons themselves are often available inexpensively and can either be blown-up by mouth (so with a mixture of air and carbon-dioxide) or inflated using helium gas cannisters that can be bought from many party supplies shops. An advantage of using helium gas is that the filled balloons are then lighter than air and so float upwards unless tied to a suitably heavy object.
• Airships: Helium gas is lighter than air, making it an attractive choice of gas with which to inflate airships (and some balloons) for lift. Helium's low weight is not it's only advantage for use in airships. Although hydrogen gas is approximately 7% more buoyant, helium is also non-flammable (in addition to being fire retardant), which is a huge advantage for reasins of safety
• Use in Rockets (for space science/exploration): Helium is used as an "ullage medium" to displace fuel and oxidizers in rocket storage tanks and to condense hydrogen and oxygen to make rocket fuel. (Ullage is the space within a fuel tank above the liquid propellant.) Helium is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles, e.g. the Saturn V booster used in the Apollo program needed about 370,000 m³ (13 million cubic feet) of helium to launch.
• Industrial leak detection: Helium gas is used to detect leaks in high-vacuum equipment and high-pressure containers because it diffuses through solids at three times the rate of air.
• Lasers: Helium-Neon lasers are a type of low-powered gas laser that produces a red beam. They have been popular in university laboratories and in certain types of research centers but laser technologies are continually developing and cheaper diode lasers are replacing gas lasers such as He-Ne lasers in many applications.
• Gas chromatography: Helium is often used as a carrier gas for gas chromatography.
• Cryogenics (Low-Temperature Physics): Helium at low temperatures is used in cryogenics.