Helium Mass Number

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Chemical properties of helium - Health effects of helium

Atomic number


Atomic mass

4.00260 g.mol -1

Electronegativity according to Pauling



0.178*10 -3 g.cm -3 at 20 °C

Melting point

- 272.2 (26 atm) °C

Boiling point

- 268.9 °C

Vanderwaals radius

0.118 nm

Ionic radius




Electronic shell

1s 2

Energy of first ionisation

2372 kJ.mol -1

Discovered by

Sir Ramsey in 1895

Helium, the lightest of the noble gases, had actually been detected and helium is the only element in the periodic table that was discovered by an astronomer. Helium is the element which you can find on the upper right side of the periodic table with atomic number 2. The PSA process is widely used to produce specification pure helium from 85+% crude helium in conjunction with cryogenic enrichment of the 50% helium raffinate. Hwang S-C et al; Noble Gases. Kirk-Othmer Encyclopedia of Chemical Technology (1999-2014). Notes: The spectroscopic data below are for the isotope 4 He. He I Ground State 1s 2 1 S 0 Ionization energy 198310.669 cm-1 (24.587387 eV) Ref. M02 He II Ground State 1s 2 S 1 / 2 Ionization energy 4 cm-1 (54.417760 eV) Ref. MK00b-1 (54.417760 eV) Ref. Helium-4 atom is the stable isotope of helium with relative atomic mass 4.002603. The most abundant (99.99 atom percent) isotope of naturally occurring helium.It contains an alpha-particle.


Gaseous chemical element, symbol: He, atomic number: 2 and atomic weight 4,0026 g/mol. Helium is one of the noble gases of group O in the periodic table. It’s the second lightest element. The main helium source in the world is a series of fields of natural gas in the United States.

Helium is a colourless, odourless, insipid and non-toxic gas. It’s less soluble in water than any other gas. It’s the less reactive element and doesn’t essentially form chemical compounds. The density and viscosity of helium vapour are very low. The thermic conductivity and the caloric content are exceptionally high. Helium can be liquefied, but its condensation temperature is the lowest among all the known substances.


Helium has many unique properties: low boiling point, low density, low solubility, high thermal conductivity and inertness, so it is use for any application which can explioit these properties. Helium was the first gas used for filling balloons and dirigibles. This application goes on in altitude research and for meteorological balloons. The main use of helium is as an inert protection gas in autogenous welding. Its biggest potential is found in applications at very low temperatures. Helium is the only cooler which is capable of reaching temperatures lower than 15 K (-434ºF). The main application of ultralow temperature is in the development of the superconductivity state, in which the resistance to the electricity flux is almost zero. Other applications are its use as pressurizing gas in liquid propellants for rockets, in helium-oxygen mixtures for divers, as working fluid in nuclear reactors cooled down by gas and as gas carrier in chemical analysis by gas chromatography.

Helium in the environment

Helium is the second most abundant element in the known universe, after hydrogen. Helium constitutes the 23% of all elemental matter measured by mass. Helium is formed in The Earth by natural radioactive decay of heavier elements. Most of this helium migrates to the surface and enters the atmosphere. It could be logical to think that the helium concentration in the atmosphere was higher than it is (5,25 parts per million at sea level). Nevertheless, its low molecular weight allows it to escape to space at the same rate of its formation. There is an about 1000 km layer in the heterosphere at 600 miles where helium is the dominant gas (although the total pressure is very low). Natural gases contain higher helium concentrations than the atmosphere.
Helium is the 71st most abundant element in the Earth's crust where it is found in 8 parts per billion (109).

Health effects of helium

Effects of exposure: The substance can be absorbed into the body by inhalation. Inhalation: High voice. Dizziness. Dullness. Headache. Suffocation. Skin: on contact with liquid: frostbite. Eyes: on contact with liquid: frostbite. Inhalation risk: On loss of containment this gas can cause suffocation by lowering the oxygen content of the air in confined areas. Check oxygen content before entering area.
Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood.

Read more on helium in water

Back to periodic chart

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››More information on molar mass and molecular weight. In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together.

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Molar mass of He = 4.002602 g/mol


Convert grams Helium to moles or moles Helium to grams

What Is Helium Mass Number

Symbol# of AtomsHeliumHe4.0026021100.000%

In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together.

A common request on this site is to convert grams to moles. To complete this calculation, you have to know what substance you are trying to convert. The reason is that the molar mass of the substance affects the conversion. This site explains how to find molar mass.

If the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. The percentage by weight of any atom or group of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100.

Helium Symbol

The atomic weights used on this site come from NIST, the National Institute of Standards and Technology. We use the most common isotopes. This is how to calculate molar mass (average molecular weight), which is based on isotropically weighted averages. This is not the same as molecular mass, which is the mass of a single molecule of well-defined isotopes. For bulk stoichiometric calculations, we are usually determining molar mass, which may also be called standard atomic weight or average atomic mass.

Atomic Number

Formula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. These relative weights computed from the chemical equation are sometimes called equation weights.

Finding molar mass starts with units of grams per mole (g/mol). When calculating molecular weight of a chemical compound, it tells us how many grams are in one mole of that substance. The formula weight is simply the weight in atomic mass units of all the atoms in a given formula.

Helium Atomic Structure

Using the chemical formula of the compound and the periodic table of elements, we can add up the atomic weights and calculate molecular weight of the substance.