Helium Mass Number

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Chemical properties of helium - Health effects of helium

Mass numbers of typical isotopes of Helium are 3; 4. The total number of neutrons in the nucleus of an atom is called the neutron number of the atom and is given the symbol N. Neutron number plus atomic number equals atomic mass number: N+Z=A. A helium atom is an atom of the chemical element helium.Helium is composed of two electrons bound by the electromagnetic force to a nucleus containing two protons along with either one or two neutrons, depending on the isotope, held together by the strong force. Helium-4 (4 He) is a stable isotope of the element helium.It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consists of two protons and two neutrons. Alpha decay of heavy elements in the Earth's crust is the source of most naturally occurring helium-4 on Earth. ››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. The nucleus of the helium atom contains 2 massive, positively charged particles. Thus, Z, the atomic number = 2.

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


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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