PROPERTIES

Helium is the lightest noble gas and the second lightest element. The chemical symbol of helium is He. Helium is a non-toxic, tasteless, non-flammable, odorless, colorless, tasteless, inert, monatomic gas. The stable isotopes of helium are helium-3 and helium-4. The isotopic abundances of helium-3 is 0.000137%. Helium-3 is composed of 2 protons and 1 neutron. Helium-4 is composed of 2 protons and 2 neutrons. The isotopic abundances of helium-4 is 99.999863%. Helium has the following unique features:

• Its boiling point of 4.22 K (-268.93 °C) is substantially lower than other known elements.
• Helium does not solidify at normal atmospheric pressure, so it has not a melting point. Other known elements have higher melting points. Helium is solid at the pressure of 25 atmospheres and the temperature of 0.95 K (-272.2 °C). As the pressure increases, the temperature at solid helium occurs, increases likewise. Solid helium occurs at room temperature, when the pressure is 114000 atmospheres.
• It is completely inert, so it does not form stable molecules.
• Helium is seven times lighter than air. It is so light that the gravity force of the Earth is not strong enough to hold on to it. Helium escapes into space, when helium is released into the  atmosphere.
• Helium has a relatively high thermal conductivity of 0.1513 W/(m·K). The thermal conductivity of helium is five times higher than the thermal conductivity of air.
• Helium has a sound velocity of 970 m/s, so helium conducts sound three times faster than air.
• Helium does not become radioactive under irradiation.
• Helium has a relatively high ionization potential due to the fact that only two electrons completely fill the lowest energy shell.
• Helium has an extremely low index of refraction of 1.000036.
• Helium has a low viscosity of 0.0186 centipoise.
• Helium becomes a superfluid at temperatures close to absolute zero. A superfluid is a fluid with a viscosity of zero, so the superfluid flows up and over the walls of containers. The state of superfluid is called Helium II. Other known elements do not become superfluid at temperatures close to absolute zero.

The physical and chemical properties of helium are presented below.