Under normal conditions, neutral helium is non-poisonous and harmless gas. But in interstellar medium, the relative presence of helium-3 with respect to helium-4 is hundred times more than that on earth.
The ratio of abundance of helium-3 and helium-4 is that for every one million atoms of helium-4, a single atom of helium-3 is found on earth. Isotopes:Īlthough there are nine isotopes of helium, the stable ones among them are two isotopes called helium-3 and helium-4. "He" is in liquid state below the absolute temperature under normal pressure. Due to the small size of monoatomic molecule, the rate of diffusion through solids is three more than that of air and 65% of that of hydrogen. Since helium has the second lowest atomic mass after hydrogen, its thermal conductivity, specific heat and sound speed in gas phase are all greater than any element except hydrogen. He is the second least reactive gas after neon, its immediate successor in the inert gas group of periodic table. It is formed due to nuclear fusion of highly active hydrogen atoms in plasma state in the atmosphere of sun and stars. Like hydrogen, the formation of He is also found out to be the immediate effect of Big Bang. It is the second most abundant element, after hydrogen, in the observable extent of universe. He is a colorless, odorless, tasteless, non-poisonous monoatomic inert gas. Unlike hydrogen which has two atoms in a single molecule, helium is a monoatomic element which means a single atom of He itself exists as a molecule. The tendency for attaining stable electronic configuration is cited as the main reason for explaining the reactivity of an element in periodic table. To elaborate more lucidly, an atom while undergoing chemical reaction tends to gain or lose electrons to attain the stable configuration of inert gas present in the same period of periodic table. The electronic configuration of inert gases is taken as the criterion for an atom other reactive elements to participate in chemical reaction to attain stable configuration. Since other inert gases have higher atomic number than helium, they are placed in the successive lower positions in the inert gases group. The number of inert gas group is 18 in the modern periodic table. Hence, helium is grouped in the list of inert gases. Since the orbital of helium is completely filled, helium atom is in a saturated state which makes it unreactive with other elements. The maximum accommodating capacity of s-orbital is two electrons. The orbital in which two electrons are present is called s-orbital.
Since there are two protons in the nucleus of helium, the atomic number of helium is two. The number of protons is called the atomic number of any element. There are two electrons which rotate around the nucleus of helium. Hence its mass number is four (the total number protons and neutrons, collectively called nucleons). It has two protons and two electrons in its nucleus. Helium is an inert gas which is the simplest and foremost compound in the group of inert gases.