Helium (He), the second element in the periodic table has a very stable atomic structure. It is the second most common element in the universe with an abundance of 72,000,000 ppb by atoms. Only hydrogen is more common. Helium atoms have an atomic radius of 128 picometers.
There are two stable isotopes of helium, helium-3 and helium-4. The nucleus of the helium-4 atom consists of two protons and two neutrons and that of helium-3 consists of two protons and one neutron. Orbiting the nucleus are two electrons, which fill the s-orbital. This gives the element the electron configuration of 1s2.
Helium-4 is the most common of the isotopes making up 99.999863% of the total.
With a full shell of electrons, helium does not form compounds with other elements in nature. In addition, helium atoms do not bond with each other, so helium is a mono-atomic gas.
The majority of the helium present in the universe is produced in stars. Within stars, the intense pressure and heat causes hydrogen atoms to combine in a process known as nuclear fusion to form helium atoms. The temperature within our own sun at which this fusion takes place is 27,000,000°F or 15,000,000°C.
On Earth, helium is a comparatively rare element. Within Earth’s crust, helium has an abundance of only 30 ppb by atoms. As it is such a light gas, it escapes our atmosphere into space. The helium present on earth comes from the breakdown of naturally radioactive elements within The Earth’s crust. The breakdown of radioactive elements produces alpha particles, beta particles and gamma radiation. An alpha particle consists of two protons held together by two neutrons. This alpha particle is essentially the nucleus of a helium atom. The positively charged alpha particle captures two negatively charged electrons to form a helium atom. Many of the electrons also come from radioactive decay, as beta particles are, in fact, electrons.
While the element has only two stable isotopes, a number of unstable isotopes have been produced. These unstable isotopes have additional neutrons within their nuclei. To date (March 2010), six unstable isotopes of helium have been formed. These isotopes are helium-5, helium-6, helium-7, helium-8, helium-9 and helium-10. All of these unstable isotopes have extremely short half-lives. Helium-6 has the longest half-life at 806.7 milliseconds.