Group 18 elements  Occurrence

  After hydrogen, He is the second most abundant element in the universe. It occurs to an extent of _7% by volume in natural gas from sources in the US and Canada, and this origin is doubtless from the radioactive decay of heavier elements. Helium is also found in various minerals containing a-emitting unstable isotopes.  Helium was first detected spectroscopically in the Sun’s atmosphere; helium is formed by nuclear fusion in the Sun. Figure 1.1 shows the relative abundances of the noble gases in the Earth’s atmosphere.

Fig. 1.1 Relative abundances of the noble gases (excluding radon, the abundance of which is 1 × 10^-12 ppb) in the Earth’s atmosphere. The data are plotted on a logarithmic scale. The units of abundance are parts per billion by volume (1 billion=10⁹).

    Argon is present to an extent of 0.92% by volume in the Earth’s atmosphere. Radon is formed by decay of ²²⁶Ra in the ²³⁸U decay chain and poses a serious health hazard in uranium mines, being linked to cases of lung cancer.