Explain the occurrence of group-$16$ elements.

(N/A) $(i)$ Oxygen: Oxygen is the most abundant of all the elements on earth. It forms about $46.6 \%$ by mass of the earth's crust. Dry air contains $20.946 \%$ oxygen by volume.
$(ii)$ Sulphur: The abundance of sulphur in the earth's crust is only $0.03-0.1 \%$. Combined sulphur exists primarily as sulphates such as gypsum $(CaSO_4 \cdot 2H_2O)$,epsom salt $(MgSO_4 \cdot 7H_2O)$,baryte $(BaSO_4)$ and sulphides such as galena $(PbS)$,zinc blende $(ZnS)$,and copper pyrites $(CuFeS_2)$. Traces of sulphur occur as hydrogen sulphide in volcanoes and in organic materials such as eggs,proteins,garlic,onion,mustard,hair,and wool.
$(iii)$ Selenium and tellurium: Selenium and tellurium are found as metal selenides and tellurides in sulphide ores.
$(iv)$ Polonium and livermorium: Polonium occurs in nature as a decay product of thorium and uranium minerals. Livermorium is a synthetic radioactive element. It has been produced only in a very small amount and has a very short half-life (only a small fraction of one second),which limits the study of its properties.