Explain occurence of group-16 elements.

Explanation

(i) Oxygen : Oxygen is the most abundant of all the elements on earth. Oxygen forms about 46.6% by mass of 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 . 2H_2O`), epsom salt (`MgSO_4 . 7H_2O`), baryte (`BaSO_4`) and sulphides such as galena (PbS), zinc blende (ZnS), copper pyrites (`CuFeS_2`).
The traces of sulphur occur as hydrogen sulphide in volcanoes, 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. Its has been produced only in a very small amount and has very short life period (only a small fraction of one second). This limits the study of properties of livermorium.