Group 16 of periodic table contains five elements namely, oxygen (O), sulphur (S), selenium (Se), tellurium (Te) and polonium (Po). These are collectively known as chalcogens or ore forming elements because many metal ores occur as oxides and sulphides. These elements belong to p-block. The first four members of group 16 are non-metals. Polonium has metallic character and is a radioactive element with a very short period.
Oxygen, the most abundant element, is a vital element of atmosphere and ocean. It composes 46.6% of earth's crust. It forms about 21% of air as the free element and 89% of the ocean by weight. Sulphur is less abundant and occurs about 0.052% of the earth's crust. Sulphur is also referred as Brim stone. The name sulphur is derived from Sanskrit word 'Sulveri' which means 'killer of copper'. Compared to oxygen and sulphur the other members of this group are rare.
Atoms of these elements have outer electronic configuration as ns2np4, where n varies from 2 to 6. The electronic configurations of the elements of group 16 are shown in the table below:
|
Elements
|
At. No.
|
Electronic configuration
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Abundance in earth's crust (ppm)
|
|
Oxygen (O)
|
8
|
[He] 2s2 2p4
|
4.66 × 105
|
|
Sulphur (S)
|
16
|
[Ne] 3s2 3p4
|
5.20 × 102
|
|
Selenium (Se)
|
34
|
[Ar] 3d10 4s2 4p4
|
9.0 × 10-2
|
|
Tellurium (Te)
|
52
|
[Kr] 4d10 5s2 5p4
|
2 × 10-3
|
|
Polonium (Po)
|
84
|
[Xe] 4f14 5d10 6s2 6p4
|
-
|
The four p-electrons of the outermost shell are arranged as px2 py1 pz1. Thus, there are two half-filled p-orbitals which are used for bonding with other elements. From the similar outer electronic configuration of these elements, it is expected that they will show similar physical and chemical properties.
Oxygen the first member of this group has very high ionization energy (1314 kJ mol-1) and differs markedly from other members of the family.
Oxygen, on the other hand, is found to exhibit remarkable resemblance with its neighbours, nitrogen and fluorine. For example, it forms strong pπ-pπ bond like nitrogen. Also it forms hydrogen bonds like nitrogen and fluorine.