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Justify the placement of O,S, Se, Te and Po in the same group of the periodic table in terms of electronic configuration, oxidation state and formation of hydride.

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(1) Electronic configuration:
These elements contain six electrons in the valence shell, two in the s-orbital and four in the p-obrital. Hence, they are p – block elements. Their valance shell electronic configuration is ns2np4

Elements Electronic configuration
O(Z = 8) [He] 2s2p4
S(Z = 16) [Ne] 3s3p4
Se(Z = 34) [Ar] 3d10 4s4p4
Te(Z = 52) [kr] 4d10 5s5p4
Po(Z = 84) [Xe] 4f14 5d10 6s6p4

(2) Oxidation state:
The elements of group-16 show two types of oxidation states.

(a) Positive oxidation state :
The elements of group-16 can attain noble gas configuration by sharing electrons. These elements show + 2, + 4 and + 6 oxidation states. Due to the absence of d-orbital in its valence shell oxygen show only + 2 oxidation state such as in OF2, hence it show as a covalent nature. Other elements of this group have vacant d - orbitals in the valence shall, hence they show + 2, + 4 and + 6 oxidation states. On moving down the group, due to the inert pair effect the stability of + 4 oxidation states increases while that of + 6 oxidation state decreases. Generally + 4 oxidation state is more stable for Se, Te and Po while for S, + 6 oxidation state is more stable.

(b) Negative oxidation state :
The elements can also attain stable inert gas configuration by gaining two electrons. Due to high electronegativity and ionisation energy oxygen shows negative oxidation state in most of its compounds, hence all metal oxides are ionic and contain O2- ions.
However in peroxides such as Na2O2, H2O2, oxygen shows (-1) oxidation state. Due to low electronegativity and low ionisation energy, other elements of this group have less tendency to show -2 oxidation state. On moving down the group, the tendancy to show – 2 oxidation state decreases. Po does not show – 2 oxidation state

(c) Formation of Hydrides :
The elements of this group form stable, volatile and bivalent hydrides which have the general formula M2H such as H2O, H2S, H2Se, H2 Te and H2P. Due to the presence of two lone pairs electrons on the central atom, they have bent (V) shapes. The central atom in these hydrides is sp3 hybridised. Some characteristics of these hydrides are given below:

I. Melting and boiling points :
Order of M.P : H2O > H2Te > H2Se > H2S
Order of B.P : H2O > H2Te > H2Se > H2S

II. Covalent Character :
H2O < H2S < H2Se < H2Te

III. Volatility and Thermal stability :
From H2O to H2S, the volatility increases suddenly and then decreases from H2S to H2Te. Hence H2O is least volatile and H2S is most volatile of group-16 elements. Order of thermal stability is
H2O > H2S > H2Se > H2Te

IV. Bond angle :
Order of bond angle is
H2O > H2S > H2Se > H2Te

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