chemistry 12th class cbse notes

Chemical Properties and Trends in Chemical Reactivity

  • The anomalous behaviour of oxygen, like other members of the p-block present in the second period, is due to its small size and high electronegativity.
  • One typical example of effects of small size and high electronegativity is the presence of strong hydrogen bonding in H2O which is not found in H2S.
  • The absence of d orbitals in oxygen limits its covalency to four and in practice, rarely exceeds two.
  • On the other hand, in case of other elements of the group, the valence shells can be expanded and covalence exceeds four.

Reactivity with hydrogen:

All the elements of Group 16 form hydrides of the type H2E (E = S, Se, Te, Po).

Thermal stability: Thermal stability of group 16 elements decreases down the group.

Reactivity with hydrogen

This is because the H-E bond length increases down the group, hence the bond dissociation enthalpy decreases down the group.

Acidic nature:

Due to the decreasing bond dissociation enthalpy, acidic character of group 16 elements increases down the group.

Acidic nature

Reducing character:

The reducing character also decreases down the group due to the decreasing bond dissociation enthalpy.

Reducing character

Reactivity towards oxygen:

All group 16 elements form oxides of the type EO2 and EO3 Reducing character of dioxides decreases down the group. Acidity also decreases down the group. Besides EO2 type, sulphur, selenium and tellurium also form EO3 type oxides. Both types of oxides are acidic in nature.

Reactivity with halogens:

Elements of Group 16 form a large number of halides of the type, EX2 EX4 and EX6, where X is a halogen. The stability of halides decreases in the order F > Cl> Br> I. This is because E-X bond length increases with increase in size. Among hexa halides, hexafluorides are the most stable because of steric reasons. Dihalides are sp3 hybridised and have tetrahedral geometry. H2O is a liquid while H2S is a gas. Because in water due to the small size and high electronegativity of O, strong hydrogen bonding is present there.


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