The elements of this group have a configuration of ns2
in their valence shell, they may attain noble gas configuration either by gaining two electrons, forming M-2
, or by sharing two electrons, thus forming two covalent bonds. Hence, these elements show both negative and positive oxidation states. The common oxidation states exhibited by the elements of group 16 include minus two, plus two, plus four and plus six.
The electro-negativity of oxygen is very high. It exhibits a negative oxidation state of -2 in all its metal oxides. In addition to -2 oxidation state, oxygen shows -1 oxidation state in peroxides and -1/2 oxidation state in superoxides.
Oxygen shows positive oxidation state only in its compounds with fluorine, since fluorine is more electro-negative than oxygen. It shows +2 oxidation state in OF2
& +1 in O2
The other elements of the group, apart from exhibiting +2 oxidation state, also exhibit +4 and +6 oxidation states due to the availability of d-orbitals in their atoms.
Oxygen differs considerably from the rest of the elements in its group. Due to it’s small size and high electro-negativity and the absence of d-orbitals in the valence shell. The important differences are,
Oxygen is diatomic and gaseous, while the other elements exist as solids.
Formation of M -2ions:
Oxygen, due to its high electro-negativity, shows negative oxidation state and does not show any positive oxidation state, except in OF2
Oxygen covalency is limited to two, but the covalency exceeds four in the other elements due to the availability of d-orbitals in them.
Hydrogen bond formation:
Because of its small size and high electro-negativity, oxygen forms strong hydrogen bonds. The other elements in the group have comparatively low electro-negativities, and do not form hydrogen bonds.
The elements of this group react with hydrogen to form hydrides of the type H2
E, where E could be oxygen, sulphur, selenium, tellurium or polonium.
+ E → H2
Hydrogen Group 16 Hydride
Physical states of hydrides:
Water is a colourless and odourless liquid, while the hydrides of all the other elements of the group are colourless, poisonous gases with unpleasant odours. The boiling point of the hydrides markedly decreases from water to hydrogen sulphide, and then increases. Water has an abnormally high boiling point because its molecules are associated with each other by means of hydrogen bonds in its solid as well as liquid states.
Acidic character of hydrides:
Acidic nature of hydrides increases from H2
O to H2
Te. The increase in acidic character is due to the decrease in the H-E bond dissociation enthalpy from H2
O to H2
Te. Except for water, all the other hydrides act as reducing agents. The reducing property of these hydrides increases from H2S to H2Te.
All the elements of this group react with oxygen to form dioxides and trioxides. Both these types of oxides are acidic in nature. Sulphur trioxide is the only important triode in this group. At room temperature, sulphur trioxide is a solid and exists in three distinct forms - alpha, beta and gamma.
Group 16 elements form a large number of halides of the type EX6
, where E is a group 16 element and X is a halogen. Among hexahalides, only hexafluoride are stable. They undergo sp3d2 hybridisation, and hence, have octahedral geometry. SF6
is extremely inert. Among the tetra fluorides SF4
is a gas, SeF4
is a liquid and TeF4
is a solid. Except for selenium, all the other elements of this group form dichlorides and dibromides.