Ø Ionisation Enthalpy: Ionisation enthalpy of these elements decreases down the group. It is due to increase in size. However, the elements of this group have lower ionisation enthalpy values compared to those of Group15 elements. This is due to the fact that Group 15 elements have extra stable half- filled p orbitals electronic configuration.
Oxygen atom has less negative electron gain enthalpy than sulphur because of the       compact nature of its shells due to which the electronic repulsion is greater.

Ø Oxidation State: They show -2, +2, +4, +6 oxidation states. Oxygen does not show +6 oxidation state due to absence of d – orbitals. Po does not show +6 oxidation state due to inert pair effect.
The stability of -2 oxidation state decreases down the group due to increase in atomic size and decrease in electronegativity. Since electronegativity of oxygen is very high, it shows only –2 oxidation state (except in the case of OF2 where its oxidation state is + 2).
+ 4 and + 6 are more common. Sulphur, selenium and tellurium usually show + 4 oxidation state in their compounds with oxygen and + 6 with fluorine. Down the group, the stability of + 6 oxidation state decreases and that of + 4 oxidation state increases (due to inert pair effect).

Ø Electronegativity: It is the tendency of an atom to attract shared paired electrons towards itself. The order of electronegativity is

O > S > Se > Te > Po (regular trend)


Ø Electron gain enthalpy: Oxygen has less negative electron gain enthalpy than S because of small size of O.
From S to Po electron gain enthalpy becomes less negative to Po because of increase in atomic size.  The order of Electron Gain Enthalpy is
S > Se > Te > Po > O

Ø Melting and boiling point: It increases with increase in atomic number. Oxygen has much lower melting and boiling points than Sulphur because oxygen is diatomic (O2) and Sulphur is Octatomic (S8).
  

ü Chemical Properties:
Ø Hydrides: All the elements of group 16 from hydrides of the general formula H2E where E is the element belonging to group 16. Following are some of the characteristics of these hydrides: H2O, H2S, H2Se, H2Te.
·       Their acidic character increases from H2O to H2Te. This is due to the decrease in bond (H–E) dissociation enthalpy down the group.
H2Te > H2Se > H2S > H2O
·       The thermal stability also decreases down the group due to increase in bond length.
                      H2Te > H2Se > H2S > H2O
·       All the hydrides except water possess reducing property and this character increases from H2S to H2Te.
                               H2S > H2Se > H2Te
·       Bond angles H2O > H2S > H2Se > H2Te. Due to repulsion between lone pairs.