(1) Hydrides : The elements of this group form hydrides such as H₂O, H₂S,H₂Se,H₂Te an H₂Po . Following are their characteristics.

(i) Physical states : Water is colourless and odourless while hydrides of the rest of the elements of this group are colourless, unpleasant smelling poisonous gases.

(ii) Volatile nature : Volatility increases from H₂O to H₂S and then decreases. The low volatility and abnormally high boiling point of water is due to the association of water molecules on account of hydrogen bonding because of strongly electronegative oxygen atom linked to hydrogen atom. thus, water is liquid while H₂S and other hydrides are gases under normal condition of temperature and pressure.

(iii) Acidic character : The hydrides of this group behave as weak diprotic acids in aqueous solution, the acidic character increasing from H₂S to H₂Te when H₂O is neutral.

(iv) Thermal stability : The thermal stability decreases from H₂O to H₂Po because the size of the central atom (from O to Po ) increases resulting in longer and weaker M – H bond consequently the bond strength decreases. This results in the decrease of the thermal stability.

(v) Reducing character : The reducing power of the hydrides increases from H₂O to H₂Po due to the decreasing bond strength from H₂O to H₂Po .

(vi) Bond angle : All these hydrides are angular molecules and the bond angle H – X – H (X is O,S,Se,Te ) decreases from H₂O to H₂Po .

Increasing order of reducing power of hydrides :

H₂O < H₂S < H₂Se < H₂Te

Increasing order of bond angles in hydrides :

H₂Te < H₂Se < H₂S < H₂O

The order of stability of hydrides :

H₂O < H₂S < H₂Se < H₂Te

The order of increasing acidic nature of hydrides :

H₂O < H₂S < H₂Se < H₂Te