Which one of the following hydride is the best reducing agent?

Down the group as the size of the atom increases affinity of hydrogen towards central atom decreases. Therefore

a) X-H bond enthalpy decreases

b) Thermal stability decreases….NH₃ is exothermic product,remaining four hydrides are endothermic products.

c) Reducing power increases

d) acidic strength (K_a) increases

e) p^K_a decreases

Down the group as vanderwaal's forces increases

Melting and boiling points increases

Expected BP and MP….NH₃<PH₃<AsH₃<SbH₃<BiH₃

Due to H-bonding in ammonia, it has higher BP and MP than expected.

BP trend…...PH₃<AsH₃<.NH₃<SbH₃<BiH₃

MP trend…..PH₃<AsH₃<.SbH₃<BiH₃<NH₃

Central atom in these hydrides contain one lone pair, according to Bent rules

As the Electronegativity of central atom decreases, bond angle decreases

VSEPR theory is required to explain the shape of Ammonia molecule, VBT can explain the shape of other hydrides;

Bond angle in water is about 107⁰ whereas it is 90⁰ in other hydrides;

Nitrogen in Ammonia molecule is sp³ hybridized whereas pure p-orbitals of other pnicogens are involved in bonding in their hydrides;

Down the group as the size of central atom increases, electron density on central atom decreases. Thus

a) Basicity decreases,Lewis base strength decreases

b) Electron donating power decreases, ligand strength and ability to form dative bond decreases.