A,D,Y,L and X are 15^th group elements belonging to Modern periodic table. The correct trend of p_ka of hydrides is AH₃ > DH₃ > YH₃ > LH₃ > XH₃. The element ‘A’ in the sequence is

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

a) X-H bond enthalpy decreases….N-H>P-H>As-H>Sb-H>Bi-H ;

b) Thermal stability decreases….NH₃>PH₃>AsH₃>SbH₃>BiH₃ ;

NH₃ is exothermic product,remaining four hydrides are endothermic products.

c) Reducing power increases….NH₃<PH₃<AsH₃<SbH₃<BiH₃ ;

d) Acidic strength (K_a) increases….NH₃<PH₃<AsH₃<SbH₃<BiH₃ ;

e) p^K_a decreases……NH₃>PH₃>AsH₃>SbH₃>BiH₃ ;

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 NH₃, 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….NH₃>PH₃>AsH₃>SbH₃>BiH₃ ;

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

a) Basicity decreases,Lewis base strength decreases…..NH₃>PH₃>AsH₃>SbH₃>BiH₃ ;

b) Electron donating power decreases……NH₃>PH₃>AsH₃>SbH₃>BiH₃ ;

c) Ligand strength and ability to form dative bond decreases….NH₃>PH₃>AsH₃>SbH₃>BiH₃ ;