(i) Di-nitrogen is a gas but phosphorous is a solid
The physical state of any element is based on the intermolecular attraction of the atoms or molecules in the matter. For the case of nitrogen it occurs in second period of the periodic table, so it has smaller atomic size.
The nuclear attraction is enough to hold the outer shell electrons and also they can easily undergo pi-overlap to give a triple bonded N2 molecule.
Phosphorous is in third period of the table and has much larger atomic size which makes it relatively less favorable for interatomic bonding and to undergo formation of a di-atomic molecule. Instead of that it can directly form σ – bond with the neighboring atoms to form a network of trivalent phosphorous as shown:
This strong σ – bond attraction leads to a greater packing of molecules leading to a solid formation.
(ii) Bond angle decreases from H2O to H2Te
Bond angle is influenced by the interatomic repulsion and the atomic size of the central atom. Oxygen has Z=8 and Te has Z=52, which shows it has larger atomic radius. Now we consider the fact that bond formation takes place by overlap of orbitals according to the Valence Bond Theory.
This overlap of orbitals gives them a definite structure and spatial geometry in space. Both of them have a V-shaped geometry due to the extra 2 lone pairs as the group 16 members have two lone pairs of electrons.
In case of oxygen due to small atomic size and higher electronegativity it can hold together those two electron pairs i.e. the electrons exist in pairs. For tellurium, however, due to very large atomic size and weaker nuclear attraction and shielding effect of the d-orbitals, the net result is that the electrons actually are isolated i.e. the spread over the large atomic radius to overcome electron repulsion.
This in turn decreases the bond angle of H ─ Te ─H. thus the bond angle decreases down the group from H2O to H2Te.
(iii) Halogens have maximum negative electron gain enthalpy
Electron gain enthalpy is the energy released or gained by an atom while accepting or donating an electron to attain a s tate of more stability. In general, half-filled or fully filled orbitals have more stability over other configurations.
Halogens have a general outer electronic configuration of ns2 np5. Thus the outermost p-orbital only needs one electron to attain a complete octate as well as a completely filled orbital. Both of these conditions are stable and halogens will accept an electron to thus release energy as it attains a stable state
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