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NCERT Class XII Chemistry
Chapter - The p-Block Elements
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Question : 3
Total: 74
Discuss the trends in chemical reactivity of group 15 elements.
Solution:  
Nitrogen has very low reactivity due to unavailability of vacantd-orbital and high bond dissociation energy of N ≡ N bond.
(a) Hydrides : General formula for hydrides is MH3,e.g.,NH3,PH3, AsH3,SbH3,BiH3. All these hydridesarecovalent in nature and have pyramidalstructure (sp3hybridized).
 Property  Down thegroup  Reason
 Basicstrength ‌3  decreases  The size of central atom increases, electrondensity decreases.
 Thermalstability of MH‌3  decreases  The size of the central atom increases,its tendency to form stable M – H bondsdecreases.
 Reducingcharacter  increases  The stability of hydrides decreases, thus thereducing character increases.
 Melting andboiling point  increases(except in N)  NH3 has high melting point and boilingpoint than PH3 due to hydrogen bonding.As the molecular size increases van derWaals forces increases.
(b) Halides : Elements of group 15 form two types of halides viz. trihalidesand pentahalides. The halides are predominantly basic (Lewis bases) innature and have lone pair of electrons (central atom is sp3hybridized).The pentahalides arethermallyless stable than the trihalides.
 Property  Gradation  Reason
 Stability of trihalidesof nitrogen  NF3>NCl3>NBr3  Large size difference between Nand the halogens
 Lewis base strength  NF3< NCl3< NBr3< NI3  Decreasing electronegativity ofhalogens
 Bond angle amongthe halides ofphosphorus   PF3< PCl3< PBr3< PI3  Due to decreased bond pair-bondpair repulsion
(c) Oxides : All the elements of this group form two types of oxidesi.e.,M2O3 and M2O5 and are called trioxides and pentoxides.
 Property  Gradation  Reason
 Acidic strengthof trioxides   N2O3>P2O3> As2O3  Electronegativity of central atomdecreases
 Acidic strengthof pentoxide  N2O5> P2O5> As2O5> Sb2O5> Bi2O5  Electronegativity of central atomdecreases.
 Acidic strengthof oxides ofnitrogen   N2O< ‌NO< N2O3<N2O4<N2O5  Oxidation state of central atomincreases
 Stability ofpentoxide   P2O5> As2O5> Sb2O5> N2O5> Bi2O5  Stability of oxides of a higheroxidation state i.e.,M2O5 decreaseswith increasing atomic number
(d) Oxoacids : The elements of this group form a number of oxoacids out of which those of N and P are more common.
Oxoacids of N :
 Formula (Name)  Ox. state of N
 H2N2O2 (Hyponitrous acid)  +1
 HNO2 (Nitrous acid)  +3
 HNO3 (Nitric acid)  +5
 Formula (Name)  Ox. state  Basicity
 H3PO3 (Phosphorous acid)  +3  2
 H3PO4 (Orthophosphoric acid)  +5  3
 HPO3 (Metaphosphoric acid)  +5  1
 H4P2O6 (Hypophosphoric acid)  +4  4
 H2P2O7 (Pyrophosphoric acid)  +5  4
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