SureDenGroup 15 Elements Nitrogen Family
Group – 15 ELEMENTS (NITROGEN FAMILY)
The general physical and chemical properties of the group are:-
(1) Electronic Configuration:- The Electronic Configuration of group 15 elements is
(i) Nitrogen N = 7 1s2, 2s22(px)12(py)12(pz)1 [He]2s22p3
(ii) Phosphorous P = 15 1s2, 2s22p6, 3s23p3 [Ne]3s23p3
(iii) Arsenic As = 33 1s2, 2s22p6, 3s23p63d104s24p3 [Ar]3d104s24p3
(iv) Antimony Sb = 51 [Kr]4d105s25p3
(v) Bismuth Bi = 83 [Xe]4f145d106s26p3
The general valence shell electronic configuration of group 15 elements is ns2np6. The general Electronic configuration is [Noble Gas] (n-2)f14 (n-1)d10ns2np3
(2) Atomic Size and Ionisation Energy:- The atomic and ionic radii of group 15 elements are smaller than group 14 elements due to increase in nuclear charge and strong attractive forces of attraction between nucleus and outermost electron. On moving down the group the increases in atomic size is reported regularly due to addition of new energy shells.
The first Ionisation energy of group 15 elements is much higher than group 14 elements due to small size high nuclear charge and stable half filled electronic configuration. On moving down the group Ionisation energy steadily decreases due to increases in size.
(3) Oxidation State and Nature of Compounds:- All the elements of group 15 have five electrons in their valence shell. Their octet can be completed by gaining three electrons. However the gain of three electrons requires a large amount of energy, yet it occurs only with N atom due to its small size and high electro negativity value. This -3 oxidation state occurs with highly electropositive elements to form Mg3N2, Ca3N2 etc. These compounds are ionic in nature. The other members form compounds in -3 O.S. such as Ca3P2, Na3As etc., Which are covalent in nature.
N generally shows a covalency of 3. It cannot show the covalency of 5 as it cannot expand its octet due of non-availability of d-orbitals, while other elements can show the covalency of 5. Because due to availability of d-orbitals they can expand their octet. Hence NCl5 does not exist but PCl5 exists. However N also shows the covalency of 4 in N2O5. All the element show both +3 and +5 Oxidation State. However on moving down the group the stability of +3 Oxidation State goes on increasing due to inert effect.
The difference between the Oxidation state and Covalency is understood by taking up the example of N2O6, where N shows as covalency of 4 but an O.S. of +5. The covalency is 1 for each shared pair of electron (Covalent of co-ordinated), but O.S. is assigned +2 for (p5 less electronegative atom in co-ordinate bond).
Inert pair Effect:- On moving down the group the stability of lower O.S. goes on increasing and that of higher O.S. decreases. It is due to decreases in the tendency of outermost s-electrons to participate in bond formation because of their poor shielding by intervening d-and f-electrons.
(4) HYDRIDES:- The elements of group 15 from hydrides having the general formula MH3, which are covalent in nature. These elements do not directly react with hydrogen to form hydride, but their salts with metals on hydrolysis give hydrides of group 15.
Ca3N2 + 6H2O → 3Ca(OH)2 + 2NH3
Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3
Zn3M2 + HCl (dil) → ZnCl2 + 2MH3 (M = As, Sb or Bi)
Another hydride of N known as hydrazine (N2H4) is also formed
2NH3 + NaOCl → N2H4 + NaCl + H2O
(Sod.Hypochlorite)
in all these hydrides the central atom (group 15) assumes sp3-hybridization state. Due to the presence of Ione pair of electrons the bond angle is less than 109° 28’. This bond angle further goes on decreasing on moving down the group.
NH3 PH3 AsH3 SbH3 BiH3
107° 94° 92° 91° 90°
The decrease in bond angle is due to the decrease in the electro negativity of the central atom. As E.N. goes on decreasing the bond pair of electrons tend to lie away from central atom hence repulsion between H and H atoms decreases and bond angle also decrease. The bond angle of 
is greater than pH3 due to absence of Ione pair of electron.
Since the bond strength of M-H bond decreases down the group their reducing character (Tendency to lose H) goes on increasing.
The boiling point of NH3 is exceptionally high among other hydrides due to presence of intermolecular hydrogen bonding in NH3. In other hydrides there are Vander Waal’s Forces. The presence of Lone pair of electrons over central atom the hydrides of group 15 acts as Lewis base. Thus basic strength goes on decreasing on moving down the group. It is because on moving down the group size of atom goes on increasing hence the electron density goes on decreasing and tendency to donate electron pair goes on decreasing.
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NH3 PH3 AsH3 SbH3 BiH3 Bond Angle Decreases → Thermal stability, Basic nature decreases → Reducing Nature increases → |
(5) Halides:- All the elements of group 15 form trihalides (MX3) and penta halides (MX5) except N which form the trihalides
Trihalides:- The trihalides of group 15 are mainly covalent in nature and they are hydrolysed with water to form hydrohalic acids. The central atom in trihalides (MX3) is sp3 hybridized and thus form pyramidal structure. Due of the presence of highly electronegative halogen atoms the Ione pair of electrons is not available for donation. Hence the trihalides of P, As, Sb (Specially fluorides and chlorides) do not act as Lewis base but act as Lewis acid as the vacant d-orbitals can accommodate the incoming electrons.
Pentahalides:- Except nitrogen all the elements of group 15 form pentahalides N does not form penta halides as it cannot expand its octet due to non availability of d-orbitals. Phosphorous forms pentahalide (PCl5) in sp3 d-hybridisation state. The penta halides also act as Lewis acids due to the availability of vacant d-orbitals. The penta halides are less stable than tri halides.
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