The 15 group of the Periodic Table consists of nitrogen, phosphorus, arsenic, antimony and bismuth. These elements are known as pnicogens and their compounds as pniconides.
Physical Properties of Group 15 Elements:
(i) Electronic configuration Their valence shell electronic configuration is ns2 np3
(ii) Metallic character: N and P are non-metals, As and Sb are metalloids and Bi is metal.
(iii) Physical state: Nitrogen is the first element after hydrogen which is diatomic gas in native form. All other elements in the group are solids.
(iv) Atomicity: N2 is diatomic while others are triatomic.
(v) Melting and boiling points: The melting point increases from nitrogen to arsenic. The boiling points increase regularly on moving down the group.
(vi) Density: It increases down the group.
(Vii) Atomic radii: It increases with increase in atomic number as we go down the group.
(viii) Allotropy All the elements (except Bi) exhibit allotropy. Nitrogens – α nitrogen, β – nitrogen.
Phosphorus – White, red, black
Arsenic – Grey, yellow, black
Antimony – Metallic yellow (explosive)
(ix) Oxidation state:
Nitrogen has a wide range of oxidation states.
The stability of +3 oxidation state increases and stability of +5 oxidation state decreases on moving down the group due to inert pair effect.
(x) The ionization enthalpy: Ionisation energy of nitrogen is very high due to its small size and half-filled highly stable configuration. The ionization energy decreases down the group.
(xi) Electronegativity: It decreases from nitrogen to bismuth.
(xii) Catenation ‘They exhibit the property of catenation but to a lesser extent due to weak E – E bond than 14 group elements.
(xiii) Reactivity Elemental nitrogen is highly unreactive because of its strong triple bond. (almost as inert as noble gases).
White phosphorus is extremely reactive and is kept in water. It is inflammable and can be ignited at 45°C.
Chemical Properties of Group 15 Elements:
(i) Hydrides: All the elements of this group form hydrides of the type EH3, which are covalent and pyramidal in shape. Some properties follow the order as mentioned:
These properties are:
1. Thermal stability,
2. Basic strength,
3. Solubility in water,
4. Bond angle NH3 (107.4°); PH3 (92°),AsH3 (91° ), SbH3(90° ),
5. Strength of M – H bond
Some properties follow the order-
NH3 < PH3 < AsH3 < SbH3 < BiH3
These properties are -
1. Reducing character
2. Covalent character
3. Rate of combustion
(ii) Halides All the elements of this group form trihalides, MX3 and except nitrogen all form pentahalides, MX5, e.g., NCi3, NI3, PCI3, BiCI3, AsCI3 , PCl5 etc. Trihalides (except of N) behaves as Lewis acid and the order of their strength is PCl3 > AsCl3 > SbCl3. Trihalides of N behave as Lewis base and has the following order of strength-
NF3 < NCl3 < NBr3 < NI3
NCl3 is an explosive compound.
(iii) Oxides All the elements of this group form oxides of the type M2O3 and M2O5.
The acidic strength of pentoxides and trioxides decrease on moving down the group, i.e.,
N2O5 > P2O5 > As2O5 > Sb2O5
BiOCl is called pearl white.
Nitrogen and its Compounds:
1. Dinitrogen (N2)
1. Nitrogen does not react with alkali metals except Li but reacts with alkaline earth metals to give metal nitride.
2. Reaction with oxygen.
3. Reaction with non-metals.
4. Reaction with CaC2.
Uses Liquid N2 is used as refrigerant. N2 is used in the manufacture of HNO2, NH2, CaCN2(calcium cyanamide) and other nitrogenous compounds. It is used for filling electric bulbs.
2. Ammonia (NH3)
(i) Lab method
2NH4Cl + Ca(OH)2 → CaCI2 + 2NH3 + 2H2O
(ii) Haber’s process
1. It is a colourless gas with characteristic pungent odour. It is extremely soluble in water due to H – bonding.
2. It is a strong Lewis base and used in the metal ion detection as-
3. Reaction with chlorine:
When NH3 is in excess, N2 is the main product.
8NH3 + 3Cl2 → 6NH4Cl + N3
When Cl2 is in excess, NCl3 is the main product.
NH3 + 3Cl2 → NCl3 + 3HCl
4. Reaction with Nessler’s reagent:
Uses: It is used as a refrigerant and to produce various nitrogenous fertilizers.
Oxides of Nitrogen
NO2 contains odd number of valence electrons. On dimerisation, it is converted to stable N2O4 molecule with even number of electrons.
3.Nitric acid (HNO3):
It is a stronger acid than H3PO4.
(i) Lab method
NaNO3 + H2SO4 (conc.) → NaHSO4 + HNO3
(ii) Ostwald’s process
Physical properties: It is a syrupy, colourless, pungent liquid usually available as 68 % and 15.7 M aqueous solution is often yellow due to small concentrations of NO2.
1. Action of nitric acid on zinc under different conditions-
2. Action of nitric acid on copper under different conditions-
3. Reaction with non-metals-
4. Brown ring test of nitrate-
5. Metals like Fe. Cr. Ni, AI or Co becomes inactive or passive due to stable oxide layers.
Structure of nitric acid-
Uses: It is used
1.In the manufacturing of fertilizers.
2. For purification of silver and gold.
3. In the manufacturing of explosives and as oxidising agent.
4. As nitrating reagent.
Phosphorus and its Compounds Allotropic Forms of Phosphorus
(i) White phosphorus
(ii) Red phosphorus
(iii) Black phosphorus
Some Points of Distinction Between White and Red Phosphorus
Black phosphorus is formed when red phosphorus is heated in a sealed tube at 803 K. It does not oxidise in air.
Match box side contains red P or P2S3+ glue and on tip of match stick. red P, KClO3 chalk and glue is deposited.
1. With non-metals-
2. With compounds-
Uses: It is used in match boxes, explosives, as rat poison, in fertilizers and alloys
1. Phosphine (PH3)
Preparation It is prepared by following methods
1. It is a colourless gas with rotten fish like smell and is highly poisonous. It explodes in contact with traces of oxidising agents like HNO3, Cl2 and Br2 vapours.
3CuSO4 + 2PH3 → CU3P2 + 3H2SO4
3HgCl2 + 2PH3 → Hg3P2 + 6HCl
2. Phosphine is weakly basic.
PH3 + HBr → PH+4+ Br–
Uses It is used to prepare smoke screens in warfare. A mixture of CaC2 and Ca3P2 is used in Holme’s signals.
2. Phosphorus Trichloride (PCl5)
Properties: It is a colourless oily liquid having pyramidal shape [sp3 – hybridised].
3.Phosphorus Pentachloride (PCl5)
P4 + 10 Cl2 → 4 PCl5
P4 + 10 SO2CI2 → 4PCl5 + 10 SO2
Structure PCl5 in gaseous and liquid phases has sp3d – hybridization and its shape is trigonal-bipyramidal. The three equatorial P – Cl bonds are equivalent while the two axial bonds are longer equatorial bonds.
Properties In solid state, PCl5 exists as an ionic solid, [PCI4]+ [PCl6]– in which, the cation, [PCI4]+ is tetrahedral and the anion [PCl6]– is octahedral.
Oxoacids of Phosphorus:
In toothpaste, CaHPO4 * 2H2O is added as mild abrasive and polish agent.