P-block elements are those elements which have last electron in p-subshell of valence cell or outermost shell are called p-block elements. Now that p- subshell has 6 electrons hence six groups are present in this block from 13 to 18. In class 12th, we have to study only elements of four groups from 15 to 18. P-block elements have general valence shell electronic configuration ns2 np1–6. The properties of p-block elements are affected by variation in atomic size, ionization energy, electron gain enthalpy and electronegativity. The first element of these groups also show anomalous behaviour from the rest of elements of same group.
Elements of Group- 15
1. Electronic Configuration:
The valence shell or outermost shell electronic configuration of the elements of this group is ns2np3. The s orbitals these elements are completely filled and p orbitals of valence shell are half-filled.
2. Atomic and Ionic Radii:
Covalent and ionic radii of the elements of this group increase down the group. There is remarkable increase in covalent radius from N to P but from As to Bi only a small increase is observed due to presence of completely filled d or d and f-subshell.
3. Ionisation Enthalpy:
Down the group, ionisation enthalpy decreases due to gradual increase in atomic size. Ionization enthalpy of Group 15 elements greater than the elements of Group 14 in a particular period due to having completely half-filled p-subshell.
4. Electronegativity:
From top to bottom in the group, electronegativity decreases, but not so much difference is found in heavier elements as expected.
5. Physical State:
• Nitrogen is a diatomic gas, while other elements of this group are polyatomic solid.
• Boiling point increases down the group, but the b.p. of Sb is greater than Bi.
• Melting point increases up to arsenic from Nitrogen and then decreases up to bismuth.
• All the elements of group 15 show allotropy except nitrogen.
6. Chemical Properties:
Oxidation State : Common oxidation state of Group 15 elements are –3, +3 and +5. The tendency to show –3 oxidation state decreases down the group due to increase in size and metallic character. Bi hardly forms any compound in –3 oxidation state. The stability of +5 oxidation state decreases and that of +3 oxidation state increases down the group due to inert pair effect. Nitrogen shows +1, +2, +4 oxidation state also when it reacts with oxygen and form oxide.
Covalency : Nitrogen shows maximum covalency of four, because only four orbitals (one s and three p) are available for bonding. The heavier element with vacant d orbital can show covalency more than four.
Formation of a hydrides: All Group 15 elements form hydrides of the type EH3. Where E= N, P, As ,Sb and Bi.
Properties of Hydrides :
Formation of Oxides: All Group 15 elements form oxides of type E2O3 and E2O5. The oxide in higher oxidation state is more acidic than in lower oxidation state. Their acidic strength decreases down the group. The oxide E2O3 of N and P are acidic, that of As and Sb are amphoteric and those of Bi are certainly basic.
Formation of Halides: Elements of group 15 react with halogen to form halide of type EX3 and EX5. N2 does not form NX5 due to absence of d-orbitals in valence shell. All the trihalides are stable except those of Nitrogen. Only NF3 is stable.
Reaction with Metals:- All these elements react with metals to form their binary compounds showing –3 oxidation state, such as, Ca3N2 .
Anomalous properties of nitrogen
Nitrogen differs from the rest of the elements of 15th group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d-orbitals.
These properties are followings:
(1) Nitrogen has unique ability to form pπ-pπ multiple bonds with itself and with other elements
(2) Due to having small size and high electronegativity. Nitrogen exists as a diatomic molecule with a triple bond (one s and two p) between the two atoms. Consequently, its bond enthalpy (941.4 kJ mol–1) is very high.
Preparation of Dinitrogen:
Dinitrogen is produced commercially by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. 77.2 K) due to having lower boiling point distils out first leaving behind liquid oxygen (b.p. 90 K).
In the laboratory, Dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite.
NH4Cl(aq) + NaNO2(aq) ⟶ N2(g) + 2H2O(l) + NaCl(aq)
Properties of Nitrogen :
(1) It is a colorless, odorless, tasteless and nontoxic gas. It has two stable isotopes N-14 and N-15.
(2) It has very low solubility in water due to being non-polar (23.2 cm3 per litre of water at 273 K and 1 bar P) and low freezing and boiling point as it has weak dispersion force among its molecules.
(3) Dinitrogen is inert at room temperature because of high bond enthalpy of N ☰ N bond. It’s reactivity increase with increases in temperature.
(4) It combines with H2 at about 773 K in presence of catalyst (Haber’s process) to form ammonia.
N2(g) + 3H2(g) ⟶ 2NH3(g)
Preparation of Ammonia :
Ammonia is present in small quantities in air and soil. It is formed by decay of nitrogenous organic compounds e.g., urea.
NH2CONH2 + 2H2O ⟶ (NH4)2CO3 ⟶ 2NH3 + H2O + CO2
On small scale, it is obtained from ammonium salts which decompose when treated with caustic soda or lime.
2NH4Cl + Ca(OH)2 ⟶ 2NH3 + 2H2O + CaCl2
(NH4)2SO4 + 2NaOH ⟶ 2NH3 + 2H2O + Na2SO4
Structure of ammonia:
Properties of Ammonia:
(1) Ammonia is a colourless gas with pungent odour. Freezing point = 198.4 K. Boiling point = 239.7 K. As it is associated through hydrogen bonding in solid and liquid states.
(2) It has higher melting and boiling point, than expected on the basis of its molecular mass.
(3) Ammonia is highly soluble in water. Its aqueous solution is weakly basic due to formation of OH– ions.
NH3(g) + H2O(l) ⟶ NH4+(aq) + OH–(aq)
(4) As a weak Lewis base, it precipitates hydroxide of many metals from their salt solutions e.g.,
ZnSO4 + 2NH4OH ⟶ Zn(OH)2 + (NH4)2SO4
2FeCl3 + 2NH4OH ⟶ Fe2O3.xH2O + 3 NH4Cl
(5) Due to having lone pair of electrons on Nitrogen atom in ammonia, it behaves like a Lewis acid and form complex compounds with some metal ions and hence these reactions are applied for detection of metal ions.
Cu2+(aq) + 4NH3 ⟶ [Cu(NH3)4]2+
(Blue) (deep blue)
Ag+ + Cl– ⟶ AgCl
(Colourless) (white)
AgCl + 2NH3 ⟶ [Ag(NH3)2]Cl
(White ppt) (Colourless)
Oxides of Nitrogen :
Preparation of Nitric acid:
(1) In laboratory HNO3 prepared by heating KNO3 or NaNO3 and conc. H2SO4 in glass retort.
NaNO3 + H2SO4 ⟶ NaHSO4 + HNO3
(2) On a large scale, it is prepared by Ostwald process in the following way:
Now NO is recycled and the aqueous HNO3 is concentrated by distillation upto 68% by mass and further concentrated to 98% by dehydration with conc. H2SO4.
Structure of Nitric acid:
Properties of Nitric acid:
(1) It is a colourless liquid.
(2) Its freezing point is 231.4 K and boiling point is 355.6 K.
(3) HNO3 contains ~68% of HNO3 by mass and has specific gravity 1.504.
(4) In aqueous solution, HNO3 behaves as strong acid giving hydronium ions and nitrate ions.
HNO3 (aq) + H2O (l) → H3O+(aq) + NO3-
(5) Concentrated nitric acid is strong oxidising agent and attacks most metals except noble metals such as gold and platinum.
(4) HNO3 is a good oxidising agent and its products depend on the concentration of the acid in the following way:
3Cu + 8HNO3(dil.) → 3 Cu(NO3)2 + 2NO + 4H2O
Cu + 4HNO3(conc) → Cu(NO3)2 + 2NO2 + 2H2O
(5) Concentrated nitric acid also oxidises non – metals and their compounds in the following way:
I2 + 10 HNO3 → 2 HIO3 + 10NO2 + 4H2O
C + 4 HNO3 → CO2 + 2H2O + 4NO2
S8 + 48HNO3 → 8H2SO4 + 48NO2 + 16H2O
Brown Ring Test for Nitrates:
This test is done by adding dil. FeSO4 to an aqueous solution containing NO3- and then adding conc. H2SO4 along the sides of test tube. Brown ring at interface between solution and H2SO4 layer indicates the presence of NO3- in solution.
NO3- + 3Fe2+ + 4H+ ⟶ NO + 3Fe3+ 2H2O
[Fe(H2O)6]2+ + NO ⟶ [Fe(H2O)5(NO)]2+ + H2O
Phosphorous and its allotrops :
Phosphorus is found in many allotropic forms, the important ones of them are white, red and black.
White phosphorus is a translucent white 
waxy solid. It is poisonous, insoluble in water but soluble in carbon disulphide and glows in dark. It dissolves in boiling NaOH solution in an inert atmosphere giving PH3.
P4 + NaOH + 3H2O ⟶ PH3 + 3NaH2PO2
Red phosphorus is obtained by heating white phosphorus at 573K in an inert atmosphere for several days. Red phosphorus possesses iron grey lustre. It is odourless, non-poisonous and insoluble in water as well as in carbon disulphide. Chemically, red phosphorus is much less reactive than white phosphorus. It does not glow in the dark.
Structure of red Phosphorous:
Black phosphorus has two forms α-black phosphorus and β-black phosphorus. α-Black phosphorus is formed when red phosphorus is heated in a sealed tube at 803K. β-Black phosphorus is prepared by heating white phosphorus at 473 K under high pressure.
Preparation of phosphine
Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl.
Ca3P2 + 6H2O ⟶ 3Ca(OH)2 + 2PH3
Properties of phosphine gas:
It is a colourless gas with rotten fish smell and is highly poisonous. It explodes in contact with traces of oxidising agents like HNO3, Cl2 and Br2 vapours. It is slightly soluble in water. The solution of PH3 in water decomposes in presence of light giving red phosphorus and H2
3CuSO4 + 2PH3 ⟶ Cu3P2 + 3H2SO4
Use of phosphine:
The spontaneous combustion of phosphine is technically used in Home’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal. It is also used in smoke screens.
Phosphorus Halides:
Phosphorus forms two types of halides, PX3 (X = F, Cl, Br, I) and PX5 (X = F, Cl, Br).
(i). Phosphorus Trichloride
Preparation of PCl3 : It is obtained by passing dry chlorine over heated white phosphorus.
P4 + 6Cl2 ⟶ 4PCl3
It can be also obtained by the action of thionyl chloride with white phosphorus
P4 + 8SOCl2 → 4PCl3 + 4SO2 + 2S2Cl2
Structure of PCl3
Properties of PCl3 It is a colourless oily liquid and hydrolyses in the presence of moisture.
PCl3 + 3H2O ⟶ H3PO3 + 3HCl
(ii). Phosphorus Penta chloride
Preparation of PCl5 Phosphorus penta chloride is prepared by the reaction of white phosphorus with excess of dry chlorine.
P4 + 10Cl2 ⟶ 4PCl5
It can be also prepared by the action of SO2Cl2 on white Phosphorus
P4 + 10 SO2Cl2 → 4PCl5 + 10SO2
Structure of PCl5
Tri gonal bipyramidal
Properties of Phosphorus penta chloride:
PCl5 is a yellowish white powder and in moist air, it hydrolyses to POCl3 and finally gets converted to phosphoric acid.
PCl5 + H2O ⟶ POCl3 + 2HCl
POCl3 + 3H2O → H3PO4 + 3HCl
When heated, it sublimes but decomposes on stronger heating.
PCl5 ⟶ PCl3 + Cl2
Oxaacids of Phosphorous
Phosphorus forms a number of oxoacids. The important oxoacids of phosphorus with their formulas given in Table.
The compositions of the oxoacids are interrelated in terms of loss or gain of H2O molecule or O-atom. The structures of some important oxoacids are given below :
Structure of oxaacids:
