Introduction

In periodic table, p-block elements are placed in Group 13 to Group 18, having general valence shell electronic configuration ns2 np1–6. Properties of p-block elements are influenced by variation in atomic size, ionization energy, electron gain enthalpy and electronegativity. The 1st element of each group shows anomalous behaviour from rest of members of same group.

Group 15 Elements

Group 15 includes nitrogen, phosphorus, arsenic, antimony, bismuth and moscovium. As we go down the group, there is a shift from non-metallic to metallic through metalloidic character. Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids, bismuth and moscovium are typical metals.

Properties of Group 15

1. Electronic Configuration

The valence shell electronic configuration of these elements is ns2np3. The s orbital in these elements is completely filled and p orbitals are half-filled.

2. Atomic and Ionic Radii

Covalent and ionic radii increase in size down the group. There is a considerable increase in covalent radius from N to P.

3. Ionisation Enthalpy

Down the group, ionisation enthalpy decreases due to gradual increase in atomic size. Also ionization enthalpy of Group 15 elements much higher than that Group 14 element in a particular period.

4. Electronegativity

Down the group electronegativity value decreases, but difference in value is not much in heavier elements.

5. Physical State

Nitrogen is a diatomic gas, while other elements are polyatomic solid. Boiling point increases down the group, but that of Sb is greater than Bi. Melting point increases upto arsenic and then decreases upto bismuth. All 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. 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.

Covalency : Nitrogen shows maximum covalency of four, as 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. All Group 15 elements form hydrides of the type EH3. 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. Elements of group 15 reacts to form halide of type EX3 and EX5. All these elements react with metals to form their binary compounds exhibiting –3 oxidation state, such as, Ca3N2 (calcium nitride).

Anomalous properties of nitrogen

Nitrogen differs from the rest of the members of 15 group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d orbitals. Nitrogen has unique ability to form pπ-pπ multiple bonds with itself and with other elements 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.

Dinitrogen

Preparation

1. Dinitrogen is produced commercially by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. 77.2 K) distils out first leaving behind liquid oxygen (b.p. 90 K).

2. 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

1. It is colourless, odourless, tasteless and nontoxic gas. It has two stable isotopes N14 and N15.

2. It has very low solubility in water (23.2 cm3 per litre of water at 273 K and 1 bar P) and low freezing and boiling point.

3. Dinitrogen is inert at room temperature because of high bond enthalpy of N ☰ N bond. With increase in temperature reactivity increases.

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)

Ammonia

Preparation

Ammonia is present in small quantities in air and soil, 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

Properties

1. It is colourless liquid.

2. 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 and nitrate ions.

5. Concentrated nitric acid is strong oxidising agent and attacks most metals except noble metals such as gold and platinum.

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

Allotropic Forms of Phosphorus

Phosphorus is found in many allotropic forms, the important ones being 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

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.

Phosphine (PH3)

Preparation

Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl.

Ca3P2 + 6H2O ⟶ 3Ca(OH)2 + 2PH3

Properties

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

Uses

The spontaneous combustion of phosphine is technically used in Holme’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 Halide

Phosphorus forms two types of halides, PX3 (X = F, Cl, Br, I) and PX5 (X = F, Cl, Br).

(i). Phosphorus Trichloride

Preparation

It is obtained by passing dry chlorine over heated white phosphorus.

P4 + 6Cl2 ⟶ 4PCl3

Properties

It is a colourless oily liquid and hydrolyses in the presence of moisture.

PCl3 + 3H2O ⟶ H3PO3 + 3HCl

(ii). Phosphorus Pentachloride

Properties

1. O3 is pale blue gas, dark blue liquid and violet black solid.

2. It has characteristic smell and in small concentration it is harmless. But in concentration above 100 ppm breathing becomes uncomfortable resulting in headache and nausea.

3. O3 is thermodynamically unstable with respect to oxygen, as its decomposition liberates heat. Its conversion to O2 is favourable as ΔG is negative.

4. Due to ease to release nascent oxygen O3 acts as strong oxidising agent, [O3 ⟶ O2 + O]

Uses

It is used as a germicide, disinfectant and for sterilising water. It is also used for bleaching oils, ivory, flour, starch, etc. It acts as an oxidising agent in the manufacture of potassium permanganate.

Allotropes of Sulphur

Rhombic sulphur (α-sulphur)

This allotrope is yellow in colour, m.p. 385.8 K and specific gravity 2.06. Rhombic sulphur crystals are formed on evaporating the solution of roll sulphur in CS2. It is insoluble in water but dissolves to some extent in benzene, alcohol and ether. It is readily soluble in CS2.

Monoclinic sulphur (β-sulphur)

Its m.p. is 393 K and specific gravity 1.98. It is soluble in CS2. This form of sulphur is prepared by melting rhombic sulphur in a dish and cooling, till crust is formed. On removing the crust, colourless needle shaped crystals of β-sulphur are formed. It is stable above 369 K and transforms into α-sulphur below it.

Properties

Sulphur dioxide is a colourless gas with pungent smell and is highly soluble in water. It liquefies at room temperature under a pressure of two atmospheres and boils at 263 K.

Sulphur dioxide, when passed through water, forms a solution of sulphurous acid.

SO2 + H2O ⟶ H2SO3

(ii). Sulphuric Acid

Preparation

Sulphuric acid is manufactured by the Contact Process which involves three steps:

1. burning of sulphur or sulphide ores in air to generate SO2.

2. conversion of SO2 to SO3 by the reaction with oxygen in the presence of a catalyst (V2O5), and

3. absorption of SO3 in H2SO4 to give Oleum (H2S2O7).

The key step in the manufacture of H2SO4 is the catalytic oxidation of SO2 with O2 to give SO3 in the presence of V2O5 (catalyst).

2SO2 + O2 ⟶ 2SO3

Lastly, SO3 obtained is dissolved in 98% H2SO4 when oleum is formed. H2SO4 of any desired concentration can be obtained from oleum by dilution with water.

H2SO4 + SO3 ⟶ H2S2O7

H2SO4 + H2O ⟶ 2H2SO4

Interhalogen Compounds

When two different halogens react with each other, interhalogen compounds are formed. They can be assigned general compositions as XX′, XX′3, XX′5 and XX′7 where X is halogen of larger size and X′ of smaller size and X is more electropositive than X′.

Preparation

The interhalogen compounds can be prepared by the direct combination or by the action of halogen on lower interhalogen compounds. The product formed depends upon some specific

conditions.

Cl2 + F2 ⟶ 2ClF

Properties

These are all covalent molecules and are diamagnetic in nature. They are volatile solids or liquids at 298 K except ClF which is a gas. Their physical properties are intermediate between those of constituent halogens except that their m.p. and b.p. are a little higher than expected.

Uses

These compounds can be used as non aqueous solvents. Interhalogen compounds are very useful fluorinating agents. ClF3 and BrF3 are used for the production of UF6 in the enrichment of 235U.

Group 18 Elements

Group 18 consists of elements: helium, neon, argon, krypton, xenon, radon and oganesson. All these are gases and chemically unreactive. They form very few compounds, because of this they are termed as noble gases.

Electronic Configuration

All noble gases have general electronic configuration ns2np6 except helium which has 1s2.

Ionisation Enthalpy

Due to stable electronic configuration, these gases have very high ionization enthalpy. Down the group it decreases with increase in atomic size.

Atomic Radii

It increases down the group with increase in atomic number.

Electron Gain Enthalpy

As noble gases have stable electronic configuration, they have no tendency to accept electron and therefore large positive value of electron gain enthalpy.

Physical Properties

1. They are monoatomic, colourless, odourless and tasteless.

2. Sparingly soluble in water.

3. Very low melting and boiling points as weak dispersion forces present.

4. Helium has lowest boiling point (4.2 K) of any known substance.

5. It has unusual property of diffusing through commonly used laboratory materials like rubber, glass or plastics.

6. Noble gases are least reactive. Their chemical inertness is due to Completely filled valence shell.

Chemical Properties

Xe(g) + F2(g) ⟶ XeF2(s)

Xe(g) + 2F2(g) ⟶ XeF4(s)

Xe(g) + 3F2(g) ⟶ XeF6(s)

XeF6 + 3H2O ⟶ XeO3 + 6HF

Summary

1. Inert Pair Effect: The inability of valence shell ns2 electrons to take part in bond formation in p-block is known as inert pair effect.

2. Disproportionation: Same element gets oxidised as well as reduced in a chemical reaction.

3. pπ-pπ bond: The chemical bond (π) formed by lateral (sidewise) overlapping of p orbitals of two atoms in a compound or ion.

4. dπ-pπ bond: The chemical bond (π) formed by lateral overlapping of p and d orbitals of two atoms in a compound or ion.

5. Allotropy: Phenomenon of existence of an element in different physical forms.

6. Catenation: Tendency of self linking of element.

7. Chalcogens: Ore forming. Given to Group 16.

8. Halogens: Salt producers. Given to Group 17.

9. Interhalogen compound: Compounds formed between two different halgoen.

10. Noble gases: Elements of Group 18, as they form very few compounds that too under certain condition.

11. Anomalous behaviour: The unique behaviour of 1st member of a particular group from other members of same group.

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