Group 15 Elements Nitrogen Family - Chemistry Class 12 - NEET PDF Download

Table of Contents
1. What are Group 15 Elements?
2. Physical Properties of Group 15 Elements
3. Chemical Properties of Group 15 Elements
4. Compounds of Nitrogen
5. Phosphorus and its Allotropic Forms
6. Compounds of Phosphorus
7. Summary
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What are Group 15 Elements?

Group 15 of the periodic table is commonly known as the Nitrogen family. The members of this group are nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi). These are p-block elements; collectively they are often called pnictogens and their binary compounds with metals or hydrogen are referred to as pnictides.

Physical Properties of Group 15 Elements

1. Electronic configuration: The general valence shell electronic configuration is ns² np³.
2. Metallic character: Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids, and bismuth is metallic.
3. Physical state: Nitrogen (N₂) is a diatomic gas under standard conditions; the remaining elements of the group are solids.
4. Atomicity: Molecular nitrogen exists as N₂ (diatomic). The elemental forms of other members are typically polymeric/atomic solids (not diatomic).
5. Melting and boiling points: Melting and boiling points generally increase down the group for the heavier elements; there are variations due to molecular structures and allotropy.
6. Density: Density increases on moving down the group.
7. Atomic radii: Atomic radius increases down the group because of the addition of electron shells.
8. Allotropy: Most members (except Bi) show allotropy.
9. Oxidation statesTypical oxidation states are -3 to +5. Nitrogen shows a wide range (-3 to +5). The stability of the +3 oxidation state increases and the stability of the +5 oxidation state decreases down the group due to the inert pair effect.
   MULTIPLE CHOICE QUESTION

   Try yourself: What catalyst is used for oxidation of ammonia to produce nitric acid?

   A

   Palladium hydride

   B

   Sodium amalgam

   C

   Platinum-Rhodium gauze

   D

   Vanadium (V) oxide

   View Solution

   Correct Answer: CHide Explanation ⌃

   Ammonia is oxidized to nitrogen (II) oxide in the presence of Pt/Rh gauze catalyst at a temperature of 500 K and a pressure of 9 bars. The nitrous oxide is then converted to nitrogen dioxide which is further reacted with water to produce nitric acid. The NO formed is recycled.

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10. Ionisation enthalpy: Nitrogen has very high ionisation energy because of its small size and the relative stability of the half-filled p³ configuration. Ionisation enthalpies decrease down the group.
11. Electronegativity: Electronegativity decreases from nitrogen to bismuth.
12. Catenation: These elements can form E-E bonds and show catenation, but to a lesser extent than Group 14 elements because E-E bonds in Group 15 are weaker.
13. Reactivity: Elemental nitrogen is relatively inert due to the strong N≡N triple bond. White phosphorus is highly reactive (kept under water to prevent oxidation and is easily ignitable at moderate temperatures).

Chemical Properties of Group 15 Elements

1. Hydrides (EH₃): All members form covalent hydrides of the type EH₃(for nitrogen, phosphorous, arsenic, antimony and bismuth the hydrides are NH₃, PH₃, AsH₃, SbH₃ and BiH₃ respectively). These hydrides are pyramidal in shape (for NH₃ the bond angle is about 107.4°). Important trends and points:
   
   • Bond angles: NH₃ ≈ 107.4°, PH₃ ≈ 92°, AsH₃ ≈ 91°, SbH₃ ≈ 90°.
   • Basic strength: Basicity (as proton acceptor) decreases down the group; ammonia (NH₃) is significantly more basic than PH₃, AsH₃ etc. (PH₃ and heavier hydrides are very weak bases in water).
   • Thermal stability: Thermal stability of EH₃ generally decreases on moving down the group (NH₃ is thermally most stable among them under ordinary conditions).
   • Reducing character and covalent character: Reducing power and covalent character increase down the group: NH₃ < PH₃ < AsH₃ < SbH₃ < BiH₃.
   • Solubility in water: Solubility decreases down the group (NH₃ is highly soluble due to hydrogen bonding; heavier hydrides are much less soluble).
   • Strength of E-H bond: E-H bond strength decreases down the group.
2. Halides: Members commonly form trihalides (MX₃) and, except nitrogen, also pentahalides (MX₅) - for example PCl₃, PCl₅, AsCl₃, SbCl₃, BiCl₃, etc. Nitrogen forms halides such as NF₃, NCl₃, NBr₃ and NI₃ (NI₃ is very unstable and explosive). Trihalides (except those of nitrogen) tend to behave as Lewis acids. For nitrogen halides, NF₃ is relatively less basic than 
   NCl₃ < NBr₃ < NI₃ in the sense of donating the lone pair on N.
3. Oxides:Members form oxides of the types M₂O₃ and M₂O₅ and other oxides (for nitrogen many oxides with varying oxidation states exist: NO, N₂O, NO₂, N₂O₃, N₂O₄, N₂O₅). Acid-base character of these oxides:
   • N₂O₅ and N₂O₄ are strongly acidic oxides; NO and N₂O are essentially neutral.
   • P₄O₁₀ (structure related to P₂O₅) is strongly acidic.
   • As₄O₆ (sometimes called white arsenic) is poisonous.
   • Acidic strength of pentoxides and trioxides generally decreases down the group: N₂O₅ > P₂O₅ > As₂O₅ > Sb₂O₅.

Compounds of Nitrogen

1. Dinitrogen (N₂)

   Preparation: Industrially obtained by fractional distillation of liquefied air; in the laboratory by heating ammonium nitrite or by decomposition of certain azides and other compounds (various laboratory methods exist).

   

   Chemical properties:

   (i) Nitrogen does not react with alkali metals under ordinary conditions (except lithium at high temperature) but reacts with some alkaline earth metals and active metals at elevated temperatures to form nitrides (e.g., Ca₃N₂).

   

   (ii) With oxygen, N₂ can react at very high temperatures (as in lightning or electric arcs) to form NO and other oxides of nitrogen.

   

   (iii) Reaction with non-metals: under suitable conditions nitrogen reacts to give nitrides or other nitrogen-containing compounds.

   

   (iv) Reaction with calcium carbide (CaC₂) forms calcium cyanamide (CaCN₂) on heating in presence of N₂.

   

   Uses: Liquid nitrogen is used as a refrigerant (cryogen). Nitrogen is used in the manufacture of nitric acid, ammonia (via the Haber process), calcium cyanamide and other nitrogenous compounds; it is also used for inert atmospheres (e.g., in electrical bulbs, food packaging, and laboratory applications).

   MULTIPLE CHOICE QUESTION

   Try yourself: Why does nitrogen show anomalous properties with respect to other elements in group 15?

   A

   Nitrogen has low ionization enthalpy

   B

   Nitrogen atomhas high inter-electronic repulsions

   C

   Nitrogen molecule bears a triple bond

   D

   Absence of vacant d-orbitals

   View Solution

   Correct Answer: DHide Explanation ⌃

   Nitrogen shows anomalous properties compared to other elements of group 15 because of the absence of vacant d-orbitals. In addition to this, anomalous properties are a result of the nitrogen atom’s smaller size, highest electronegativity and highest ionization energy with respect to all the elements in group 15.

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2. Ammonia (NH₃)

   Preparation:

   (i) Laboratory method: NH₄Cl + Ca(OH)₂ → CaCl₂ + 2 NH₃ + 2 H₂O (commonly written as 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O).

   (ii) Industrial method (Haber process): N₂ + 3 H₂ ⇌ 2 NH₃ (high pressure, moderate temperature, catalyst such as Fe with promoters).

   

   Properties:

   (i) Ammonia is a colourless gas with a characteristic pungent odour; it is highly soluble in water due to hydrogen bonding.

   (ii) Ammonia is a strong Lewis base (it donates its lone pair on nitrogen) and forms coordination complexes with many metal ions; it is used as a reagent in qualitative inorganic analysis to detect metal ions.

   

   Reaction with chlorine:

   When NH₃ is in excess, the principal product is nitrogen: 
   8 NH₃ + 3 Cl₂ → 6 NH₄Cl + N₂.

   When Cl₂ is in excess, nitrogen trichloride (NCl₃) is produced:
    NH₃ + 3 Cl₂ → NCl₃ + 3 HCl.

   Reaction with Nessler's reagent: Nessler's reagent gives a brownish colour or brown precipitate with trace amounts of ammonia/ammonium and is used for qualitative and semi-quantitative detection of NH₃/NH₄⁺.

   
   

   Uses: As refrigerant (historically), in the manufacture of fertilizers (urea, ammonium nitrate), nitric acid (via oxidation to NO and further conversion), and many nitrogenous compounds.

   MULTIPLE CHOICE QUESTION

   Try yourself: How many unshared pair of electrons does an ammonia molecule have?

   A

   1

   B

   2

   C

   3

   D

   4

   View Solution

   Correct Answer: AHide Explanation ⌃

   The nitrogen atom in NH₃ (ammonia) bears one lone pair of electron. Each single electron of hydrogen is bonded to the nitrogen atom. Three of out five electrons of nitrogen are involved in bonding whereas the unbounded two electrons make up the single unshared pair of electron.

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   Oxides of nitrogen

   Some oxides of nitrogen are:

   NO (nitric oxide), NO₂ (nitrogen dioxide), N₂O (nitrous oxide), N₂O₃, N₂O₄, N₂O₅. NO₂ contains an odd number of valence electrons and is a radical; on dimerisation it forms N₂O₄, which has an even number of electrons and is more stable.

   
   MULTIPLE CHOICE QUESTION

   Try yourself: Which of the following is not an alternative name of dinitrogen trioxide?

   A

   Nitrogen sesquioxide

   B

   Nitrogen (III) oxide

   C

   Anhydride of nitrous acid

   D

   Nitrogen peroxide

   View Solution

   Correct Answer: DHide Explanation ⌃

   Dinitrogen trioxide is an oxide of nitrogen which can also be called as nitrogen sesquioxide, nitrogen (III) oxide or anhydride of nitrous acid. Nitrogen dioxide is also known as nitrogen peroxide.

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3. Nitric acid (HNO₃)

   General: Nitric acid is a strong oxoacid of nitrogen and is typically available as a 68% aqueous solution; concentrated HNO₃ may appear yellow due to dissolved NO₂.

   Preparation:

   (i) Laboratory method: NaNO₃ + H₂SO₄ (conc.) → NaHSO₄ + HNO₃.

   (ii) Industrial method: Ostwald process - oxidation of NH₃ to NO (using Pt-Rh catalyst), followed by oxidation to NO₂ and absorption in water to give HNO₃.

   

   Physical properties: A colourless, corrosive liquid with a pungent odour; concentrated nitric acid is a powerful oxidising agent.

   MULTIPLE CHOICE QUESTION

   Try yourself: What is the catalyst used in the industrial manufacture of nitric acid?

   A

   Powdered iron (III) oxide

   B

   Vanadium (V) oxide

   C

   Zinc-mercury amalgam

   D

   Platinum-Rhodium gauze sheet

   View Solution

   Correct Answer: DHide Explanation ⌃

   Pt-Rh gauze sheet is widely used as the catalyst in ammonic oxidation, the first step of Ostwald’s process. Fe₂O₃ is used in Haber’s process; V₂O₅ in contact process and Zn (Hg) is used in Clemmensen reduction of aldehydes.

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   Chemical reactions (selected):

   (i) Action of nitric acid on zinc under different conditions - behaviour varies with concentration of HNO₃ (dilute vs concentrated) and with temperature (different reduction products such as NO, NO₂, NH₄NO₃ may form).

   

   (ii) Action of nitric acid on copper under different conditions - copper is oxidised and various nitrogen oxides can be produced depending on conditions (dilute or concentrated acid).

   

   (iii) Reaction with non-metals - nitric acid acts as an oxidising acid (e.g., oxidises sulphur, carbon, phosphorus, etc.) and may produce NO₂ or other oxides.

   

   (iv) Brown ring test for nitrate: A characteristic brown ring at the junction of two layers is produced when an aqueous solution of a nitrate is treated with freshly prepared FeSO₄ and then carefully layered with concentrated H₂SO₄; this forms the complex [Fe(H₂O)₅(NO)]²⁺ (brown ring).

   

   (v) Some metals (Fe, Cr, Ni, Al, Co) become passive in concentrated nitric acid because of the formation of a stable protective oxide layer.

   Structure of HNO₃:

   

   Uses:

   • Manufacture of fertilizers (e.g., ammonium nitrate).
   • Purification of precious metals such as gold and silver (part of aqua regia when mixed with HCl).
   • Manufacture of explosives and as an oxidising agent.
   • As a nitrating reagent in organic synthesis.
     
     MULTIPLE CHOICE QUESTION

     Try yourself: How many allotropes does nitrogen have?

     A

     Zero

     B

     One

     C

     Two

     D

     Three

     View Solution

     Correct Answer: AHide Explanation ⌃

     The nitrogen atom does not exhibit allotropy. This is because of its relatively small size and high inter-electronic repulsion due to which the N – N becomes extremely weak. However, theoretically, there are two predicted forms of allotropes of nitrogen. First, alpha nitrogen assumes a cubic structure and exists only below 35.6 K which is a whooping -237.55°C. Beta nitrogen exists from 35.6 K to 63.15 K. After that, it melts.

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Phosphorus and its Allotropic Forms

Phosphorus exhibits several allotropes; the most important are white (or yellow) phosphorus, red phosphorus and black phosphorus. Their properties differ markedly because of differences in molecular and crystal structure.

White phosphorus is molecular (P₄ tetrahedral units), very reactive, glows in air on slow oxidation (phosphorescence), and is stored under water. Red phosphorus is polymeric, much less reactive and is used on matchbox striking surfaces (safety matches). Black phosphorus is the most thermodynamically stable allotrope; it is obtained by heating red phosphorus in a sealed tube at elevated temperature (about 803 K) and has layered structure and is conductive.

Black phosphorus can exist in two crystalline forms commonly referred to as α- and β-phosphorus (different stacking and electronic properties).

Household matches: the striking surface of a matchbox contains red phosphorus and an oxidising agent (e.g., potassium chlorate), while the match head typically contains an oxidiser, binder (glue), filler (e.g., chalk) and a combustible material. Red phosphorus on the striking surface converts to white phosphorus momentarily to initiate ignition under frictional heat.

Types of Black Phosphorous

Chemical properties of phosphorus

1. Reactions with non-metals:Phosphorus reacts with oxygen, halogens, sulphur and others to give a range of phosphorus compounds (e.g., P₄O₁₀, PCl₃, PCl₅, P₂S₅).
   
2. Reactions with compounds:Phosphorus reacts to form many organophosphorus and inorganic phosphorus compounds; examples include formation of phosphides with metals and various derivatives via chlorination, oxidation and reduction.
   

Uses of phosphorus

Phosphorus and its compounds are used in the manufacture of match heads and matchbox striking surfaces, explosives, rat poisons, fertilizers (phosphate fertilisers), and in many alloys and industrial reagents.

Compounds of Phosphorus

1. Phosphine (PH₃)

   Preparation: Phosphine can be prepared by the action of water on metal phosphides (e.g., Ca₃P₂ + 6 H₂O → 3 Ca(OH)₂ + 2 PH₃) or by other laboratory methods.

   

   Properties:

   • Colourless gas with a characteristic rotten-fish odour; highly poisonous.
   • Phosphine may ignite or explode on contact with traces of oxidising agents (HNO₃, Cl₂, Br₂ vapours).
   • Reacts with metal salts to give phosphides or metal phosphine complexes; example reactions include formation of insoluble metal phosphides with Cu²⁺ or Hg²⁺:
   • 3 CuSO₄ + 2 PH₃ → Cu₃P₂ + 3 H₂SO₄
   • 3 HgCl₂ + 2 PH₃ → Hg₃P₂ + 6 HCl
   • Phosphine is a weak base: PH₃ + HBr → PH₄⁺ Br⁻.
   • Uses: It is used to prepare smoke screens in some military applications and certain phosphine derivatives are used as fumigants; a mixture of CaC₂ and Ca₃P₂ was historically used for signalling (Holmes' signals).
   • MULTIPLE CHOICE QUESTION

     Try yourself: What is the hybridization of phosphine?

     A

     sp² hybridized

     B

     sp³ hybridized

     C

     sp hybridized

     D

     No hybridization

     View Solution

     Correct Answer: DHide Explanation ⌃

     The hybridization of phosphine seems like sp² but in reality the molecule has no hybridization as it forms all bonds using its pure p orbitals. This can be proved from its bong angle data which shows that its bond angles are 93.5°.

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2. Phosphorus trichloride (PCl₃)

   Preparation: By chlorination of phosphorus: P₄ + 6 Cl₂ → 4 PCl₃.

   

   Structure and properties: PCl₃ is a colourless, fuming oily liquid with a pyramidal geometry around phosphorus (approximately sp³ hybridisation with one lone pair on P). It is an important intermediate in the manufacture of organophosphorus compounds and phosphorus oxychlorides.

   
   

3. Phosphorus pentachloride (PCl₅)

   Preparation: P₄ + 10 Cl₂ → 4 PCl₅.
   P₄ + 10 SO₂CI₂ → 4PCl₅ + 10 SO₂

   Structure: In the gaseous and liquid phases PCl₅ has sp³d hybridisation and a trigonal bipyramidal shape; three equatorial P-Cl bonds are equivalent and shorter, while two axial P-Cl bonds are longer.
   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.

   

4. Oxoacids of phosphorus

   Phosphorus forms a number of oxyacids; the most important are phosphorous acid (H₃PO₃) and phosphoric acid (H₃PO₄), plus related polyphosphoric acids (P₂O₅ ↔ H₄P₂O₇ etc.). Their structures and acid strengths differ: H₃PO₄ is a triprotic acid (three O-H groups) while H₃PO₃ is diprotic (one P-H bond and two P-OH groups).

   
   
   
   

   Example application in everyday products: calcium hydrogen phosphate dihydrate (CaHPO₄·2H₂O) is used in toothpaste as a mild abrasive and polishing agent.

Summary

The Group 15 (pnictogen) elements show systematic trends in electronic configuration (ns² np³), bonding and properties across the series N → P → As → Sb → Bi. Key points for students to remember include the wide range of oxidation states (especially for nitrogen), differences in metallic character, allotropy of phosphorus, characteristic hydride and halide chemistry, and important industrial processes such as the Haber process (NH₃ synthesis) and the Ostwald process (HNO₃ manufacture). Many compounds of phosphorus and nitrogen have large practical importance in fertilizers, explosives, reagents and everyday materials.