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

What are Group 15 Elements?

Group 15 elements are also called the Nitrogen family includes nitrogen phosphorus, arsenic, antimony and bismuth elements. The p-block elements are also known as the Representative Elements which is placed on the right side of the main periodic table. 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

1. Electronic configuration: 

   Their valence shell electronic configuration is ns² np³
   

2. Metallic character: 

   N and P are non-metals, As and Sb are metalloids and Bi is metal.

3. Physical state: 

   Nitrogen is the first element after hydrogen which is diatomic gas in native form. All other elements in the group are solids.

4. Atomicity: 

   N₂ is diatomic while others are triatomic.

5.  Melting and boiling points: 

   The melting point increases from nitrogen to arsenic. The boiling points increase regularly on moving down the group.

6. Density: 

   It increases down the group.

7. Atomic radii: 

   It increases with an increase in atomic number as we go down the group.

8. 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)

9. Oxidation state:

   Nitrogen has a wide range of oxidation states. The stability of the +3 oxidation state increases and the stability of the +5 oxidation state decreases on moving down the group due to the inert pair effect.
   
   Question for Group 15 Elements: Nitrogen Family

   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

   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. The ionization enthalpy: 

    The 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.

11. Electronegativity: 

    It decreases from nitrogen to bismuth.

12. Catenation: 

    ‘They exhibit the property of catenation but to a lesser extent due to weak E – E bond than 14 group elements.

13. 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

1. 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:
   (i) Thermal stability,
   (ii) Basic strength,
   (iii) Solubility in water,
   (iv) Bond angle NH₃ (107.4°); PH₃ (92°),AsH₃ (91° ), SbH₃(90° ),
   (v) Strength of M – H bond
   
   Some properties follow the order: NH₃ < PH₃ < AsH₃ < SbH₃ < BiH₃
   These properties are :
   (i) Reducing character
   (ii) Covalent character
   (iii) Rate of combustion

2. 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 < NI_3. 
   NCl₃ is an explosive compound.

3. Oxides: 

   All the elements of this group form oxides of the type M₂O₃ and M₂O₅.
   N₂O₅ and N₂O₄ are strongly acidic, whereas, NO and N₂O are neutral.
   P₄O₁₀ is also strongly acidic.
   As₄O₆ is called white arsenic and is a poison.
   The acidic strength of pentoxides and trioxides decrease on moving down the group, i.e., N₂O₅ > P₂O₅ > As₂O₅ > Sb₂O_5.
   BiOCl is called pearl white.

Compounds of Nitrogen

1. Dinitrogen (N₂)

   
   Preparation:Properties:
   (i) Nitrogen does not react with alkali metals except Li but reacts with alkaline earth metals to give metal nitride.
   (ii) Reaction with oxygen
   (iii) Reaction with non-metals
   (iv) Reaction with CaC₂
   Uses:
    Liquid N₂ is used as a refrigerant. N₂ is used in the manufacture of HNO₂, NH₂, CaCN₂(calcium cyanamide) and other nitrogenous compounds. It is used for filling electric bulbs.
   
   Question for Group 15 Elements: Nitrogen Family

   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

   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) Lab method: 2NH₄Cl + Ca(OH)₂ → CaCI₂ + 2NH₃ + 2H₂O
   (ii) Haber’s process
   Properties:
   (i). It is a colourless gas with a characteristic pungent odour. It is extremely soluble in water due to H – bonding.
   (ii). 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.
   
   Question for Group 15 Elements: Nitrogen Family

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

   • A.

     1
   • B.

     2
   • C.

     3
   • D.

     4

   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

   NO₂ contains an odd number of valence electrons. On dimerisation, it is converted to a stable N₂O₄ molecule with an even number of electrons.
   
   Question for Group 15 Elements: Nitrogen Family

   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

   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): 

   It is a stronger acid than H₃PO₄.
   Preparation:
   (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.
   
   Question for Group 15 Elements: Nitrogen Family

   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

   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:

   (i). Action of nitric acid on zinc under different conditions:(ii). Action of nitric acid on copper under different conditions:(iii). Reaction with non-metals:(iv). Brown ring test of nitrate:(v). 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.
   
   Question for Group 15 Elements: Nitrogen Family

   Try yourself:How many allotropes does nitrogen have?

   • A.

     Zero
   • B.

     One
   • C.

     Two
   • D.

     Three

   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 & its Allotropic Forms

Phosphorus is found in many allotropic forms, the important ones being white, red and black.

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. The black phosphorous is also of further 2 types: α and β.

Matchbox side contains red P or P₂S₃⁺ glue and on tip of a match stick. red P, KClO₃ chalk and glue is deposited.

Types of Black Phosphorous

Chemical properties:

1. With non-metals:
   
2. With compounds:
   

Uses

It is used in matchboxes, explosives, as rat poison, in fertilizers and alloys

Compounds of Phosphorous

1. Phosphine (PH₃)

   
   Preparation It is prepared by following methods:Properties
   (i). It is a colourless gas with a 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
   (ii). 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.
   
   Question for Group 15 Elements: Nitrogen Family

   Try yourself:What is the hybridization of phosphine?

   • A.

     sp² hybridized
   • B.

     sp³ hybridized
   • C.

     sp hybridized
   • D.

     No hybridization

   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:
   
   Properties: It is a colourless oily liquid having a pyramidal shape [sp³ – hybridised].

3. Phosphorus Pentachloride (PCl₅)

   
   Preparation:
   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.
   
   

4. Oxoacids of Phosphorus

In toothpaste, CaHPO₄^.2H₂O is added as a mild abrasive and polish agent.