Elements of p-Block II Group 15 | Notes and Definition

Elements of p-Block II
Elements of p-Block II Group 15 | Notes and Definition

Elements of p-Block II– NCERT class 12th Elements of p-Block II notes

Elements of p-BlockElements of p-Block II Group 15 | Notes and Definition II- Group 15

(Nitrogen Family)

p-block elements are placed in group 13 to 18 of the periodic table. The electronic configuration of valence shell is ns2 np1-6 (except He which has 1s2 configuration).

The first member of each of the groups 13-17 of the p-block elements differ in many respects from rest of the members of their respective groups. Their respective groups. This anomalous behavior is due to small size, high electronegativity  and inability to expand its octet due to the absence of d– orbitals on the valence shell.

Group 15 Elements: Nitrogen Family

Group-15 of the long form of the periodic table consists of nitrogen (N), phosphorus (P), arsenic (As), bismuth (Bi) and antimony (Sb). Nitrogen and phosphorus are non-metal, arsenic and antimony are sub-metals and bismuth is a metal. These are collectively called pnicogens.


  • Nitrogen is about 78% by volume of the atmosphere.
  • It is also found in form of proteins in living organism also as nitrates in earth crust.
  • Arsenic, antimony and bismuth are mainly present as sulphides.
  • Phosphorus occurs in minerals like Ca3(PO4)6, CaX2 and in bones etc.

General Properties of elements of Group-15

  • Electronic configuration– The general electronic configuration of these elements in np2np3.
  • Ionisation enthalpy– Ionistation enthalpy of these elements is higher than the corresponding group-14 elements because of their extra stable half-filled configuration and smaller size.

Physical Properties

  • All elements except nitrogen are polyatomic, nitrogen is diatomic.
  • Nitrogen is a gas, while all others are solid.
  • Except nitrogen, all other members of group 15 show allotropy.
  • In general, boiling points increases from top to bottom in the group but, melting point increase upto arsenic and then decreases upto bismuth.

Oxidation State– The general oxidation states of these elements are -3, +3 and +5.

Nature of hydrides– Elements of this group form hydrides of the type MH3.

Decreasing order of thermal stability of hydrides:

{NH}_{3} > {PH}_{3} > {AsH}_{3} > {SbH}_{3} > {BiH}_{3}

Abnormal Behaviour of Nitrogen

Nitrogen shows abnormal behaviour because of its smaller size, high ionization energy, high values of electronegativity and absence of d-orbitals in its valence shell. Due to absence of d-orbitals nitrogen cannot expand its octet. Therefore, NF5 does not form.

Properties of Nitrogen

  • Nitrogen is a colourless, odourless, tasteless and non toxic gas.
  • Nitrogen is highly inert at room temperature due to high bond energy of N \equiv N .

Oxidation of Nitrogen

The elements of group -15 combine with oxygen and form different types of oxides, as N2O, NO, N2O5, etc. Acidic character of oxides decreases on going down the group. Nitrous oxide is known as laughing gas it is neutral in nature. It has no effect on Litmus Paper.

Ostwald’s process

Nitric acid, HNO3 on large scale is prepared by Ostwald’s process. This is the method which is based upon catalytic oxidation of NH3 by atmospheric oxygen. It is a strong oxidising agent and attacks most metal except noble metals (Au and Pt).

{ 4NH }_{ 3 }(g) + { 5O }_{ 2 }(g)\xrightarrow [ Pt/\quad Rh\quad gauge\quad catalyst ]{ 500K,\quad 9\quad bar\quad } { 4NO }(g) + { 6H }_{ 2 }{ O }(g)

{ 2NO }(g) + { O }_{ 2 }(g)\rightleftharpoons { 2NO }_{ 2 }(g)

{3NO}_{2}(g) + {{H}_{2}O}(l) \longrightarrow {2HNO}_{3}(aq) + {NO}(g)

Fuming nitric acid

Concentrated nitric acid in which oxide of Nitrogen remains dissolved, is called fuming nitric acid or Aqua regia the resulting mixture volumetrically one part concentrated nitric acid and three parts concentrated hydrochloric acid is called aqua-regia.

Reaction of Gold with  Aqua Regia

\underset { Gold }{ Au } +\quad \underset { Nitric   acid }{ { conc.HNO }_{ 3 } } +\quad \underset { Hydrochloric  acid }{ conc.3HCl } \quad \longrightarrow \underset { Auric   chloride }{ { AuCl }_{ 3 } } +\underset { Nitric oxide }{ NO } +\underset { Water }{ 2H_{ 2 }O }

\underset { Auric   chloride }{ AuCl_{ 3 } } +\underset { Hrdrocloric  acid }{ conc.HCl }\longrightarrow \quad { H[AuCl }_{ 4 }]

Oxo-acids of Phosphorus

Phosphorus forms a number of oxoacids. Their basicity is equal to the number of P—OH bonds in the molecules. All these oxoacids contain atleast one P=O and one P—OH bond. Hypophosphorus acid, H3PO2 is a good reducing agent.

Halides of Phosphorus

Phosphorus forms two types of halides. PX3 and PX5. In PX3, P is sp3 hybridised and it has pyramidal shape. In PCl5, P is sp3 —d hybridised and it has trigonal bioyramidal structure. In this, three equatorial bonds are equivalent, whereas two axial bonds are greater than equatorial bonds. PCl5 acts as an ionic solid [PCl4][PCl6] in solid state.

Chemical Test of Ammonia and Phosphine

  • Phosphine precipitate cupric phosphine from cupric sulphate solution, when ammonia reacts with the solution of cupric sulphate and form complex salt [Cu(NH3)SO4] of dark blue colour which is soluble in water.
  • Phosphine reduces solution of silver nitrate into silver ammonia forms complex salt with silver nitrate, which is soluble in water.

Important reactions

Ammonia (NH3)

Preparation–      {NH}_{2}{CONH}_{2} + {2H}_{2} {O} \longrightarrow {{(NH}_{4})}_{2} {CO}_{3}

Properties \underset {White  ppt}{AgCl (s)} + {2NH}_{3} (aq) \longrightarrow \underset {(Colourless)}{[{{Ag(NH}_{3} )}_{2}]} Cl (aq)

{Cu}^{2+}(aq) + {4NH}_{3} (aq) \rightleftharpoons  \underset {(Deep  Blue)}{[{Cu{({NH}_{3})}_{4}]}}^{2+} (aq)

{NH}_{3} + {NaOCl} \longrightarrow {NaNH}_{2} + {HCLO}

Nitric Acid (HNO3)

Preparation {NaNO}_{3} + {{H}_{2} {SO}_{4}} \xrightarrow {Heat} {NaHSO}_{4} + {HNO}_{3}


{Cu} + {{4HNO}_{3}{(dil.)}} \longrightarrow {3Cu(NO}_{3})_{2} + {2NO} + {4H}_{2}{O}

{I}_{2} + {10HNO}_{3} \longrightarrow {2HIO}_{3} + {10NO}_{2} + {4H}_{2} {O}

{C} + {4HNO}_{3} \longrightarrow {CO}_{2} + {{2H}_{2}O} + {4NO}_{2}

{P}_{4} {20  HNO}_{3} \longrightarrow {{4 H}_{3}{PO}_{4}} + {20 NO}_{2} +{{4H}_{2} O}

Phosphorus  trichloride (PCl3)

Preparation–            {P}_{4} + {8SOCl}_{2} \longrightarrow {4PCl}_{3} + {4SO}_{2} + {2S}_{2}{Cl}_{2}


{PCl3} + {3H}_{2} {O} \longrightarrow {{H}_{3}{PO}_{3}} + {3HCl}

{3CH}_{3}{COOH} + {PCl}_{3}  \longrightarrow {3CH}_{3}{COCl} + {H}_{3}{PO}_{3}

Phosphorus Pentachloride (PCl5)

Preparation          {P}_{4} + {10SO}_{10}{Cl}_{2} \longrightarrow  {4PCl}_{5} + {10 SO}_{2}


{PCl}_{5} + {{H}_{2} O} \longrightarrow {POCl}_{3} + {2HCl}

{PCl}_{5} \xrightarrow{ \Delta} {PCl}_{3} + {Cl}_{2}

{2Ag} + {PCl}_{5} \longrightarrow {2AgCl} + {PCl}_{3}


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