The P-Block Elements

The elements of the group 13 – 18 come under the p – block elements. In these elements the last electron enters in the outermost p – orbital. They have ns2np1-6 electronic configuration in valence shell, helium being an exception. These elements show the maximum oxidation state equal to the sum of electrons in the outermost shell or valence shell. Most of the elements of the p – block form covalent compounds although some elements form ionic compounds (such as halogens) and coordination compounds as well. p-block contains elements which are either metals, non – metals or metalloids. p-block elements include the group of halogens and inert gases. First member of each family of the p-block elements is given below in the table with their general electronic configuration and oxidation states. p-block has the most electronegative element which is fluorine. Elements of p-block generally form acidic oxides. Many elements such as C, Si, Ge, O, N etc. also show phenomena of allotropy. Property of catenation is also shown by many elements.  

Group 

13

14

15

16

17

18

First Member of the Group

He

General Electronic Configuration 

ns2np1

ns2np2

ns2np3

ns2np4

ns2np5

ns2np6

Group Oxidation State 

+3

+4

+5

+6

+7

+8


We have covered the Boron Family (Group -13 elements) and the Carbon Family (Group – 14 elements) in another article of p-block elements (Class XI, Chemistry). In this article, we will cover the Nitrogen Family or Group 15 Elements of p-block elements (Class XII, Chemistry).

 

Group 15 Elements: The Nitrogen Family 

Group 15 is the third group of p-block elements. The first element of the group is nitrogen, that’s why it is also known as the Nitrogen Family. 

Elements of the Group - 15

Atomic Number 

Symbol 

Metal/Nonmetal/metalloid

Color & State 

Electronic Configuration 

Density g/cm3 at 298 K

Atomic and Ionic Radii 

Ionization Enthalpy 

7

N

Non - metal 

Colorless gas 

[He] 2s2 2p3

0.879 (at 63K)

Increases on moving from top to bottom in the group

(Exception – Antimony, Bismuth)

Decreases on moving from top to bottom in the group due to gradual increase in size of elements.

As group 15 elements have half filled p-orbital so their ionization enthalpy is much greater than that of group 14 elements.  

15

P

Non- Metal 

White (purest form).

Solid 

[Ne] 3s2 3p3

1.823

33

As

Metalloid 

Grey (Most common).

Solid 

[Ar] 3d10 4s2 4p3

5.778

51

Sb

Metalloid

Silvery gray with luster. 

Solid 

[Kr] 4d10 5s2 5p3

6.697

83

Bi

Metal

Brownish silver with luster.

Solid 

[Xe] 4f14 5d10 6s2 6p3

9.808


Elements of the Group 15 – Physical Properties 

Symbol 

Atomic Number 

Atomic Mass (g mol-1)

Melting Point (K)

Boiling point (K)

Density 

Ionic Radius 

Electronegativity 

N

7

14 

63

Increases on moving from top to bottom (Exceptions – Sb, Bi)

77.2

Increases on moving from top to bottom in the group(Exceptions – Bi)

Increases on moving from top to bottom in the group 

Increases on moving from top to bottom in the group (Exceptions – Sb, Bi)

Decreases on moving from top to bottom in the group, although heavier elements of the group has very minute difference. 

P

15

30.97

317 

554

As

33

74.92

1089

888

Sb

51

121.75

904

1860

Bi

83

208.98

544

1837

  • All elements of the group 15 are polyatomic.

  • All elements of the group 15 show allotropy, except nitrogen. 


Elements of the Group 15 – Chemical Properties

Oxidation Number 

Group -15 elements generally exhibit -3, +3 and +5 oxidation states.


As we move top to bottom in the group metallic character increases, so only top elements exhibit -3 oxidation state.


Nitrogen exhibits +1, +2 and +4 oxidation states as well. Phosphorus also shows +1 and +4 oxidation states.


Reactivity Towards Oxygen  

All elements of nitrogen family forms two types of oxides – E2O3, E2O5

Acidic character of the oxides decreases on moving down the group.

N2O3 and P2O3 – Acidic 

As2O3 and Sb2O3 – Amphoteric 

Bi2O3 – Basic 

Reactivity with Hydrogen 

All elements of nitrogen family react with hydrogen and forms EH3 type hydrides. 

The stability of hydrides decreases and reducing character increases on moving down the group. 

Order of basicity of these hydrides –

NH3 > PH3 > AsH3 > SbH3 > BiH3

Reactivity Towards Metals 

All elements of nitrogen family react with various metals and form binary compounds. 

In the binary metallic compounds group 15 element exhibit -3 oxidation state. 

Examples – Ca3N2, Mg3Bi2 etc. 

Reaction Towards Halogens 

All elements of nitrogen family react with halogens. 

Group – 15 elements for following two types of halides – EX3 & EX5

Nitrogen does not form pentahalide and its trihalide is not stable. 


Anomalous Properties of Nitrogen 

  • Nitrogen differ from other elements of the group – 15 due to its high electronegative character, small size and high ionization enthalpy. 

  • Nitrogen can form multiple bonds with itself and other elements. it forms p – p multiple bonds. Other heavier elements of the group 15 do not form p – p multiple bonds

  • d- orbitals are not found in the valence shell of the nitrogen element. It cannot form d – p bond while other heavier elements can. For example, phosphorous forms R3P=CH2


Compound 

Preparation 

Properties 

Uses 

Dinitrogen 

Chemical Formula – N2

Commercial method – It is prepared by liquefaction and fraction distillation of air as air contains high amount of nitrogen. 

Laboratory method - In labs it is prepared by reaction of aq. Ammonium chloride and sodium nitrite. 

Reaction - NH4CI(aq) + NaNO2(aq) → N2(g) + 2H2O(l) + NaCl (aq)

By thermal decomposition – Pure nitrogen can be formed by thermal decomposition of barium azide. 

Reaction - Ba(N3)2→ Ba + 3N2

🡪It is colorless, odorless gas. 

🡪It is a non – toxic gas which is present in atmosphere at higher concentrations. 

🡪It is tasteless. 

🡪It is reactive at high temperature although inert at room temperature. 

🡪It reacts with Li and forms lithium nitride. 

6Li + N2 + Heat→ 2Li3N


In manufacturing of ammonia, to create an inert atmosphere, to prevent food items from spoiling. 

Ammonia 

Chemical Formula – NH3

By decay of nitrogenous organic matter – 

NH2CONH2 + 2H2O 🡪 (NH4)2CO3 <🡪 2NH3 + H2O + CO2

By decomposition of ammonium salt - 2NH4Cl + Ca(OH)2 → 2NH3 + 2H2O + CaCl2

By Haber’s process – N2 + 3H2 2NH3

🡪It is a colorless gas. 

🡪It has a pungent odor. 

🡪Its freezing point is 198.4K and boiling point is 239.7K. 

🡪it is highly soluble in water.

🡪it behaves as a Lewis base as nitrogen atom of ammonia has a lone pair of electrons. 



Used in manufacturing of nitrogenous fertilizers, inorganic nitrogenous compounds and nitric acid. 

Liquid ammonia is used as a coolant. 

Nitric acid 

Chemical Formula – HNO3

Laboratory method – It is prepared in the labs by heating sodium nitrate with conc. sulfuric acid in a glass retort flask. 

Reaction – 

NaNO3 + H2SO4 🡪 NaHSO4 + HNO3

Commercial production method – Commercially it is produced by Ostwald’s process. In this process catalytic oxidation of ammonia is carried out through atmospheric oxygen. 

Reaction – 4NH3 + 5O2 Pt/Rhgauge catalyst→

4NO + 6H2O

2NO + O2 2NO2

3NO2 + H2O 🡪 2HNO3 + NO

🡪it is a colorless liquid compound.

🡪 Its freezing point is 231K and boiling point is 355K.

🡪It is a strong acid. On dissolving in water, it gives hydronium ion and nitrate ion. 

HNO3 + H2O 🡪 H3O+ + NO3-

🡪 It is a strong oxidizing agent.

🡪 Conc. nitric acid can oxidize non -metals as well. for example – 

I2 + 10HNO3 🡪 2HIO3 + 10NO2 + 4H2O

Used for manufacturing of ammonium nitrate for fertilizers.

Used in preparation of nitroglycerin, trinitrotoluene and other compounds used in explosives and pyrotechnics. 

Used in stainless steel, rocket fuels and etching of metals.  


Brown Ring Test: Test for Nitrate Ion

Aim- To detect nitrate in the given sample. 

Requirements- Test tubes, freshly prepared iron sulphate solution, concentrated sulphuric acid.  

Principle- The test is based on the fact that nitrate ion acts as an oxidizing agent. In the reaction mixture reduction of nitrate ion takes place by iron (II) and iron(II) gets oxidized to iron (III) Nitric oxide is reduced to NO- and forms nitrosonium complex which forms a brown ring at the junction of two layers. 


Brown Ring Test Procedure 

Brown ring test experiment for nitrate ions can be performed by using following steps-

  • Take a sample in which you suspect nitrate is present in a test tube. 

  • Prepare fresh iron sulphate solution. 

  • Now mix freshly prepared iron sulphate solution in the sample.

  • Add concentrated sulphuric acid solution along the side to the bottom of the test tube with the test tube tilted which containing the mixture of sample and freshly prepared iron sulphate solution. Concentrated sulphuric acid is added in such a way that it forms a layer below the aqueous solution. 

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Observation 

A brown ring is formed at the junction of two layers (one layer of mixture of iron sulphate solution + sample solution and second layer of concentrated sulphuric acid solution). 

Result 

Brown ring confirms the presence of nitrate in the sample. 

Brown Ring Test reactions 

Following reactions take place during brown ring test – 

2HNO3 + 3H2SO4 + 6FeSO4 🡪 3Fe2(SO4)3 + 2NO + 4H2

[Fe(H2O)6]SO4        +            NO            🡪         [Fe(H2O)6]SO4                     +                H2

                                                                          Nitrosonium complex 


Applications of Brown Ring Test 

Brown ring test is mainly used for the detection of nitrate. It is used to detect the presence of nitrate in many food samples, soil and water. As high quantity of nitrate in food causes food poisoning and high quantity of nitrate in soil makes it very acidic which is harmful for plants and farming. If nitrate compounds are present in water more than 10miligrams per liter then it’s not drinkable.


Phosphorus 

Allotropes of Phosphorus – Important allotropic forms of phosphorus are red, white and black. 

Allotropes of Phosphorus

White Phosphorus 

Red Phosphorus 

Black Phosphorus 

It is white in color and found in the form of waxy solid.

It is translucent, poisonous and insoluble in water.

It is soluble in CS2 and shows the property of chemiluminescence (glows in dark).

It is more reactive than other allotropes of carbon. On burning it gives white fumes of P4O10

White phosphorus crystal structure (Tetrahedral)-

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It is obtained from white phosphorus. White phosphorus is heated at 573K for many days in an inert atmosphere to get red phosphorus. 

Red phosphorus is odorless, red in color, non-poisonous and possesses lustre. 

It is insoluble in water and CS2.

It is less reactive than white phosphorus. 

It has chains consisting of P4 tetrahedra linked together. 

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Black phosphorus has two forms: alpha black phosphorus and beta black phosphorus. 

Alpha black phosphorus has opaque monoclinic or rhombohedral crystal structure. It is obtained by heating red phosphorus in a sealed tube at 803K. 

Beta black phosphorus is not very reactive. It is prepared by heating white phosphorus at 473K under high pressure. 



Compounds of Phosphorus 

Name of Phosphorus Compound 

Preparation 

Properties 

Uses 

Phosphine 

Chemical Formula – PH3

By the reaction of calcium phosphide and water or dil. hydrochloric acid. 

Reaction – 

Ca3P2 + 6H2O 🡪3Ca(OH)2 + 2PH3

Ca3P2 + 6HCl → 3CaCl2 + 2PH3

Laboratory method – In labs it is prepared by the reaction of white phosphorus and conc. sodium hydroxide solution, in the inert atmosphere of carbon dioxide. 

Reaction – 

P4 + 3NaOH + 3H2O 🡪 PH3 + 3NaH2PO2

🡪It is highly poisonous and has an odor like rotten fish. 

🡪It is a colorless gas.

🡪 It is soluble in water. 

🡪 It is weakly basic in nature. On reaction with acids, it gives phosphonium compounds.  

Reaction – 

PH3 + HBr 🡪 PH4Br

🡪Reaction with copper sulphate –

3CuSO4 + 2PH3 🡪 Cu3P2 + 3H2SO4

It is used in smoke screens.

It is used in Holme's Signals. 

Phosphorus Halides 

General Formula – PX3, PX5

(X = halides F, Cl, Br)

  • Phosphorus Trichloride 

Chemical Formula – PCl3

-Dry chlorine gas is passed over white phosphorus.

Reaction - 

P4 + 6Cl2 🡪 4PCl3

- By the reaction of thionyl chloride with phosphorus.

Reaction – 

P4 + 8SOCl2 🡪 4PCl3 + 4SO2 + 2S2Cl2

🡪It is a colorless oily liquid. 

🡪In phosphorus trichloride phosphorus is sp3 hybridized. 

🡪It has a trigonal pyramidal shape.

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🡪It gets hydrolyzed in presence of moisture. 

Reaction – 

PCl3 + 3H2O 🡪 H3PO3 + 3HCl

🡪 Reaction with organic compounds –

3CH3COOH + PCl3 🡪 3CH3COCl + H3PO3

Used for the synthesis of many compounds such as PCl5, POCl3 etc. 

Used as a precursor to triphenylphosphine in Wittig reaction. 

  • Phosphorus Pentachloride 

Chemical Formula – PCl5

By the reaction of white phosphorus and dry chlorine. Dry chlorine is used in excess. 

Reaction – 

P4 + 10Cl2 🡪 4PCl5

In another method of preparation, white phosphorus reacts with SO2Cl2.

Reaction – 

P4 + 10SO2Cl2 🡪 4PCl5 + 10SO2


🡪It is found as light-yellow colored powder. 

🡪In presence of moisture, it gets hydrolyzed to POCl3 which gets converted to phosphoric acid. 

Reaction – 

PCl5 + H2O 🡪 POCl3 + 2HCl

POCl3 + 3H2O 🡪 H3PO4 + 3HCl 

🡪it reacts with organic compounds which contain the hydroxy functional group and converts them into chloro derivatives. 

Reaction – 

C2H5OH + PCl5 🡪 C2H5Cl + POCl3 + HCl

🡪It reacts with metals and gives respective chlorides. 

Reaction –

2Ag + PCl5 🡪 2AgCl + PCl3

🡪It has a trigonal bipyramidal shape.

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Used as a chlorinating agent, catalyst. 

Oxoacids of Phosphorus 

An oxoacid is an acid that contains oxygen. Oxoacids of phosphorus contain oxygen and phosphorus. For examples, H3PO2, H3PO3, H3PO4 etc. 

🡪 Different oxoacids of phosphorus are prepared by different methods. 

In H3PO2, the oxidation state of phosphorus is +1. 

In H3PO3, the oxidation state of phosphorus is +3. 

In H3PO4, the oxidation state of phosphorus is +5. 


Used as color stabilizer, catalyst, Wetting agent, reducing agent etc. 


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