Elements of Group 15 (Nitrogen family)


Elements of group 15 are Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi).

 

Following table describes the atomic and physical properties of group 15 elements:

Property

N

P

As

Sb

Bi

Atomic number

7

15

33

51

83

Electronic configuration

[He]

2s22p3

[Ne]

3s23p3

[He]3d10

4s24p3

[Ke]4d10

5s25p3

[Xe]4f14

5d106s26p3

Atomic mass/g mol-1

14.01

30.97

74.92

121.75

208.98

Ionisation enthalpy

iH/KJ mol-1)

I

II

III

 

1402

2856

4577

 

1012

1903

2910

 

947

1798

2736

 

834

1595

2443

 

703

1610

2466

Electro-negativity

3.0

2.1

2.0

1.9

1.9

Covalent
radius/pma 

70

110

121

141

148

Ionic radius/pm

171b

212b

222b

76c

103c

Melting point/K

63*

317d

1089e

904

544

Boiling point/K

77.2*

554d

888f

1860

1837

Density/[g cm-3(298 K)]


0.879g


1.823


5.778h


6.697


9.808

From the above table it is clear that physical properties of the elements like atomic mass, covalent and ionic radius, density, boiling point and melting point increases down the group while atomic properties like ionization enthalpy and electronegativity decreases down the group due to increasing atomic size.

There is a considerable increase in covalent radius from N to P while from As to Bi only a small increase is due to the presence of completely filled d and/or f orbitals. The ionization enthalpy of group 15 elements is much greater than that of group 14 elements in the corresponding period is due to the extra stability of half-filled p-orbitals in group 15 elements. The order of successive enthalpies, as expected is:

ΔiH1 < ΔiH2 < ΔiH3            (see above table)

(i) Occurrence:

Nitrogen: N2 comprises 78% by volume of atmosphere. In earth?s crust, it is found as NaNO3 (chile saltpeter) and KNO3 (Indian Saltpetre).

Phosporus: Occurs as minerals in the phosphate rocks in the form of Ca2(PO4)6.CaX2(x = f, Cl or OH). It is an essential constituent of plants and animals.

Arsenic, antimony and bismuth occur as sulphide minerals.

(ii) Physical Properties: Dinitrogen (N2) is a diatomic gas while all other elements of this group is polyatomic and solid. Metallic character increases down the group due to decrease in ionisation enthalpy and increase in atomic size. Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a metal.

Generally, the boiling point increases from top to bottom while the melting point increases upto arsenic and then decreases upto bismuth. All elements of this group show allotropy except nitrogen.

(iii) Chemical Properties:

A. Hydrides: They form covalent hydrides with the formula EH3 , where E stands for element.

(i) Thermal stability: It decreases down the group because the size of the atom increases and hence the bond length (EH) increases.

(ii) Reducing character: It increases down the group due to decrease in bond dissociation enthalpy. Except NH3 , all are strong reducing agents.

(iii) Basic character: It decreases down the group because as atomic size increases, electron density decreases on central atom E i.e., the order is NH3 > PH3 > AsH3 > SbH3 > BiH3 .

(iv) Boiling points: Boiling point of  NH3 is greater than  PH3 because of hydrogen bonding. Boiling points increase from  PH3 onwards because of increase of mol. mass and hence van der Waals? forces.

Reactions:

Uses: Hydrazine and its derivatives are used as rocket fuels.

B. Halides: Trihalides: All of them directly combine with halogens to form trihalides of the type  EX3 Except  NBr3 and  NI3 , all are stable and have pyramidal shape. They are easily hydrolysed by water

They also act as lewis acids.

Pentahalides: P, As and Sb form pentachlorides of the formula  ECl5 N does not form pentachloride because of non-availability of d-orbital in its valence shell. Bi does not form pentachloride due to inert pair effect. Pentachlorides involve sp3d hybridisation and have trigonal bipyramidal shape.

C. Oxides and Hydroxides: They combine with oxygen directly or indirectly to form a number of oxides.

Oxides of N

Formula (Name) Ox. State of N

 N2O(Nitrous oxide) +1

(Laughing gas)

NO (Nitric oxide) +2

 N2O3 (Dinitrogen trioxide) +3

 N2O4 (Dinitrogen tetraoxide) +4

 N2O5 (Dinitrogen pentoxide) +5

Preparation:

Oxides of P : Two important oxides of P are  P4O6 (a dimer of  P2O3 )  P4O10 (a dimer of  P2O5 ). These are obtained as follows:

Oxides of other elements:

 As4O6, As2O5, Sb4O6, Sb2O5, Bi2Oand Bi2O5 

Properties: Trioxides of N, P and As are acidic. The acidic character decreases down the group. Oxide of Sb is amphoteric while that of Bi is basic. All pentoxides are acidic. Acidic character decreases down the group. N2O5  is the strongest acidic oxides while Bi2O5  the weakest.

D. Oxo-acids: The elements of this group form a number of oxo-acids out of which those of N and P are more common.

Oxo-acids of N:

Formula (Name) Ox. State of N

H2N2O2 (Hyponitrous acid) +1

HNO2 (Nitrous acid) +3

HNO3 (Nitric acid) +5

Oxo-acids of P:

Formula (Name) Ox. State Basicity

H3PO3 (Phosphorous acid) +3 2

H3PO4 (Orthophosphoric acid) +5 3

HPO3 (Metaphosphoric acid) +5 1

H4P2O6 (Hypophosphoric acid) +4 4

H2P2O7 (Pyrophosphoric acid) +5 4

Structures:

Preparation of H3PO4 

(iv) Nitrogen and its Compounds:

Nitrogen, due to its smaller size, high electronegativity, high ionisation enthalpy and non-availability of d-orbitals, differs from other members of this group and shows anomalous properties.

Unlike heavier elements of this group, nitrogen has unique ability to form pr-pr multiple bonds with itself (N2) and with other elements having small size and high electronegativity (e.g., c, 0).

Thus, nitrogen exists as diatomic molecule with a triple bond (one s and two p) between two atoms, while single bonds are formed other elements like P-P, As-As and Sb-Sb and in case of bismuth, metallic bonds in elemental state.

The catenation (self linking of atoms) tendency of nitrogen is weaker due to small bond length and high interelectronic repulsion of non-bonding electrons.

A. Dinitrogen (N2) :

(a) Preparation: laboratory method

By thermal decomposition of ammonium dichromate

For obtaining very pure

Commercially, N2 is produced by the liquefaction and fractional distillation of air. Liquid N2 (b.p. 77.2 k) distills out first leaving behind liquid O2 (b.p. 90 k).

(b) Properties: Normally dinitrogen is inert due to high bond enethapy of bond but at higher temperature if directly combines with some metals to form ionic nitrides and with non-metals, covalent nitrides.

 

For example

(c) Uses of N2 : Dinitrogen is mainly used in manufacture of NH3 and other industrial chemicals like calcium cyanamide. Liquid N2 is used as a refrigerant to preserve food items, biological materials and in cryosurgery.

B. Ammonia (NH3) :

(a) Preparation: On a small scale, NH3 is obtained as follows:

by decay of urea,

Commercially, ammonia is manufactured by Haber?s process. In Haber?s process. N2 combines with H2 at 773 K to form NH3 (ammonia),

(b) Properties: Ammonia, being a colourless gas with a pungent smell, is highly soluble in water. Its freezing and boiling points are 198.4K and 239.7K respectively. Its melting and boiling points are higher because it forms H-bond in solid and liquid states, like water.

Ammonia molecule is trigonal pyramidal in shape. It has three bond pairs and one lone pair as shown in following structure.

In aqueous solution, NH3 acts as weak base and gives OH- ions,

As a weak base, it precipitates the hydroxides of many metals from their salt solutions. For example,

(c) Uses of : For the production of various nitrogenous fertilizers like urea, ammonium nitrate, ammonium phosphate etc., and for production of nitrogen compounds like HNO3 ammonia is used.

C. Nitric acid :

(a) Preparation: In laboratory, HNO3  is prepared by heating and and conc. ,

Commercially, nitric acid is prepared by Ostwald?s process which involves the catalytic oxidation of NH3  by atmospheric O2

(b) Properties: Nitric acid is a strong acid which contains about 68% of the HNO3  by mass and a concentration upto 98% can be obtained by dehydration with conc. H2SO4 

Freezing point = 231.4 K

Boiling point = 355.6 K

Specific gravity = 1.504

In gaseous state, HNO3  exists as planar molecules with structure as follows:

? Oxidation of metals by HNO3  :

Concentrated HNO3  acts as oxidizing agent and oxidizes most metals except noble metals like gold and platinum. For example:

Thus, we see that the products of oxidation of metals are different with different concentration of HNO3  . Actually, the products also depend on the temperature and nature of materials undergoing oxidation.

? Oxidation of non-metals by :

? Brown ring test: This test is done for detection of nitrates which depends on the ability of Fe2+ to reduce nitrates to nitric oxide. Nitric Oxide reacts with Fe2+ to form brown coloured complex.

(c) Uses of : It is mainly used for manufacture of ammonium nitrate fertilizers and other nitrates in explosives and pyrotechnics. It is also used in pickling of stainless steel, etching of metals and as an oxidiser in rocket fuels.

(d) Oxides of Nitrogen:

Table : Oxides of Nitrogen

Name

Formula

Oxidation state of nitrogen

Common methods of preparation

Physical appearance and
chemical nature

Dinitrogen oxide [Nitrogen
(I) oxide]

N2O

+1

colourless gas, neutral

Nitrogen monoxide [Nitrogen
(II) oxide]

NO

+2

colourless gas, neutral

Dinitrogen trioxide [Nitrogen
(III) oxide]

N2O3

+3

blue solid, acidic

Nitrogen dioxide [Nitrogen
(IV) oxide]

NO2

+4

brown gas, acidic

Dinitrogen tetroxide [Nitrogen
(IV) oxide]

N2O4

+4

colourless solid/
liquid, acidic

Dinitorgen pentoxide [Nitrogen
(V) oxide]

N2O5

+5

colourless solid, acidic

 

 

Table : Structures of Oxides of Nitrogen

(v) Phosphorus and its compounds: Phosphorus is found in many allotropic forms among which white, red and black phosphorus are important.

White phosphorus: The structure of white phosphorous (P4) is tetrahedral. White phosphorous is less stable and therefore, more reactive than the other solid phases because of angular strain in the (P4) molecule where the angles are only 60°

Fig. White Phosphorus

White phosphorus shows chemiluminescence i.e., it glows in dark. It is translucent white waxy solid which is poisonous and insoluble in water but soluble in C S2 (carbon disulphide).

Red phosphorus: The structure of red phosphorus is shown below. It consists of chains of P4 tetrahedral linked together and exists as a polymer.

Fig. Red Phosphorus

White phosphorus on heating at 573 K in an inert atmosphere for several days, converts into red phosphorus. Red phosphorus is odourless, nonpoisonous and insoluble in water as well as in carbon disulphide. It does not show chemiluminescence. It is much less reactive than white phosphorus.

Black phosphorus: When red phosphorus is heated under high pressure, a series of phases of black phosphorus are formed. These are of two forms, α-black phosphorus and β -black phosphorus.

When red phosphorus is heated in a sealed tube of 803 K, α -black phosphorus is formed.

Red phosphorus -black phosphorus

When white phosphorus is heated at 473 K under high pressure, β -black phosphorus is formed.

White phosphorus -black phosphorus.

α -black phosphorus has opaque nonoclinic or rhombohedral crystals which can be sublimed in air. β -black phosphorus doesn?t burn in air upto 673 K.

A. Phosphine (PH3) :

(a) Preparation: In laboratory, phosphine is prepared by heating white phosphorus with conc. NaOH solution in the presence of

(b) Properties: Phosphine is highly poisonous colourless gas with rotten fish smell. It explodes in presence of oxidising agents like HNO3, Cl2 and Br2 vapours. It is slightly soluble in water. It decomposes to give red phosphorus and H2 in presence of light.

Phosphine is weakly basic and gives phosphonium compounds with acids. For example,

(c) Uses of PH3 : The spontaneous combustion of phsphine is technically used in Holem?s signals and in smoke screens.

B. Phosphorus halide: There may be two types of phosphorus halides,

Px3 where x = F, Cl, Br, I

and Px5 where x = F, Cl, Br

 

 

(a) Phosphorus trichloride (PCl3) :

Preparation: PCl3 is prepared by passing dry chlorine over heated white phosphorus,

Properties: It is a colourless oily liquid that hydrolyses in presence of moisture,

(b) Phosphorus pentachloride :

Preparation: PCl5 is prepared by the reaction of white phosphorus with excess of dry chlorine.

Properties: PCl5  is yellowish white powder, it hydrolyses to POCl5  in moist air and finally converted into phosphoric acid.

On strong heating, it decomposes into and

It reacts with organic compounds containing ?OH group and converts them to chloro derivatives:

It is used in synthesis of some organic compounds like C2H5Cl, CH3COCl

(c) Oxoacids of Phosphorus: Some important oxoacids of phosphorus are described as follows:

Name


Formula

Oxidation state of phosphorus

Characteristic bonds and their number

Preparation

Hypophos-phorous

 

H3PO2 

+1

One P ? OH

Two P ? H

One P = O


white p4 + alkali

Orthophos-phorous

(Phospho-nic)(Phosphonic)

H3PO3

+3

Two P ? OH

One P ? H

One P = O

P2O3 + H2O

Pyrophos-phorous

H4P2O5 

+3

Two P ? OH

Two P ? H

Two P = O

PCl3 + H3PO3

Hypophos-phoric

H4P2O6 

+4

Four P ? OH

Two P = O

One P ? P

red P4 + alkali

Orthophos-phoric

H3PO4 

+5

Three P ? OH

One P = O

P4O10 + H2O

Pyrophos-phoric

H4P2O7 

+5

Four P ? OH

Two P = O

One P ? O ? P

Heat
phosphoric
acid

Metaphos-phoric*

(HPO3)n

+5

Three P ? OH

Three P =