Class 12 Chemistry P Block Elements Chemical properties of group 15

Chemical properties of group 15:

  1. Reaction with hydrogen: Whenever any substance reacts with hydrogen they form respective hydrides.

Like whenever 15 group elements react they form hydrides of order EH3 that is:

  NH3         PH3          AsH3     SbH3    BiH3

Ammonia Phosphine  Arsine  Stibine Bismuthine

In this NH3 is called ammonia and PH3 is called phosphine.

All these hydrides are called Lewis bases.

These can be formed by reacting metal hydrides with water that is:

Mg 3N2  +  H2O -->Mg(OH)2           +            NH3

(Magnesium nitride) (magnesium hydroxide)( ammonia)

Likewise for preparing phosphine we take calcium phosphide:

Ca3P2 + H2O -->    Ca (OH)2    +              PH3

(Calcium phosphide) (calcium hydroxide) (phosphine)

Ammonia can be industrially prepared by Haber’s process:

In this nitrogen and hydrogen are taken in 1:3 ratios and catalyst Fe and MoO is used. This reaction can take place at conditions of low temperature and high pressure. The structure of these hydrides is pyramidal and hybridization sp3.

The properties of hydrides are:

  • Bond angle: The bond angle decreases down the group. The order of decrease of angle is:

NH3 > PH3 > AsH3 > SbH3 > BiH3

The reason behind this is that when size of central atom increases, lone pair will push closer to bond pair-bond pair. Due to this bond angle decreases.

  •  Basic character: It refers to ability of molecule to donate its lone pair .the order of increase in basic character is :

                NH3 > PH3 > AsH3 > SbH3 > BiH3

             Most basic                           least basic

Due to large size of bismuth the lone pair density is less closer to bismuth therefore tendency to lose electron decreases.

  •  Stability: The stability of hydrides depends upon the comparability of size. The order of stability of hydrides is:

                  NH3 > PH3 > AsH3 > SbH3 > BiH3

Due to increase in size, bond length increase due to which bond dissociation energy decrease therefore stability decrease.

  • Reducing nature: The order of reducing character of hydrides is:

                  NH3  < PH3  < AsH3 < SbH3 < BiH3

Out of them in case of BiH3, the bond strength is low therefore reducing character is lowest.

  •  Boiling point: The boiling point depends upon Vander wall force and this Vander wall force increases with increase in size.

The order of their boiling points for 15 group hydrides is:

NH3  > PH3   < AsH3  < SbH3   < BiH3

In case of NH3 and PH3, ammonia has higher boiling point than phosphine because of hydrogen bonding.

  •  Solubility: The solubility of hydrides in water for group 15 is:

                NH3 > PH3   > AsH3  > SbH3   > BiH3

 Ammonia has higher solubility due to formation of     hydrogen bonds.

  1. Reaction with oxygen: Whenever group 15 elements react with oxygen they form oxides of order E2O3 and E2O5.

E2O3- trioxides

E2O5. - Penta oxides

Nitrogen form maximum types of different oxides ranging from +1 to +5.

The oxides of nitrogen are:

  • N2O (laughing gas ) oxidation state +1
  • NO oxidation state +2
  • N2O3 oxidation state +3
  • N2O4 oxidation state +4
  • N2O5 oxidation state +5(most acidic)

In all the oxides pπ-pπ bonding takes place.

  • P-forms oxide in oxidation state of +3 (P4O6), +4 (P4O8), +5 (P4O5)
  • As forms oxide as As2O3, As2O5 (Arsenic trioxide and Arsenic pentaoxide).
  • Sb forms oxide as Sb2O3 and Sb2O5 (Antimony trioxide and Antimony pentaoxide).
  • Bi forms oxide as Bi2O5 and Bi2O3 (Bismuth trioxide and Bismuth pentaoxide).

Nature of their oxides:

Nitrogen and phosphorous forms acidic oxides.

Arsenic and antimony forms amphoteric oxide.

Bismuth forms basic oxide. 

“Higher is the oxidation state more acidic is the oxide”

  1. Reaction with halogens

The group 15 elements combine with halogen to form their respective halides of general formula EX3 and EX5.

Nitrogen does not form pent halides due to non availability of d orbital but form trihalides.

The structure of trihalides is distorted tetrahedral of hybridization sp3.

The geometry is given below:


Nitrogen trichloride

Characteristics of their trihalides:

  • They are covalent and down the group ionization increase
  • Trihalides of nitrogen are least stable
  • Nitrogen triflourides are maximum stable
  • The order of stability of nitrogen halides are :


                    The reason behind is incomparable sizes.

  • Trihalides of phosphorous arsenic and antimony are good Lewis bases.
  • Down the group trihalides of group 15 stability increases due to inert pair effect.

Characteristics of pentahalides:

  • Their general formula is EX
  • Their hybridization is sp3
  • The geometry of pentahalides is trigonal bipyramidal as shown below:


Phosphorous pentachloride


  • Down the group stability of pent halides decreases due to inert pair effect.
  • All pent halides are Lewis acids.


Please note that: The existence of PCl5 (phosphorous pentachloride)is gas but in solid state it exist as dimer {[PCl4]+[PCl6]-}

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