Genaral Properties, Hydrides and Oxides in Group15 Elements

 

Group 15 Elements

            Group 15 contains five elements namely Nitrogen(N), Phosphorus(P), Arsenic(As), Antimony(Sb) and Bismuth(Bi). They are known as Nitrogen family. These elements do not occur very abundantly in nature except Nitrogen & Phosphorus. Nitrogen is a very well known elements & is an essential constituent of fertilizers, explosive and proteins.

Electronic Configuration:-

            The electronic configuration of these elements are presented as:-

Elements	Atomic Number	Electronic Configuration
N	7	[He] 2S2 2P3
P	15	[Ne] 3S2 3P3
As	33	[Ar] 3d10 4S2 4P3
Sb	51	[Kr] 4d10 5S2 5P3
Bi	83	[Xe] 4f14 5d10 6S2 6P3

 

            General outer electronic configuration of group 15 elements is ns² nP³.

            The P orbitals in the outer shells are only half filled and they are most stable.

General properties of Group 15 Elements

1:- Atomic Radius:-

            Atomic radius increases from top to bottom because extra shell is added to each element from top to bottom.

2.:- Metallic and non-metallic Character:-

            The change from non-metallic to metallic character with increase in atomic number in the same group. First two elements Nitrogen and Phosphorus are non-metals, Arsenic and Antimony are semi-metals and the last element Bismuth is a metal.

3.:- Physical State:-

            The first element Nitrogen is a gas and other elements P, As, Sb & Bi are solid.

4.:- Ionization Energy:-

            The ionization energy of Nitrogen is very high. The values for the succeeding elements decreases on moving down the group.

5.:- Melting & Boiling Points:-

            The melting(except Antimony and Bismuth) and boiling points increases with increase in atomic number in the same group.

6.:- Electronegativity:-

            Electro negativity decreases gradually as we move down the group.

7.:- Positive Oxidation States:-

N	P	As	Sb	Bi
+1, +2	+5	+3	+3	+3
+3, +4	+3, +4	+5	+5	+5

 

            The electronic configuration of group 15 elements ns² nP³ for the outer shell suggested that these can have two positive oxidation state i.e. +3 & +5, eg. M₂O₃ and M₂O₅.

            The +3 state become more and more common on moving down the group from P to Bi. Nitrogen does not form compounds in +5 oxidation like NF₅, NCl₅ etc because it does not have d orbital in its outer shell. Other members shows +5 oxidation state as in PF₅ and SbF₅.

            Nitrogen and Phosphorus show +4 oxidation state because of the ability to form dative bond with Lewis acid (:NH₃, PH₃).

            Nitrogen can exist in other oxidation state in combination with oxygen, it is found in +1(N₂O), +2(NO) and +4(NO₂) oxidation state.

8.:- Negative Oxidation States:-

N	P	As	Sb	Bi
-3	-3	-3	-3	-3

 

            The oxidation state of Nitrogen in Ammonia(NH₃) is -3, Phosphorus in PH₃ is -3. The tendency of the elements to pass into -3 oxidation state decreases on moving down the group. In fact the last member of the group, hardly exhibits negative oxidation state.

BiH_3­ exist but less stable.

 

9.:- Electron donor Properties:-

            All the elements in the tripositive state(+3) have a tendency to act as electron pair donors. Nitrogen shows this tendency to the maximum extent.

 

10.:- Electron-acceptor Properties:-

            Nitrogen does not have d orbitals and hence it cannot act as electron acceptor. Other elements, having vacant d orbitals in their valency shell, do not act as electron acceptor.

 

11.:- Catenation:-

            All the elements of group 15 shows this property but to a much smaller extent than Carbon.

            The decrease in tendency to form chains is mostly due to a decrease in the bond energies.

 

12.:- Allotropy:-

            All the elements(except Bismuth) show allotropy. Thus two solid forms of Nitrogen known as α-Nitrogen and β- Nitrogen.

            The α-Nitrogen has cubic crystalline structure and β- Nitrogen has hexagonal crystalline structure.

            Phosphorus exist in a number of allotropic forms such as white Phosphorus, red Phosphorus, Scarlet Phosphorus, Metallic or α – black Phosphorus β – black Phosphorus and Violet Phosphorus.

            Arsenic exists in three allotropic forms i.e. Gray, Yellow and Black Arsenic.

            Antimony exists in three different form i.e. Metallic form, yellow or α – antimony and black or β – Antimony.

 

Hydrides of Group 15 Elements

            -All the elements of this group form volatile trihydrides of the formula MH₃ i.e. NH₃(Ammonia), PH₃(Phosphine), AsH­₃(Arsine), SbH₃(Stibine) and BiH₃(Bismuthine).

            The higher elements also form hydrides of the formula M₂H₄ i.e. N₂H₂(hydrazine), P₂H₄(diphosphine) and As₂H₄(darsine).

-          Nitrogen forms hydrides of the formula HN₃, this is called hydrozoic acid.

-          The most important hydrides of elements of group 15 are the trihydrides.

 

 

Preparation

             The trihydrides of group 15 elements are prepared from binary metal compounds i.e. Mg₃N₂, Ca₃P₂, Zn₃As₂ etc.

 

Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃

Ca₃P₂  + 6H₂O → 3Ca(OH)₂ + 2H₃

Zn₃As₂ + 6HCl → 3ZnCl₂ + 2AsH₃

-          They can be prepared by the reduction of trichlorides of these elements MCl₃ with LiAlH.

 

MCl₃ + 3 LiAlH₄ → MH₃ + 3LiCl + 2AlH₃

(M= N, P, As, Sb)

-          The most important trihydride i.e. NH₃ is prepared on an industrial scale by the Haber Process.

 

N₂ + 3H₂     2NH₃

-          The BiH₃ is difficult to prepare because it is extremely unstable.

 

Structure and Bond Angle

 

            The trihydrides of group 15 elements have pyramidal structure. The central atom undergo SP³ hybridization.

            The bond angle decreases as we go down the group.

NH3	PH3	AsH3	SbH3
107.30	93.60	91.80	91.3

 

            The variation in bond angle may be explained on the basis of the size & electronegativity of the central atom.

Properties

 

1.:- Boiling Points:-  The boiling points of the trihydrides of group 15 elements increases as we move down the group except NH₃.

NH3	PH3	AsH3	SbH3	NH3
-34.50 C	-87.50 C	-62.40 C	-18.40 C	16.80C

 

            The abnormally high boiling point of Ammonia is due to the association of its molecules through hydrogen bonding.

 

2.:- Basic Character:-

            All the trihydrides have one lone pair of electron on the central atom and are called as Lewis bases. On going down the group, the basic character of these hydrides decreases.

-          NH₃ is distinctly basic

-          Ph₃ is also base but much weaker than NH₃.

 

3.:- Thermal Stability:-

            The thermal stability of the trihydrides of group 15 elements decreases from NH₃ to BiH₃.

-          NH₃ is most stable

-          PH, AsH₃, SbH₃, are less stable.

-          BiH₃ is unstable.

 

4.:- Reducing Character:-

            Reducing character of the trihydrides increases as we move down the group, thus Ammonia is not good reducing agent & other act as good reducing agent.

 

5.:- Formation of Complexes:-

            Unlike Ammonia, other trihydrides of group 15 elements do  not form any complexes with transition metals.

 

Oxides of Group 15 Elements

            All the elements of group 15 are capable of forming trioxides and pentaoxides of the general formula R₂O₃ and R₂O₅.

(a):- Trioxides:- (R₂O₃)

 

1:- Nitrogen Trioxide(N₂O₃):-

-          It exists pure only in solid state, at low temperature.

-          In liquid state, It dissociated into NO & NO₂.

-          In vapour state, It exist in equimolar mixture of NO & NO₂.

-          It is obtained by interaction of stochiometric quatities of NO & NO₂. On cooling a mixture below -20⁰C, N₂O₃ is obtained.

-          It is acidic in Nature.

-          Structure of N₂O₃:-

The structure of N₂O₃ is not definitely known. The spectroscopic studies reveal that its structure is as shown below.

                                   

 

2.:- Phosphorus Trioxide:-

-          It exists in dimer P₄O₆.

-          It is prepared by the combinations of white phosphorus in a limited supply of air.

-          It is acidic in nature.

-          Structure of P₄O₆:-

P₄O₆  has tetrahedral structure.

The P atoms are situated at each corner of tetrahedron

Each O atoms are situated in between two P atom.

Each P atom is covalently bonded to three oxygen atom and each O atom is covalently bonded to two P atoms.

           

 

3.:- Trioxides of Others:-

-          Trioxides of Arsenic, Antimony and Bismuth are obtained by the direct reaction of metal with Oxygen.

-          It has tetrahedral structure.

-          The trioxides of N, P and As are acidic, trioxides of Sb is amphoteric and trioxides of Bi are basic in character.

 

 

(b):- Pentaoxides(R₂O₅):-

 

1.:- Nitrogen Pentaoxide (N₂O₅):-

-          It is prepared by the dehydration of HNO₃ with Phosphorus Pentaoxide.

-          Structure:-

 

-          All the terminal NO bond distance are equal due to resonance between N=O & N→O bonds in the molecule.

2.:- Phosphorus Pentaoxide(P₄O₁₀)

-          It is obtained by burning P in a free supply of air or Oxygen.

-          Structure of P­₄O₁₀:-

 

 

-          P atoms are situated at each corners of tetrahedron.

-          Six O atoms are situated in between the two P atoms and remaining four O atoms on the four P atoms.

 

3.:- Pentaoxides of Others:-

-          As₂O₅& Sb₂O₅ are obtained by the oxidation of metal with conc. HNO₃.

-          Bi₂O₅ is obtained by the action of strong oxidizing agent on Bi₂O₅.

-          It is unstable.

-          All the pentaoxides are acidic in nature but acidity decreases from N to Bi. N₂O₅ is strongest acidic oxide & Bi₂O₅ is the weakest.