INTRODUCTION

The p-block contains several elements of great social and economic importance as well as chemical interest. Examples include the use of aluminium as a structural material, the importance of silicon and germanium as semiconductors, and the use of sulphur, phosphorus and nitrogen in fertilizers. The elements belonging to groups 13 to 18 of the periodic table are the p-block elements. They are also called as representative elements or normal elements. The chemistry of p-block elements plays an important role in our daily life. The three p-block elements oxygen, silicon and aluminium are most abundant in earth's crust. Carbon and sulphur are the two p-block elements that occur in free state.The p-block elements have relatively little in common except that their outer elections are in p orbitals. This contrasts with the considerable similarities in properties of the elements of the s-block and also between those of the d-block. The p-block is more notable for trends. Each group in the p-block has its own characteristic properties and hence they are called as representative elements. Depending upon the number of valence electrons, the elements of the p-block exhibit variable oxidation states. . The general electronic configuration of these elements is ns2np1-6. They involve the filling up of 'p' orbitals of their outermost shells, when their 's' orbitals are already filled. Some of the 'p' block elements such as boron, carbon, nitrogen, and oxygen, form very important compounds. The occurrence, preparation and properties of these elements and the chemistry of their compounds are focused here

Group 15 elements (Nitrogen family)Occurrence and Uses of PhosphorusPhosphorus is widely distributed in nature. It does not occur in free state since it can be very easily oxidised. It is usually present in rocks in the combined state as phosphates. The main phosphate rocks are: Phosphorite: Ca3(PO4)2Fluorapatite: 3Ca3(PO4)2 . CaF2 Chlorapatite: 3Ca3(PO4)2 . CaCl2Phosphorus compounds play an important role in life processes. P is an essential constituent of animal and plant matter. It is present in bones, blood and brain of animal body and also in living cells. Several of its compounds

have also industrial applications. The most important of these chemicals are orthophosphoric acid and phosphatic fertilizers

Atomic Properties- Electronic ConfigurationThe atoms of group 15 have five electrons in the outermost shell, two in s and three in p sub-shell. The general electronic configuration of this group may be expressed as ns2np3. Electronic configuration of group 15 elements

Physical Characteristics of group 15 Elements

The important physical constants of group 15 elements are given below: Some important physical constants of group 15 elements

   N  P  As  Pb  Bi

 Atomic radius (pm)  70  110  120  140  150

 Ionic radius (pm)  171 (N3 - )  212 (P3 - )  222 (As3 - )  76 (Sb3 + )  

 Ionisation energy IE1(k.J mo1-1)

 1402  1012  947  834  703

 IE2  2856  1903  1798  1594  1610

 IE3  4577  2910  2736  2443  2446

 Electronegativity  30  2.1  2.0  1.9  1.9

 Melting point (K)  63  317.1  1089  904  544.4

 Boiling point (K)  77  553.5  888  1860  1837

 Density (g cm- 3)  0.879  0.879  5.778  6.697  9.808

 The important physical characteristics are discussed below:Atomic and ionic radiiThe atomic and ionic radii of group 15 elements are smaller than the atomic radii of the corresponding group 14 elements. This is because of increased nuclear charge. On going down the group, the atomic radii increase due to the increase in number of shells.Melting and boiling pointsMelting points (except for antimony and bismuth) and boiling points increase on going down the group from N to Bi.Ionization energiesThe first ionization energies of the group 15 elements are higher than the corresponding members of the group 14 elements.Explanation: The larger ionization energy is due to greater nuclear charge, small size and stable configuration of the atoms of group 15 elements. The electronic configuration of atoms of group 15 is half filled,

energies. On going down the group, the ionization energies decrease. This is due to increase in atomic size and screening effect, which overweigh the effect of increased nuclear charge.ElectronegativityThe electronegativity values of elements (of group 15 are higher than the corresponding elements of group 14.Explanation: The elements of group 15 have smaller size and greater nuclear charge of atoms and therefore they have higher electronegativity values. On going down the group the electronegativity value decreases. This is due to increase in size of the atoms and shielding effect of inner electron shells on going down the group.Metallic characterThe elements of group 15 are less metallic. However on going down the group, the metallic character increases from N to Bi. For e.g.,, N and P are non-metallic, As and Sb are partly non-metallic while Bi is a metal.

CatenationThe elements of group 15 also show a tendency to form bonds with itself known as catenation. All these elements show this property but to a much smaller extent than carbon. For e.g., hydrazine (H2N-NH2) has two N atoms

bonded together, hydrazoic acid, (N3H), has three N-atoms, azide ion, , has also three N atoms bonded together, while diphosphine (P2H4) has two phosphorus atoms bonded together. The lesser tendency of elements of group 15 to show catenation in comparison to carbon is their low (M-M) bond dissociation energies.

Bond C - C N - N P - P As - As

Bond energy 353.3 163.8 201.6 147.4

AllotropyExcept nitrogen and bismuth, all other elements of this group show allotropy.For e.g., Phosphorus exists as - white, black or red phosphorusArsenic exists as - yellow or grey arsenic Antimony exists as - yellow or silvery greyallotropic forms. Oxidation statesThese elements have five electrons in the valence shell. The loss of five electrons is quite difficult because of energy considerations. Hence they do not form ionic compounds by loss of 5 electrons. On the other hand, these elements can also gain three electrons to complete their octets. But gain of 3 electrons is also not energetically favorable. However, they do form N3-and P3- ions by gaining three electrons from highly electropositive elements, e.g. Mg3N2, Ca3P2.In addition to - 3 oxidation state, the elements of group 15 exhibit +3 and +5 oxidation states. For e.g., phosphorus forms pentahalides such as PF5, PCl5 (+5 oxidation state) and trihalides PCl3, PF3 (+3 oxidation state). Nitrogen exhibits various oxidation states from -3 to +5 in its hydrides, oxides and oxo acids.

For e.g.,

Compound Oxidation state

 NH3 Ammonia  - 3

 N2H4 Hydrozine  - 2

 N2 Nitrogen  0

 N2O Nitrous Oxide  + 1

 NO Nitric Oxide  + 2

 N2O3 Nitrogen trioxide  + 3

 N2O4 Nitrogen tetraoxide  + 4

 N2O5  + 5

Chemical Properties

Elemental state and chemical reactivityNitrogen is a colorless gas and exists as diatomic. The two nitrogen atoms are held together by triple bond and have very high bond dissociation energy (945 kJ mol-1).

Due to the presence of triple bond, which has very high bond dissociation energy, the nitrogen molecule bas very low reactivity. However, the tendency to form multiple bonds (pp-pp) is limited only to nitrogen. On the other hand, phosphorus, arsenic and antimony exist in various forms containing single bonded atoms. For e.g., phosphorus exists as tetrahedral P4 molecules.

Structure of P4 molecule

In this case, four P atoms lie at the comers of a regular tetrahedron. Each P is bonded to three P atoms by single P - P bonds. Therefore, it exists as solid. Therefore, nitrogen exists as gas while phosphorus exists as solid.

Hydrides

The elements of groups 15 form trihydrides of the general formula MH3 such as

These hydrides can be obtained by different chemical reactions:

StructureAll these hydrides are covalent in nature and have pyramidal structure. These involve sp3 hybridization of the central atom and one of the tetrahedral position is occupied by a lone pair.

Shape of NH3 moleculeDue to the presence of lone pair, the bond angle in NH3 is less than the normal tetrahedral angle. It has been found to be 107o. Down the group the bond angle decreases as:

Explanation:

In all these hydrides, four electron pairs, three bond pairs and one lone pair surround the central atom. Now, as we move down the group from N to Bi, the size of the atom goes on increasing and its electronegativity decreases. Consequently, the position of bond pair shifts more and more away from the central atom in moving from NH3 to BiH3. For e.g., the bond pair in NH3 is close to N in N-H bond than the bond pair in P-H bond in PH3. As a result, the force of repulsion between the bonded pair of electrons in NH3 is more than in PH3. In general, the force of repulsion between bonded pairs of electrons decreases as we move from NH3 to BiH3 and therefore, the bond angle also decreases in the same order.Basic strengthAll these hydrides have one lone pair of electrons on their central atom. Therefore, they act as Lewis bases. They can donate an electron pair to electron deficient species (Lewis acids). Down the group, the basic character of the hydrides decreases. For e.g., NH3 is distinctly basic; PH3 is weakly basic; AsH3, SbH3 and BiH3 are very weakly basic.Explanation: Nitrogen atom has the smallest size among the hydrides. Therefore, the lone pair is concentrated on a small region and electron density on it is the maximum. Consequently, its electron releasing tendency is maximum. As the size of the central atom increases down the family, the electron density also decreases. As a result, the electron donor capacity or the basic strength decreases down the group.Thermal stabilityThermal stability of the hydrides of group 15 elements decreases as we go down the group. Therefore, NH3 is most stable and BiH3 is least stable. The stability of the hydrides of group 15 elements decreases in the order:NH3 > PH3 > AsH3 > SbH3 > BiH3Explanation: This is due to the fact that on going down the group, the size of the central atom increases and therefore, its tendency to form stable covalent bond with small hydrogen atom decreases. As a result the M~H bond strength decreases and therefore thermal stability decreases.Reducing characterThe reducing character of the hydrides of group 15 elements increases from NH3 to BiH3. Thus, increasing order of reducing character is as follows: NH3< PH3< AsH3 < SbH3 < BiH3Explanation: The reducing character depends upon the stability of the hydride. The greater the instability of an hydride, the greater is its reducing character.

Since the stability of group 15 hydrides decreases from NH3 to BiH3, hence the reducing character increases.Boiling pointsAmmonia (240 K) has a higher boiling point than phosphine (190 K) and then the boiling point increases down the group because of increase in size.

Explanation:The abnormally high boiling point of ammonia is due to its tendency to form hydrogen bonds.

In PH3 and other hydrides, the intermolecular forces are Van der Waals' forces. These van der Waals' forces increase with increase in molecular size and therefore, boiling points increase on moving from PH3 to BiH3. The main trends are summarized below.

Oxides

The elements of group 15 combine with oxygen either directly or indirectly to form oxides. The important oxides of group 15 elements along with their oxidation states are listed in table. Thus, it is nitrogen, which forms all oxides having oxidation states +1 to +5. All the oxides of nitrogen except N2O and NO and phosphorus are strongly acidic: oxides of arsenic are weakly acidic; oxides of antimony are amphoteric while those of bismuth are weakly basic.There is an important difference between the oxides of nitrogen and other group congeners in their structures. The nitrogen has the ability to form pp-pp multiple bonds and this is present in its structures. On the other hand the reluctance of P, As, Sb and Bi to form pp-pp multiple bonds-leads to the

cage structures for their oxides. The structures of some oxides are given in Fig 9 (a) and 9 (b). Oxides of group 15 elements in different oxidation states

Structures of oxides of N and P

HalidesGroup 15 elements form two series of halides of the type MX3, (trihalides) and MX5, (pentahalides). The trihalides are formed by all the elements while pentahalides are formed by all the elements except nitrogen. Nitrogen cannot form pentahalides due to the absence of vacant d-orbitals in its outermost shell. Similarly the last element, Bi has little tendency to form pentahalides because +5 oxidation state of Bi is less stable than +3 oxidation state due to inert pair effect (a) Trihalides(i) The trihalides have pyramidal structure and have one lone pair. In these cases, the central atom involves sp3 hybridization.(ii) The trihalides are mainly covalent with the exception of BiF3, which is ionic.

Structure of PCl5 (iii) The trihalides are easily hydrolyzed by water. However, the products are different in hydrolysis of different chlorides.

Antimony and bismuth trichlorides are only partially hydrolyzed to form oxychlorides.

The trihalides of P, As and Sb (especially fluorides and chlorides) act as Lewis acids and combine with Lewis bases:

(b) Pentahalides(i) In pentahalides the P undergoes sp3d hybridization and have trigonal bipyramidal geometry.(ii) The pentahalides are thermally less stable than the trihalides. For e.g., PCl5, exists as molecules in the gas phase but exist as [PCl5]+[PCl6]- in the crystalline state. PBr5, and PI5, also exist in the ionic form as [PBr4]+[PBr6]- and [PI4]+I- respectively in the solid state.

Structure of PF5

Allotropes of Phosphorus

Elemental phosphorus is obtained by heating phosphate rock with coke and silica in an electric furnace at about 1770 K. The phosphorus so formed is white phosphorus. The reaction may be represented as:

There are three principal allotropic forms of Phosphorus. These are white phosphorus, red phosphorus and black phosphorus. (a) White phosphorus or Yellow phosphorus(i) It is a soft waxy solid with garlic smell.(ii) It is poisonous in nature. (iii) It turns yellowish on exposure to light. For this reason, it is also called yellow phosphorus.(iv) It is not soluble in water but soluble in carbon disulphide. (v) It undergoes spontaneous combustion in air and produces greenish glow.(vi) It exists as P4 molecules both in solid and vapor state. The four atoms in P4 molecule occupy the corners of regular tetrahedron as shown in Fig. 7 (b) Red PhosphorusIt is prepared by heating white phosphorus to about 540 K in an inert atmosphere of nitrogen for several hours.

(i) It is a hard crystalline solid without any smell.(ii) It is non-poisonous in nature. (iii) It is insoluble in water as well as in carbon disulphide.(iv) It is more stable and relatively less reactive. (v) It consists of tetrahedral units of P4 linked to one another to constitute linear chains.(c) Black PhosphorusIt is prepared by heating white phosphorus to about 470 K under high pressure of 1200 atmospheres in inert atmosphere. 

 (i) It has metallic lustre.(ii) It is most inactive form of phosphorus.(iii) It has a layer type structure in which each layer consists of phosphorus atoms.Some physical properties of three forms of Phosphorus are given below.

Property White phosphorusRed phosphorus

Black Phosphorous

ColourWhite, but turns yellow on exposure

Dark red Black

ColourWhite, but turns yellow on exposure

Dark red Black

StateWaxy solid, can be cut with knife

Brittle powderCrystalline with greasy touch

Smell Garlic smell Odorless  

Density 1.84 2.1 2.69

Ignition temperature

307 K 533 K 673 K

Melting point 317 K Does not melt 860 K

 

The elements of group 15 form a large number of oxoacids.

(i) Oxoacids of nitrogenThe important oxoacids of nitrogen are given in the table below.Oxo acids of nitrogen

 Name  Formula Oxidation state of N

 Nature

 Nitroxylic acid  H4N2O4 +2 Highly explosive, difficult to get in pure state

 Nitrous acid  HNO2  +3  Weak acid and unstable

 Nitric acid  HNO3  +5  Strong acid and stable

 Peroxynitric acid

 HNO3  +5  Unstable and explosive

 Out of the oxoacids of nitrogen, nitric acid is the most important. It is very strong oxidising agent and is quite useful. (ii) Oxoacids or phosphorusPhosphorus forms a number of oxoacids as given in below table.Oxoacids or phosphorus

Name Formula Oxidation state of P

Hypophosphorus acid H3PO2 +1

Phosphorus acid H3PO3 +3

Hypophosphoric acid H4P2O6 +4

Orthophosphoric acid H3PO4 -

Diphosphoric acid (Pyrophosphoric acid) H4P2O7 +5

Metaphosphoric acid HPO3 +5

Peroxophosphoric acid H3PO5 +7

Among the oxoacids of phosphorus, orthophosphoric acid is the most important and is used in the manufacture of phosphate fertilizers.The structures of oxoacids of phosphorus are systematically given ahead:

The formulae of oxoacids of P can be remembered as:The prefixa) meta acid is used for the acid obtained by the loss of one water molecule.b) pyro acid is used for the acid obtained by heating two molecules with loss of one water molecule.c) hypo is generally used for the acid having lower oxygen content than the parent acid.However. it may be noted that metaphosphoric acid does not exist as simple monomer; it exists are cyclometaphosphoric acid or polymetaphosphoric acid. 

 Phosphatic fertilizersThe fertility of soil can be enhanced by using chemical fertilizers, which provide the essential plant nutrients, potassium, nitrogen and phosphorus.The most important phosphatic fertilizer is the superphosphate of lime, Ca(H2PO4)2.This is produced directly from phosphate rock by treatment with concentrated sulphuric acid. In this way the insoluble phosphate rock is rendered soluble in water to improve the release of Phosphorus to the soil for the uptake by the plants.

Phosphatic rock generally contains fluoride, which reacts with H2SO4 to give hydrogen fluoride, which in turn generates other side products. The gaseous side-products are removed by washing with water in a scrubber. Almost 90% of the phosphate rock mined goes into the production of phosphatic fertilizers; the remaining 10% is used for the production of elemental phosphorus.Treatment of phosphate rock with phosphoric acid yields triple super phosphate, Ca(H2PO4)2.H2O which is free from calcium sulphate and hence contains a greater percentage of Phosphorus: