Unit-7:
Nitrogen family:
It contains five elements mainly nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). This group is regarded as nitrogen family.
Electronic Configurations: – The group 15 elements have five electrons in the outermost shell, two in s and three in p sub-shell. The general electronic configuration of group-15 may be expressed as ns2np3.
Nitrogen (Z=7): 1s22s22p3
Phosphorus (Z=15): 1s22s22p63s23p3
Arsenic (Z=33): 1s22s22p63s23p64s23d104p3
Antimony (Z=51): 1s22s22p63s23p64s23d104p65s24d105p3
Bismuth (Z=83): 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p3
Occurrence:-
The elements of group 15, except phosphorus do not occur very abundantly in nature. Though nitrogen comprises about 78% of the earth’s atmosphere, it is not very abundant in earth’s crust. Since nitrates are very soluble in water so these are not widespread in the earth’s crust. The only major minerals are KNO3 (nitre, salt petre) and NaNO3 (sodanitre, chili salt petre). The major deposits of salt petre (KNO3) occur in India. Nitrogen is also an important constituent of proteins and amino acids. The continuous interchange of nitrogen between the atmosphere and biosphere is called nitrogen cycle.
Phosphorus is the eleventh element in order of abundance in crystal rocks of the earth. All its known minerals are orthophosphates. Major amounts of phosphorus occur in a single mineral family known as apatites, which have the general formula, 3Ca3(P04)2 CaX2 or Ca10(PO4 )6X2. Where X= F, Cl or OH. The common minerals of phosphorus are-
- Phosphorite: Ca3 (PO4)2
- Fluorapatite: 3Ca3(PO4)2. CaF2
- Chloroapatite: 3Ca3(PO4)2. CaCl2
- Hydroxyapatite: 3Ca3(PO4)2. Ca(OH)2
Phosphorus is essential for life, both as a structural material in animals and plants. About 60% bones and teeth are Ca3(P04)2 or [3{Ca3(P04)2}. CaF2].
The elements As, Sb and Bi are not very abundant. Their important source is as sulphides occurring as traces in other ores e.g. arsenopyrites (FeAsS), stibnite (Sb2S3), bismuth (Bi2S3). These are obtained as metallurgical by-products from roasting sulphide ores.
Uses:
Nitrogen is used in large amounts as an inert atmosphere in laboratory and in industrial processes such as in iron and steel industry and in oil refineries. Liquid N2 is used as refrigerant. Large amounts of nitrogen are used in the manufacture of ammonia, calcium cyanamide, etc.
Vast amounts of phosphates are used in fertilizers. Phosphorus is used for the manufacture of matches and as a rat poison. It may be noted that red variety of phosphorus is preferred to yellow variety for matches because of its non-poisonous nature. Phosphorus is also used for making phosphorus bronze which is hard and not corroded by water. It is also used for the manufacture of tracer bullets, incendiary bombs and for producing smoke screens. Phosphorus is also used for preparation of other important compounds such as phosphoric acids, phosphorus chlorides and hypophosphites for their use in industry and medicines.
Arsenic is used to form alloys with many metals. Compounds of arsenic are used for killing weeds and in medicines.
Antimony metal is also used in alloys with Sn and Pb. It is also used to electroplate steel present rusting. Antimony compounds are used as fire retardants, in foam fillings for furniture and mattresses.
Bismuth is also largely used for making alloys of low melting points. Some of these alloys melt even below 100°C and are called fusible alloys. For example, woods metal, rose metal, etc. These alloys are used mainly for making automatic electrical fuses, automatic fire alarms, automatic sprinklers, automatic safety plugs of boilers, etc.
General Trends in Physical Properties:-The important physical properties are as follows-
1) Atomic and Ionic radii: – The 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 increases due to increase in number of shells. However, from As to Bi only a small increase in covalent radius is observed because of the presence of completely filled d and f orbitals in the higher members.
2) Ionisation enthalpies: – The first ionisation enthalpies of the group 15 elements are higher than the corresponding members of the group 14 elements. This is due to greater nuclear charge, small size and stable configuration of the atoms of group 15 elements. The electronic configurations of atoms of group 15 are half filled npx1, npy1, npz1 and are stable. Therefore, they have high ionisation enthalpies. On going down the group, the ionisation enthalpies decrease. This is due to increase in atomic size and screening effect which overweigh the effect of increased nuclear charge.
3) Electronegativity: – The electronegativity values of elements of group 15 are higher than the corresponding elements of group 14. 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.
4) Metallic character: – The elements of group 15 are less metallic. However, on going down the group, the metallic character increases from N to Bi. For example, N and P are non-metallic, As and Sb are partly non-metallic (metalloid) while Bi is a metal.
5) Catenation: – The 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 example, hydrazine (H2NNH2) has two N atoms bonded together, hydrazoic acid (N3H) has three N-atoms, azide ion, N–3 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.
6) Melting and boiling points: – Melting points (except for antimony and bismuth) and boiling points increase on going down the group from N to Bi.
7) Oxidation States: – The elements of group-15 have five electrons in their 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 favourable. However, they do form N3- and P3- ions by gaining three electrons form highly electropositive elements, e.g. Mg3N2, Ca3P3.
In addition to -3 oxidation state, the elements of group 15 exhibit + 3 and + 5 oxidation states. For example, 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 oxoacids. For example, NH3 (-3), N2H4 (-2), N2 (0), N2O (+1), NO (+2), N2O3 (+3), N2O4 (+4) and N2O5 (+5).
General Trends in Chemical Properties:-The important chemical properties are as follows-
Nitrogen differs from rest of the members of the group due to its small size, high electronegativity, high ionisation enthalpy and non availability of d-orbitals in the valence shell.
Nitrogen has unique tendency to form pπ— pπ multiple bonds with itself and with other elements having small size and high electronegativity e.g. C and H. However, the heavier elements of this group do not form pπ— pπ bonds because of their atomic orbitals are so large and diffuse that they cannot have effective overlapping. This differences between nitrogen and other members of the group result into anomalous properties of nitrogen as explain below:
Nitrogen is a colourless 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).
: N≡N:
Due to the presence of triple bond, which has very high bond dissociation energy, the nitrogen molecule has very low reactivity. However, the tendency to form multiple bonds (pπ— pπ) is limited only to nitrogen. On the other hand, phosphorus, arsenic and antimony exist in various forms containing single bonded atoms. For example, phosphorus exists as tetrahedral P4 molecules. In this case, four P atoms lie at the corners of a regular tetrahedron. Each P is bonded to three P atoms by single P—P bonds. However, the single N—N bond is weaker than the single P—P bonds because of high interelectronic repulsion of non bonding electrons owing to small bond length (109) pm). Therefore, nitrogen exists as gas while phosphorus exists as solid.
White phosphorus is more reactive than nitrogen. It catches fire when exposed to air, burning to form the oxide, P4O10. It is stored under water to prevent it. Red P is stable in air at room temperature but reacts on heating.
Arsenic and antimony both occur in two forms. The most reactive is yellow form which contains M4 tetrahedral units and resembles white phosphorus. Arsenic is stable in dry air but tarnishes in moist air giving first a bronze and then a black tarnish.
Antimony is less reactive and is stable towards water and air at room temperature. On heating in air, it forms Sb4O6, Sb4O3 or Sb4O10. Bismuth forms Bi2O3 on heating.
Another factor which affects the chemistry of nitrogen is the absence of d-orbitals in its valence shell. Besides restricting its covalency to four, nitrogen cannot form pπ— dπ as the heavier elements can e.g. R3P = O or R3P = CH2 (R = alkyl group). Phosphorus and arsenic can form dπ— dπ bond also with transition metals when their compounds like P(C2H5)3 and As(C6H5)3 act as ligand.
Reactivity towards Hydrogen (Formation of hydrides):
The elements of group 15 form trihydrides of the general formula MH3, where M = N, P, As, Sb, or Bi) such as:
NH3 (Ammonia), PH3 (Phosphine), AsH3 (Arsine), SbH3 (Stibine), BiH3 (Bismuthine)
Structure:
All 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. The structure of NH3 molecule is shown below. Due to the presence of lone pair, the bond angle in NH3 is less than the normal tetrahedral angle. It has been found to 107°. As we go down the group the bond angle decreases as:
NH3 (107°), PH3 (94°), AsH3 (92°), SbH3 (91°), BiH3 (90°)
Explanation:
In all these hydrides, the central atom is surrounded by four electron pairs, three bond pairs and one lone pair. 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 example, 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.
Characteristics of Hydrides:-
The important characteristics of these hydrides are-
1) Basic strength: –
All 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). As we go down the group, the basic character of these hydrides decreases. For example, 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.
2) Thermal Stability:
Thermal 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 > BiH3
Explanation:
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.
3) Reducing Character:
The 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 <BiH3
Explanation:
The reducing character depends upon the stability of the hydride. The greater the unstability of a hydride, the greater is its reducing character. Since the stability of group 15 hydrides decreases from NH3 to BiH3, hence the reducing character increases.
4) Boiling and melting points:
Ammonia has a higher boiling point than phosphine and then the boiling point increases down the group because of increase in size. Similar behaviour is observed for melting point.
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 increases with increase in molecular size and therefore, boiling points increase on moving from PH3 to BiH3.
Reactivity towards Oxygen (Formation of oxides):
All these elements form two types of oxides, M2O3 and M2O5. The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state. Their acidic character decreases down the group. The oxides of the type M2O3 of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth is basic in nature. E.g.
N2O (nitrous oxide)
NO (nitric oxide)
N2O3 (nitrogen trioxide) P4O6 As2O3 Sb2O3 Bi2O3
N2O4 (nitrogen tetra oxide) P4O8
N2O5 (nitrogen penta oxide) P4O10 As2O5 Sb2O5 Bi2O5
Reactivity towards Halogens (Formation of halides):
Group 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. Pentahalides are more covalent than trihalides. All the trihalides of these elements except those of nitrogen are stable. In case of nitrogen, only NF3 is known to be stable. Trihalides except BiF3 are predominantly covalent in nature.
Structures of Oxides of Nitrogen:
Preparation of nitrogen (N2):
In the laboratory, nitrogen is prepared by heating equimolar aqueous solution of NaNO2 and ammonium chloride (or ammonium sulphate) in a round bottomed flask.
NH4Cl + NaNO2 —–>NH4 NO2 + NaCl
NH4 NO2 ——> N2 + H2O
From Ammonia:
Nitrogen can be prepared by oxidation of NH3 with red-hot cupric oxide (CuO) or bleaching powder.
NH3 + CuO —–> Cu + N2 + H2O
NH3 + CaOCl2 —–> CaCl2 + N2 + H2O
From Ammonium dichromate, (NH4)2 Cr2O7:
Nitrogen can also be prepared by heating (NH4)2 Cr2O7 .
(NH4)2 Cr2O7 —-> Cr2O3 + N2 + H2O
Properties of Nitrogen:
Nitrogen is colourless, tasteless, odourless gas. It is slightly lighter than air and sparingly soluble in water. Nitrogen is not poisonous; however, animals die in an atmosphere of nitrogen for want of oxygen.
Chemically nitrogen is very inactive at room temperature. Its chemical reactivity, however, increases with raise in temperature.
1) Action of Hydrogen:
Nitrogen combines with hydrogen to give ammonia under the influence of electric spark.
N2 + H2 —> NH3
2) Action of Oxygen:
Nitrogen combines with oxygen to give nitric oxide under the influence of electric lightening.
N2 + O2 —-> NO
3) Action of Metals:
Nitrogen is heated with metals like Li, Mg and Al give the corresponding metal nitrides. For examples,
N2 + Al —> AlN
N2 + Mg —> Mg3N2
N2 + Li —-> Li3N
Uses of Nitrogen:
Nitrogen is used—
- In the manufacture of ammonia and industrial chemicals containing nitrogen like calcium cyanamide.
- As a refrigerant agent to preserve food items, biological materials and in cryosurgery.
- To create an inert atmosphere in the metallurgical operation, e.g. iron and steel.