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P-Block Elements
The nitrogen family is element group 15 of the periodic table. The nitrogen family consists of nitrogen phosphorus, arsenic, antimony and bismuth. Nitrogen family elements consists of atoms having 5 electrons in their outer energy level. A pnictogen is one of the chemical elements in group 15 of the periodic table. This group is also known as the nitrogen family. Z Element No. of electrons/shell 7 nitrogen 2, 5 15 phosphorus 2, 8, 5 33 arsenic 2, 8, 18, 5 51 antimony 2, 8, 18, 18, 5 83 bismuth 2, 8, 18, 32, 18, 5 Like other groups, the members of this family show similar patterns in electron configuration, especially in the outermost shells. This group has the defining characteristic that all the component elements have 5 electrons in their outermost shell, that is 2 electrons in the s subshell and 3 unpaired electrons in the p subshell. They are therefore 3 electrons short of filling their outermost electron shell in their non-ionized state. The most important elements of this group are nitrogen (N), which in its diatomic form is the principal component of air, and phosphorus (P), which, like nitrogen, is essential to all known forms of life. The pnictogens consist of two nonmetals (one gas, one solid), two metalloids, one metal, and one element with unknown chemical properties. All the elements in the group are solids at room temperature, except for nitrogen which is gaseous at room temperature. Nitrogen and bismuth, despite both being pnictogens, are very different in
their physical properties. For instance, at STP nitrogen is a transparent nonmetallic gas, while bismuth is a silvery-white metal. Nitrogen Nitrogen can be produced by fractional distillation of air. Nitrogen can also be produced in a large scale by burning hydrocarbons or hydrogen in air. On a smaller scale, it is also possible to make nitrogen by heating barium azide. Additionally, the following reactions produce nitrogen: NH 4 + + NO 2 −^ → N 2 + 2H 2 O 8NH 3 + 3Br 2 → N 2 + 6NH 4 +^ + 6Br− 2NH 3 + 3CuO → N 2 + 3H 2 O + 2Cu Phosphorus The principal method for producing phosphorus is to reduce phosphates with carbon in an electric arc furnace. Arsenic Most arsenic is prepared by heating the mineral arsenopyrite in the presence of air. This forms As 4 O 6 , from which arsenic can be extracted via carbon reduction. However, it is also possible to make metallic arsenic by heating arsenopyrite at 650 to 700 °C without oxygen. Antimony With sulfide ores, the method by which antimony is produced depends on the amount of antimony in the raw ore. If the ore contains 25% to 45% antimony by weight, then crude antimony is produced by smelting the ore in a blast furnace. If the ore contains 45% to 60% antimony by weight, antimony is obtained by heating the ore, also known as liquidation. Ores with more than 60% antimony by weight are chemically displaced with iron shavings from the molten ore, resulting in impure metal.
Properties of Group 15 Element Group 15 Element There are two allotropic elements in Group 15, phosphorus andarsenic. Phosphorus exists in several allotropic forms. The main ones (and those from which the others are derived) are white, red, and black (the thermodynamically stable form at room temperatu re). Only white and red phosphorus are of industrial importance. Phosphorus was first produced as the common white phosphorus, which is the most volatile , most reactive, and most toxic, but the least thermodynamically stable form of phosphorus, α - P
- White phosphorus is a waxy, nonconductor and reacts with air —the phosphorescent reaction of oxygen with the vapour above the solid producing the yellow-green chemiluminescent light, which gives phosphorus its name (after the Greek god, Eosphoros, the mornin g star, the bringer of light). The phosphorus in commercial use is amorphous red phosphorus, produced by heating white phosphorus in the absence of air at about 300°C. It melts Element/Symbol Atomic Number Mass Electron Configuration Covalent Radius (pm) Electronegativity First Ionizaton Energy (kJ/mol) Common Physical Form(s) Nitrogen (N) 7 14.01 1s^2 2s^2 2p^3 75 3 1402 Colorless Gas Phosphorus (P) 15 30.97^ [Ne]3s^2 3p^3 110 2.1^1012 White Solid / Red Solid Arsenic (As) 33 74.92^ [Ar] 3d^10 4s^2 4p^3 121 2 947 Yellow Solid / Gray Solid Antimony (Sb) 51 121.76 [Kr] 4d^10 5 s^2 5p^3 140 1.9 834 Yellow Solid / Silver- White Metallic Solid Bismuth (Bi) 83 208.98 [Xe] 4f^14 5d^10 6s^2 6p^3 155 1.9 703 Pink- White Metallic Solid
around 600°C and was long thought to contain polymers formed b y breaking a P-P bond of each P 4 tetrahedron of white phosphorus then linking th e "opened" tetrahedral. A variety of crystalline modifications (tetragonal red, triclinic red, cubic red), possibly with similar polymeric structures can also be prepared by heating amorphous red phosphorus at over 500°C. The most thermodynamically stable, and least reactive, form of phosphorus is black phosphorus, which exists as three crystalline (orthorhombic - , rhombohedral- and metallic, or cubic) and one amorphous, allotrope. All are polymeric solids and are practically nonflammable. Both orthorhombic and rhombohedral phosphorus appear black and graphitic, consistent with their layered structures. Figure a.Linkage of P 4 units in red phosphorus. A violet crystalline allotrope, monoclinic phosphorus, or Hittorf's phosphorus, after its discoverer, can be produced by a complicated thermal electrolytic procedure. The structure is very complex, consisting of tubes of
nitride, and it is reduced to ammonia by certain microorganisms .Few binary molecular compounds of nitrogen are formed by direct reaction of the elements. At elevated temperatures, N 2 reacts with H 2 to form ammonia, with O 2 to form a mixture of NO and NO 2 , and with carbon to form cyanogen (N≡C–C≡N); elemental nitrogen does not react with the halogens or the other chalcogens. Nonetheless, all the binary nitrogen halides (NX 3 ) are known. Except for NF 3 , all are toxic, thermodynamically unstable, and potentially explosive, and all are prepared by reacting the halogen with NH 3 rather than N 2. Both nitrogen monoxide (NO) and nitrogen dioxide (NO 2 ) are thermodynamically unstable, with positive free energies of formation. Unlike NO, NO 2 reacts readily with excess water, forming a 1:1 mixture of nitrous acid (HNO 2 ) and nitric acid (HNO 3 ) 2NO 2 (g) + H 2 O(l) → HNO 2 (aq) + HNO 3 (aq) Nitrogen also forms N 2 O (dinitrogen monoxide, or nitrous oxide), a linear molecule that is isoelectronic with CO 2 and can be represented as −N=N+=O. Like the other two oxides of nitrogen, nitrous oxide is thermodynamically unstable. The structures of the three common oxides of nitrogen are as follows: Physical Properties And Oxidation States Physical properties include physical state, metallic character, melting and boiling points, density, and allotropy. Nitrogen is a diatomic gas, while the remaining elements are solids. As we move down a group, metallic character increases & the ionisation enthalpy of the elements dec rease with an increase in their atomic size.
Trends in melting and boiling points:
The melting point increases from nitrogen to arsenic due to the gradual increase in atomic size. The very low melting point of nitrogen is due to its discrete diatomic molecules. On the other hand, the high melting point of arsenic is attributed to its giant layered structure in which the layers are closely packed. Although the atomic size increases from arsenic to antimony, there is a decrease in their melting points. Alth ough antimony has a layered structure, it has a low melting point than arsenic because of the relatively loose packing of atoms. Furthermore, the melting point of bismuth is less than antimony due to the loose packing of atoms by metallic bonding. On the other hand, the boiling point gradually increases from nitrogen to bismuth. Allotropy: All the elements in group fifteen, except for bismuth, show allotropy. Nitrogen exists in two allotropic forms, that is, alpha nitrogen and beta nitrogen. Phosphorus exists in many allotropic forms. Of these, the two important allotropic forms are white phosphorus and red phosphorus. Arsenic exists in three important allotropic forms - yellow, grey and black. Antimony also has three important allotropic forms, namely, yellow, explosive and metallic. Oxidation states: All the elements of group 15 have 5 electrons in their outermost orbit. They need only 3 electrons to complete their octet configuration. The octet can be achieved either by gaining 3 electrons or by sharing 3 electrons by means of covalent bonds. As a result, the common negative oxidation state of these elements is - 3. As we move down the group, the tendency to exhibit - 3 oxidation state decreases. This is due to the increase in atomic size and metallic c haracter. Group 15 elements also show positive oxidation states of +3 & +5 by forming covalent bonds. Due to the inert pair affect the stability of +5 oxidation state decreases down the group, while that of +3 oxidation state increases. Nitrogen has only s- and p-orbitals, but no d-orbitals in its valance shell.
considering the case of arsenic, antimon y and bismuth, the +3 state is stable with respect to disproportionation. Nitrogen has only 4 electrons in its outermost shell (one in s orbital and 3 in p) which is available for bonding, hence it exhibits a maximum covalency of
- The heavier elements have a vacant d orbital in the valence shell which is used for bonding. Anamolous properties of Nitrogen - The bond enthalpy of nitrogen molecule is very high due to the presence of triple bond between the two atoms. Other elements form only single bonds with their own atoms and other atoms too. The N-N is weaker than P-P because of high interelectronic repulsions of the non-bonding electrons in N-N due to the small size of N-atom. Reactivity towards hydrogen:- All the elements of Group 15 form hydrides of the type E H 3 where E = N, P, As, Sb or Bi. The stability of hydrides decreases from NH3 to BiH3 due to decrease in their bonddissociation enthalpy. Consequently, the reducing character of the hydrides incre ases. Ammonia is only a mild reducing agent while BiH 3 is the strongest reducing agent amongst all the hydrides. Basicity also decreases in the order Facts of the hydrides NH 3 > PH 3 > AsH 3 > SbH 3 > BiH 3. Basicity decreases because electron density around the central atom decreases. Reactivity towards oxygen All these elements form two types of oxides: E 2 O 3 and E 2 O 5. 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 E 2 O 3 of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth is predominantly basic.
Reactivity towards halogens
- These elements react to form two series of halides : EX 3 and EX 5.
- Nitrogen does not form pentahalide due to non - availability of the d orbitals in its valence shell.
- Pentahalides are more covalent than trihalides.
- In case of nitrogen, only NF 3 is known to be stable due to high polarizing nature of F-atom
- Trihalides except BiF 3 are predominantly covalent in nature. BiF 3 is ionic due to high matallic character of Bismuth. Reactivity towards metals- All these elements react with metals to form their binary compounds exhibiting – 3 oxidation state 1.PH 3 has lower boiling point than NH 3 becausePH 3 molecules do not form intermolecular hydrogen bonding in liquid state. That is why the boiling point of PH 3 is lower than NH 3.
- NH 3 is soluble in water whereas PH 3 is insoluble. Preparation, Properties and Uses – Dinitrogen Dinitrogen makes up to 78% of the earth’s atmosphere and it is the most abundant element present in the air and the seventh most abundant uncombined element found in the universe.This element was first discovered
In laboratory, dinitrogen is obtained by reacting aqueous solution of ammonium chloride with sodium nitrite. NH 4 Cl(aq) + NaNO 2 (aq) → N 2 (g)+ 2H 2 O(l) + NaCl(aq) The products obtained consists of impurities such as NO and HNO3 which can be removed by thermal decomposition of ammonium dichromate. Another method to remove the impurities is to pass the gaseous mixture through sulphuric acid containing potassium dichromate. (NH 4 ) 2 Cr 2 O 7 → N 2 + 4H 2 O+ Cr 2 O 3 Decomposition of sodium or barium azide in the presence of high temperature also results in the formation of pure nitrogen. Physical properties of Dinitrogen :
- Nitrogen is a colourless, odourless and diamagnetic in nature.It is a non-toxic gas.
- It is sparingly soluble in water.
- Nitrogen undergoes condensation to form a colourless liquid which on solidification results in the formation of snow like mass. Chemical properties of Dinitrogen :
- Dinitrogen has a high bond enthalpy due to the N = N bond. Due to this it is inert at room temperature. However the reactivity increases as the temperature increases. Athigh temperatures, nitrogen molecules react with
metals to form respective ionic nitrides and with non - metals to form covalent nitrides. 6Li +N 2 heat → 2Li 3 N At about 773 K it reacts wi th hydrogen to form ammonia in Haber’s Process. N 2 (g) + 3H 2 (g) 773k ↔ 2NH 3 (g)
- Nitric oxide is formed when nitrogen molecule reacts with oxygen molecule at a temperature of 2000 K. N 2 (g) + O 2 (g)↔heat 2NO(g) Uses of Dinitrogen:
- It is mainly used in the industrial manufacturing of compounds such as ammonia, calcium cynamide etc.
- It is used in the manufacturing industries such as iron and steel to obtain an inert atmosphere.
- Liquid nitrogen is used in food industries as a preservativ e and as a refrigerant. Ostwald process The Ostwald process is a chemical process for making nitric acid (HNO 3 ). Wilhelm Ostwald developed the process, and he patented it in 1902 The Ostwald process is a mainstay of the modern chemical industry, and it provides the main raw material for the most common type of fertilizer
- pressure between 4 and 10 atmospheres (approx. 400 - 1010 kPa or 60- 145 psig) and
- temperature is about 500 K (approx. 217 °C or 422.6 °F.). Compounds of nitrogen Red phosphorus Red phosphorus structure Red phosphorus may be formed by heating white phosphorus to 300 °C ( °F) in the absence of air or by exposing white phosphorus to sunlight. Red phosphorus exists as an amorphous network. Upon further heating, the amorphous red phosphorus crystallizes. Red phosphorus does not ignite in air at temperatures below 240 °C (464 °F), whereas pieces of white phosphorus ignite at about 30 °C (86 °F). Ignition is spontaneous at room temperature with finely divided material. Heating red phosphorus in the presence of moisture creates phosphine gas, which is both highly flammable and toxic. Applications Red phosphorus can be used as a very effective flame retardant, especially in thermoplastics (e.g. polyamide) and thermosets (e.g. epoxy resins or
polyurethanes). The flame retarding effect is based on the formation of polyphosphoric acid. Together with the organic polymer material, this acid creates a char which prevents the propagation of the flames. The safety risks associated with phosphine generation and friction sensitivity of red phosphorus can be effectively reduced by stabilization and micro - encapsulation. For easier handling, red phosphorus is often used in form of dispersions or masterbatches in various carrier systems. Reactions of violet phosphorus It does not ignite in air until heated to 300 °C and is insoluble in all solvents. It is not attacked by alkali and only slowly reacts with halogens. It can be oxidised by nitric acid to phosphoric acid. If it is heated in an atmosphere of inert gas, for example nitrogen or carbon dioxide, it sublimes and the vapour condenses as white phosphorus. If it is heated in a vacuum and the vapour condensed rapidly, violet phosphorus is obtained. It would appear that violet phosphorus is a polymer of high relative molecular mass, which on heating breaks down into P2molecules. On cooling, these would normally dimerize to give P4 molecules (i.e. white phosphorus) but, in vacuo, they link up again to form the polymeric violet allotrope.
atom is bonded to three other atoms. Black and red phosphorus can also take a cubic crystal lattice structure. A recent synthesis of black phosphorus using metal salts ascatalysts has been reported. Diphosphorus molecule The diphosphorus allotrope (P 2 ) can normally be obtained only under extreme conditions (for example, from P 4 at 1100 kelvin). In 2006, the diatomic molecule was generated in homogenous solution under normal conditions with the use of transition Diphosphorus is the gaseous form ofphosphorus, and the thermodynamically stable form between 1200 °C and 2000 °C. The dissociation of tetraphosphorus (P 4 ) begins at lower temperature: the percentage of P 2 at 800 °C is ≈ 1%. At temperatures above about 2000 °C, the diphosphorus molecule begins to dissociate into atomic phosphorus. Phosphine. Phosphine (IUPAC name: phosphane) is the compound with the chemical formula PH 3. It is a colorless, flammable, toxic gas. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphane (P 2 H 4 ). W ith traces of P 2 H 4 present, PH 3 is spontaneously flammable in air, burning with a luminous flame. Phosphines are also a group of organophosphoruscompounds with the formula R 3 P (R = organic derivative). Organophosphines are important in catalysts where they complex to various metal ions; complexes derived from a chiral phosphine can catalyze reactions to give chiral,enantioenriched products.
Structure and properties PH 3 is a trigonal pyramidal molecule with C 3 v molecular symmetry. The lengthof the P-H bond is 1.42 Å, the H-P-H bond angles are 93.5°. The dipole moment is 0.58 D, which increases with substitution of methyl groups .The aqueous solubility of PH3 is slight; 0.22 mL of gas dissolve in 1 mL of water. Phosphine dissolves more readily in non - polar solvents than in water because of the non-polar P-H bonds. It is technically amphoteric in water, but acid and base activity is poor. Proton exchange proceeds via a phosphonium(PH 4 +) ion in acidic solutions and via PH 2 −^ at high pH, with equilibrium constants Kb = 4 × 10−28 and Kz = 41.6 × 10−29. Phosphine burns producing a dense white cloud of phosphorus pentoxide: 2 PH 3 + 4 O 2 → P 2 O 5 + 3 H 2 O Preparation Phosphine may be prepared in a variety of ways. Industrially it can be made by the reaction of white phosphorus with sodium or potassium hydroxide, producing sodium or potassium hypophosphite as a by - product. 3 KOH + P 4 + 3 H 2 O → 3 KH 2 PO 2 + PH 3 Alternatively the acid-catalyzed disproportioning of white phosphorus yieldsphosphoric acid and phosphine. Both routes have industrial significance; the acid route is preferred method if further reaction of the phosphine to substituted phosphines is needed. The acid route requires purification and pressurizing. It can also be made (as described above) by the hydrolysis of a metal phosphide, such as a luminium phosphide or calcium
