The p-Block Elements

Unit 7 Objectives After studying this Unit, you will be able to • appreciate general trends in the chemistry of elements of groups 15,16,17 and 18; • learn the preparation, properties and uses of dinitrogen and phosphorus and some of their important compounds; • describe the preparation, properties and uses of dioxygen and ozone and chemistry of some simple oxides; • know allotropic forms of sulphur, chemistry of its important compounds and the structures of its oxoacids; • describe the preparation, properties and uses of chlorine and hydrochloric acid; • know the chemistry of interhalogens and structures of oxoacids of halogens; • enumerate the uses of noble gases; • appreciate the importance of these elements and their compounds in our day to day life. The p -Block Elements Diversity in chemistry is the hallmark of p–block elements manifested in their ability to react with the elements of s–, d– and f–blocks as well as with their own. In Class XI, you have learnt that the p-block elements are placed in groups 13 to 18 of the periodic table. Their valence shell electronic configuration is ns2np1–6 2 (except He which has 1s configuration). The properties of p-block elements like that of others are greatly influenced by atomic sizes, ionisation enthalpy, electron gain enthalpy and electronegativity. The absence of dorbitals in second period and presence of d or d and f orbitals in heavier elements (starting from third period onwards) have significant effects on the properties of elements. In addition, the presence of all the three types of elements; metals, metalloids and non-metals bring diversification in chemistry of these elements. Having learnt the chemistry of elements of Groups 13 and 14 of the p-block of periodic table in Class XI, you will learn the chemistry of the elements of subsequent groups in this Unit. 7.1 Group 15 Elements Group 15 includes nitrogen, phosphorus, arsenic, antimony, bismuth and moscovium. As we go down the group, there is a shift from nonmetallic to metallic through metalloidic character. Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids, bismuth and moscovium are typical metals. 7.1.1 Occurrence Molecular nitrogen comprises 78% by volume of the atmosphere. In the earth’s crust, it occurs as sodium nitrate, NaNO3 (called Chile saltpetre) and potassium nitrate (Indian saltpetre). It is found in the form of proteins in plants and animals. Phosphorus occurs in minerals Chemistry 170 of the apatite family, Ca9(PO4)6. CaX2 (X = F, Cl or OH) (e.g., fluorapatite Ca9 (PO4)6. CaF2) which are the main components of phosphate rocks. Phosphorus is an essential constituent of animal and plant matter. It is present in bones as well as in living cells. Phosphoproteins are present in milk and eggs. Arsenic, antimony and bismuth are found mainly as sulphide minerals. Moscovium is a synthetic radioactive element. Its symbol is Mc, atomic number 115, atomic mass 289 and electronic 14 10 2 3 configuration [Rn] 5f 6d 7s 7p . Due to very short half life and availability in very little amount, its chemistry is yet to be established. Here, except for moscovium, important atomic and physical properties of other elements of this group along with their electronic configurations are given in Table 7.1. Table 7.1: Atomic and Physical Properties of Group 15 Elements Property N Atomic number 7 –1 Atomic mass/g mol (∆iH/(kJ mol ) Electronegativity Covalent radius/pm a g 3 83 121.75 10 2 3 10 208.98 2 3 14 10 2 3 [Ne]3s 3p [Ar]3d 4s 4p [Kr]4d 5s 5p [Xe]4f 5d 6s 6p 1402 1012 947 834 703 II 2856 1903 1798 1595 1610 III 4577 2910 2736 2443 2466 3.0 2.1 2.0 1.9 1.9 70 110 b d 1089 212 Melting point/K 63* 317 77.2* –3 0.879 EIII single bond (E = element); b E 3–; c E 3+; At 63 K; hGrey α-form; * Molecular N2. d g d 141 222 171 Boiling point/K 121 b Ionic radius/pm Density/[g cm (298 K)] 51 74.92 2 Bi [He]2s 2p I –1 3 Sb 33 30.97 2 Ionisation enthalpy As 15 14.01 Electronic configuration a P b 76 e f 554 888 1.823 5.778 White phosphorus; e h c 148 103 c 904 544 1860 1837 6.697 9.808 Grey α-form at 38.6 atm; f Sublimation temperature; Trends of some of the atomic, physical and chemical properties of the group are discussed below. 2 3 7.1.2 Electronic Configuration The valence shell electronic configuration of these elements is ns np . The s orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable. 7.1.3 Atomic and Ionic Radii Covalent and ionic (in a particular state) radii increase in size down the group. There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled d and/or f orbitals in heavier members. 7.1.4 Ionisation Enthalpy Ionisation enthalpy decreases down the group due to gradual increase in atomic size. Because of the extra stable half-filled p orbitals electronic configuration and smaller size, the ionisation enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods. The order of successive ionisation enthalpies, as expected is ∆iH1 < ∆iH2 < ∆iH3 (Table 7.1). 7.1.5 Electronegativity The electronegativity value, in general, decreases down the group with increasing atomic size. However, amongst the heavier elements, the difference is not that much pronounced. 7.1.6 Physical Properties All the elements of this group are polyatomic. Dinitrogen is a diatomic gas while all others are solids. Metallic character increases down the group. Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a metal. This is due to decrease in ionisation enthalpy and increase in atomic size. The boiling points, in general, increase from top to bottom in the group but the melting point increases upto arsenic and then decreases upto bismuth. Except nitrogen, all the elements show allotropy. 7.1.7 Chemical Properties Oxidation states and trends in chemical reactivity The common oxidation states of these elements are –3, +3 and +5. The tendency to exhibit –3 oxidation state decreases down the group due to increase in size and metallic character. In fact last member of the group, bismuth hardly forms any compound in –3 oxidation state. The stability of +5 oxidation state decreases down the group. The only well characterised Bi (V) compound is BiF5. The stability of +5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group. Besides +5 oxidation state, nitrogen exhibits + 1, + 2, + 4 oxidation states also when it reacts with oxygen. However, it does not form compounds in +5 oxidation state with halogens as nitrogen does not have d-orbitals to accommodate electrons from other elements to form bonds. Phosphorus also shows +1 and +4 oxidation states in some oxoacids. In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution. For example, 3HNO2 → HNO3 + H2O + 2NO Similarly, in case of phosphorus nearly all intermediate oxidation states disproportionate into +5 and –3 both in alkali and acid. However +3 oxidation state in case of arsenic, antimony and bismuth becomes increasingly stable with respect to disproportionation. Nitrogen is restricted to a maximum covalency of 4 since only four (one s and three p) orbitals are available for bonding. The heavier elements have vacant d orbitals in the outermost shell which can be used for – bonding (covalency) and hence, expand their covalence as in PF6. Anomalous properties of nitrogen Nitrogen differs from the rest of the members of this group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d orbitals. Nitrogen has unique ability to form pπ -p π multiple bonds with itself and with other elements having small size and high electronegativity (e.g., C, O). Heavier elements of this group do not form pπ -pπ bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping. Thus, nitrogen exists as a diatomic molecule with a triple bond (one s and two p) between the two atoms. Consequently, its bond enthalpy –1 (941.4 kJ mol ) is very high. On the contrary, phosphorus, arsenic and antimony form single bonds as P–P, As–As and Sb–Sb while bismuth forms metallic bonds in elemental state. However, the single N–N bond is weaker than the single P–P bond because of high interelectronic repulsion of the non-bonding electrons, owing to the small bond length. As a result the catenation tendency is weaker in nitrogen. 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 dπ –pπ bond 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 ligands. (i) Reactivity towards hydrogen: All the elements of Group 15 form hydrides of the type EH3 where E = N, P, As, Sb or Bi. Some of the properties of these hydrides are shown in Table 7.2. The hydrides show regular gradation in their properties. The stability of hydrides decreases from NH3 to BiH3 which can be observed from their bond dissociation enthalpy. Consequently, the reducing character of the hydrides increases. Ammonia is only a mild reducing agent while BiH3 is the strongest reducing agent amongst all the hydrides. Basicity also decreases in the order NH3 > PH3 > AsH3 > SbH3 > BiH3. Due to high electronegativity and small size of nitrogen, NH3 exhibits hydrogen bonding in solid as well as liquid state. Because of this, it has higher melting and boiling points than that of PH3. Table 7.2: Properties of Hydrides of Group 15 Elements Property NH 3 PH 3 AsH3 SbH 3 Melting point/K 195.2 139.5 156.7 185 Boiling point/K 238.5 185.5 210.6 254.6 290 (E–H) Distance/pm 101.7 141.9 151.9 170.7 – HEH angle (°) 107.8 93.6 91.8 91.3 – –46.1 13.4 66.4 145.1 V –1 ∆f H /kJ mol V –1 ∆dissH (E–H)/kJ mol 389 322 297 255 BiH3 – 278 – (ii) Reactivity towards oxygen: All these elements form two types of oxides: E2O3 and E2O5. 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 E2O3 of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth predominantly basic. (iii) Reactivity towards halogens: These elements react to form two series of halides: EX3 and EX5. Nitrogen does not form pentahalide due to non-availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides. This is due to the fact that in pentahalides +5 oxidation state exists while in the case of trihalides +3 oxidation state exists. Since elements in +5 oxidation