The p-block elements are the elements in which the last electron enters the outermost p-orbital. They include elements of groups 13 to 18 of the periodic table, ranging from the metallic elements on the left, through the metalloids, to the non-metals and noble gases on the right. The general electronic configuration of p-block elements is ns^2 np^1-6. These elements exhibit the greatest diversity of properties among all the blocks, and this chapter focuses on the chemistry of groups 15, 16, 17, and 18.
The chemistry of p-block elements is strongly influenced by two anomalies: the inert pair effect and the small size of the first element of each group. The inert pair effect explains why heavier members of groups 13 and 14 show lower oxidation states, such as the stability of +1 for thallium and +2 for lead. The first member of each group, being small with high electronegativity, differs markedly from the rest of the group, showing a greater tendency to form pi bonds, hydrogen bonding, and multiple bonds.
This chapter covers the nitrogen family (group 15) with its important compounds ammonia, nitric acid, and the oxides of nitrogen; the oxygen family (group 16) with dioxygen, ozone, and sulphur and its oxoacids; the halogen family (group 17) with their strong oxidising character and the interhalogen compounds; and the noble gases (group 18) whose inertness was historically explained by their complete octet configuration. The chemistry of these elements is fundamental to atmospheric chemistry, industrial processes, and biological systems.
The group 15 elements are nitrogen, phosphorus, arsenic, antimony, and bismuth. The general electronic configuration is ns^2 np^3. There is a regular increase in atomic radius down the group, with a decrease in ionisation enthalpy. The electronegativity of nitrogen is higher than expected because of its small size.
Dinitrogen (N2) is a colourless, odourless, and relatively inert gas due to the very strong N-N triple bond (bond enthalpy 946 kJ/mol). It is prepared industrially by the fractional distillation of liquid air. Dinitrogen combines with hydrogen to give ammonia in the Haber process: $$N_2 + 3H_2 \rightleftharpoons 2NH_3, \qquad \Delta H = -46.1\ \text{kJ/mol}$$ The Haber process uses a promoted iron catalyst, a temperature of about 700 K, and a pressure of about 200 atmospheres.
Ammonia is a colourless gas with a pungent smell, a strong Lewis base, and a trigonal pyramidal shape. It forms ammonium salts with acids and acts as a good ligand in coordination compounds. Ammonia is used in the manufacture of fertilisers, nitric acid, and explosives.
Nitric acid is manufactured by the Ostwald process, in which ammonia is oxidised to nitric oxide and then to nitrogen dioxide, which is absorbed in water: $$4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O$$ $$2NO + O_2 \rightarrow 2NO_2$$ $$3NO_2 + H_2O \rightarrow 2HNO_3 + NO$$ Nitric acid is a strong oxidising agent and its oxidising power is responsible for the fact that it produces NO2 with metals rather than liberating hydrogen. In the Ostwald process, nitric oxide formed as a by-product is recycled.
Nitrogen forms a series of oxides: N2O, NO, N2O3, NO2, N2O4, and N2O5. Nitric oxide (NO) is a colourless gas that is oxidised to the brown NO2. These oxides show a range of oxidation states from +1 to +5 and are important in atmospheric chemistry.
Group 16 consists of oxygen, sulphur, selenium, tellurium, and polonium, with the general configuration ns^2 np^4. Oxygen is the most abundant element in the earth's crust. The group shows a regular increase in atomic size and metallic character down the group.
Dioxygen (O2) is a colourless, odourless gas essential for respiration and combustion. It is prepared industrially by the fractional distillation of liquefied air and in the laboratory by heating potassium chlorate with MnO2: $$2KClO_3 \xrightarrow{MnO_2} 2KCl + 3O_2$$ Dioxygen is paramagnetic because of two unpaired electrons in its molecular orbitals. It is a powerful oxidising agent but generally reacts slowly at room temperature.
Ozone (O3) is an allotropic form of oxygen. It is formed in the upper atmosphere by the action of UV radiation on dioxygen: $$O_2 \xrightarrow{UV} O + O, \qquad O_2 + O \rightleftharpoons O_3$$ Ozone is a pale blue gas with a characteristic smell, and it is an extremely powerful oxidising agent. It is used in water purification and bleaching, and the ozone layer protects the earth from harmful UV radiation. Ozone is thermodynamically unstable and decomposes to dioxygen.
Sulphur is a yellow crystalline solid with a molecular formula S8 in the rhombic form. It is oxidised to sulphur dioxide when burnt in air: $$S_8 + 8O_2 \rightarrow 8SO_2$$ Sulphur dioxide is a colourless, pungent gas used in the manufacture of sulphuric acid, bleaching, and as a preservative. Sulphur forms oxoacids such as sulphurous acid and sulphuric acid. Sulphuric acid, manufactured by the Contact process, is called the king of chemicals because of its enormous industrial importance.
The halogens are fluorine, chlorine, bromine, iodine, and astatine, with the general configuration ns^2 np^5. They are non-metallic, highly reactive, and exist as diatomic molecules. Fluorine is the most electronegative element, and the halogens are the strongest oxidising agents among the elements of their periods.
Halogens combine with hydrogen to form hydrogen halides, which dissolve in water to give strong acids (HF is a weak acid because of hydrogen bonding). Chlorine reacts with water to give hydrochloric and hypochlorous acids: $$Cl_2 + H_2O \rightarrow HCl + HOCl$$
Chlorine is prepared in the laboratory by the oxidation of hydrochloric acid with manganese dioxide: $$MnO_2 + 4HCl \rightarrow MnCl_2 + Cl_2 + 2H_2O$$
Chlorine is used for bleaching, water purification, and the manufacture of PVC and pesticides.
Interhalogen compounds are formed when two different halogens combine, with the formula XY, XY3, XY5, and XY7, where X is the less electronegative (larger) halogen. Examples are ClF, ClF3, BrF5, and IF7. Their properties are intermediate between the constituent halogens. The higher interhalogens have the larger halogen as the central atom.
The halogens form several oxoacids in which the halogen exhibits different oxidation states. For example, chlorine forms HOCl (+1), HClO2 (+3), HClO3 (+5), and HClO4 (+7). The acidic strength increases with the number of oxygen atoms attached.
The noble gases are helium, neon, argon, krypton, xenon, and radon, with the general configuration ns^2 np^6. They were historically called inert gases because of their very low reactivity, which was attributed to their complete octet. However, the discovery of xenon compounds with fluorine in 1962 by Bartlett disproved the complete inertness.
Noble gases are monoatomic, colourless, odourless, and have very low boiling points. Their ionisation enthalpies are very high and decrease down the group. The reactivity increases down the group because ionisation enthalpy decreases.
Only the heavier noble gases form compounds. Xenon forms fluorides XeF2, XeF4, and XeF6, and oxides XeO3 and XeO4. Xenon difluoride is a linear molecule, XeF4 is square planar, and XeF6 has a distorted octahedral structure. Krypton forms only KrF2.
Helium is used in balloons, cryogenics, and deep-sea diving mixtures. Argon is used in inert atmosphere welding and light bulbs. Neon is used in discharge tubes and advertising signs. Radon is used in radiotherapy.
| Property | Group 15 | Group 16 | Group 17 | Group 18 |
|---|---|---|---|---|
| Outer configuration | ns2 np3 | ns2 np4 | ns2 np5 | ns2 np6 |
| Atomic radius | Increases | Increases | Increases | Increases |
| Ionisation enthalpy | Decreases | Decreases | Decreases | Decreases |
| Electron gain enthalpy | Less negative | Less negative | Less negative | Near zero/positive |
| Typical oxidation states | -3 to +5 | -2 to +6 | -1 to +7 | 0, +2 to +8 |
| Process | Reactants | Catalyst | Product | Use |
|---|---|---|---|---|
| Haber process | N2 + H2 | Iron | Ammonia | Fertilisers |
| Ostwald process | NH3 + O2 | Platinum | Nitric acid | Fertiliser, explosives |
| Contact process | SO2 + O2 | V2O5 | Sulphuric acid | King of chemicals |
| Manufacture of Cl2 | MnO2 + HCl | None | Chlorine | Bleaching, purification |
The p-block elements demonstrate the full spectrum of chemical behaviour, from the strongly reducing and oxidising tendencies of the halogens to the almost complete inertness of the noble gases. The group trends, modulated by size, electronegativity, and the inert pair effect, explain the oxidation states and reactivities observed across groups 15 to 18. The industrial processes described in this chapter, the Haber, Ostwald, and Contact processes, supply ammonia, nitric acid, and sulphuric acid that are essential for fertilisers, explosives, and countless chemical manufactures. Interhalogen compounds and noble gas compounds expand the range of covalent chemistry beyond the simple diatomic molecules. An understanding of the p-block provides the foundation for atmospheric chemistry, industrial chemistry, and the rational design of compounds ranging from refrigerants to semiconductors.