Visual Elements Periodic Table
The elements of Group 15 are:
The most important members of this Group are nitrogen and phosphorus. The other elements will mostly not be considered here.
The appearance of the Group 15 elements varies widely. Nitrogen is a colourless, odourless gas; phosphorus exists in white, red and black solid forms; arsenic is found in yellow and grey solid forms; antimony is found in a metallic or amorphous grey form; and finally bismuth is a white, crystalline, brittle metal. These appearances reflect the changing nature of the elements as the Group is descended, from non-metal to metal.
The elements of Group 15 show a marked trend towards metallic character on descending the Group. This trend is reflected both in their structures and in their chemical properties, as for example in the oxides which become increasingly basic.
Nitrogen is found in the atmosphere, and makes up 78% of the air by volume. Phosphorus is not found free in nature, but occurs in several minerals and ores such as phosphate rock. The other elements are all found in the elemental form in the earths crust, but more frequently as minerals.
The physical properties of this Group vary widely as nitrogen is a gas, and the other elements are solids of increasingly metallic character.
Nitrogen exists as the diatomic molecule N2. It is a colourless, odourless gas, which condenses to a colourless liquid at 77K. The strength of the bond and the short bond length provide evidence for the bond between the N atoms being a triple bond.
Phosphorus has at least two allotropes, red and white phosphorus. White phosphorus is a solid composed of covalent tetrahedral P4 molecules, and red phosphorus is an amorphous solid which has an extended covalent structure.
The covalent radii of the atoms increase on descending the Group. However, the N atom is anomalously small and so it can multiple-bond to other N, C and O atoms.
Both nitrogen and phosphorus exist in oxidation states +3 and +5 in their compounds. Nitrogen is very unreactive, mainly because its bond enthalpy is very high (944 kJ mol-1). The only element to react with nitrogen at room temperature is lithium, to form the nitride Li3N. Magnesium also reacts directly, but only when ignited. Some micro-organisms, however, have developed a mechanism for reacting directly with nitrogen gas and building it into protein - this is called nitrogen fixation, and is an important early step in the food chain.
Phosphorus is more reactive than nitrogen. It reacts with metals to form phosphides, with sulfur to form sulfides, with halogens to form halides, and ignites in air to form oxides. It also reacts with both alkalis and concentrated nitric acid.
There are five oxides of nitrogen, with N ranging in oxidation number from +1 to +5; N2O, NO, N2O3, NO2, N2O5. There are also two important oxoacids of nitrogen - nitric(III) acid (nitrous acid) HONO, and nitric(V) acid (nitric acid) HNO3. Nitric acid is highly reactive, and behaves as an oxidising agent and a nitrating agent as well as an acid.
There are many oxoacids of phosphorus, the most important being phosphoric acid (HO)3PO. This is produced on a large scale commercially as it is used in the manufacture of fertilisers.
The nitrogen halides N(hal)3 all have covalent, pyramidal structures. More important are the two series of phosphorus halides, P(hal)3 and P(hal)5.
Compounds with hydrogen
The most important of these is ammonia, NH3. During the industrial synthesis of ammonia the inert nitrogen obtained from the air is made into a reactive compound, ammonia, thus making atmospheric nitrogen available for many reactions. Ammonia is a reducing agent, but can be reduced by stronger reducing agents such as sodium metal. Ammonia is also a Lewis base, as it has a lone pair of electrons.
Phosphine, PH3, is also a Lewis base but is less soluble in water than ammonia because it does not form hydrogen bonds.
Oxidation States and lonisation Energies
The elements of this Group have the general electron configuration ns2np3. They all exhibit an expected oxidation state of +3 arising from the three unpaired p electrons, but also an oxidation state of +5. In all the elements except N this is made possible by promotion of an s electron to an available d orbital. Nitrogen is remarkable for its wide range of oxidation states ranging from -3 to +5.
In this Group the first five ionisation energies are relatively low, reflecting the removal of the two s and three p electrons. There is a larger increase between the 5th and 6th ionisation energies as an electron is removed from the inner, filled quantum shell.
For industrial use nitrogen is obtained by fractional distillation of the air. It is used for the manufacture of ammonia by the Haber-Bosch Process:
N2(g) + 3H2(g) ® 2NH3(g)
A catalyst of finely-divided iron is required for this equilibrium reaction. The mixture is heated to 450°C at 250 atm of pressure. The yield of ammonia is increased by working at high pressure, but this adds to the cost of the plant, and a compromise between cost and yield is needed. The reaction is exothermic so the yield of ammonia is increased by low temperatures, but this slows up the reaction so again a compromise is needed. At the temperatures and pressures used in practice about 15% conversion is attained. The ammonia is condensed and removed from the plant and the unreacted gases are recirculated.
Ammonia has numerous uses - approximately 100 megatonnes are produced worldwide each year. It is used as a fertiliser both directly and after conversion to other fertilisers such as ammonium nitrate. It is also a raw material for nitric acid manufacture and for the production of nylon.
Gaseous nitrogen is used to provide an inert atmosphere for reactions which cannot be carried out in oxygen. It is also used as a carrier gas in liquid-gas chromatography. Phosphorus is used in match-heads and on safety match boxes.
For further information look up the individual elements.
Ionisation Energies/kJ mol-1