Chemistry of group 15 elements

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Chapter: Essentials of Inorganic Chemistry : Group 15 Elements

Nitrogen makes up 78% (by volume) of air, whereas phosphorus can be found in several minerals and ores.


Chemistry of group 15 elements

 

Occurrence and extraction

Nitrogen makes up 78% (by volume) of air, whereas phosphorus can be found in several minerals and ores. Phosphorus is an essential constituent of plants and animals, being present in deoxyribonucleic acid (DNA), bones, teeth and other components of high biological importance. Phosphorus does not occur in its elemental state in nature, as it readily oxidises and therefore is deposited as phosphate rock. The remaining elements of group 15 are mostly obtained from minerals, but can also be found in their elemental form in the earth’s crust. Arsenic is mostly presented in nature as mispickel (FeAsS), realgar (As4S4) and orpiment (As2S3). Bismuth occurs as bismuthinite (Bi2S3) as well as in its elemental form.

Dinitrogen (N2) is extracted by fractional distillation of liquid air. By-products such as dioxygen (O2) are removed by addition of H2 and the use of a Pt catalyst. Elemental phosphorus is extracted from phosphate rock by reacting with sand and coke in an electrically heated industrial oven. Phosphorus vapour is isolated and condensed under water – white phosphorus is extracted.

2Ca3(PO4)2 + 6SiO2 + 10C P4 + 6CaSiO3 + 10CO

Elemental arsenic is extracted mainly from FeAsS by heating and subsequent condensation of arsenic in the absence of air.

FeAsS FeS + As (in absence of air)

Antimony is obtained from stibnite (Sb2S3) after reduction with iron. Bismuth is extracted from its sulfide or oxide ores via a reduction with carbon.

Sb2S3 + 3Fe 2Sb + 3FeS

 

Physical properties

The physical properties of group 15 elements vary widely, from nitrogen being a gas to the remaining elements being solids with increasing metallic character. Nitrogen exists as a diatomic molecule N2 and is a colourless and odourless gas (condensation at 77 K). Nitrogen forms relatively strong and short bonds, resulting in the formation of a triple bond in the N2 molecule. Furthermore, nitrogen has an anomalously small covalent radius and therefore can form multiple bonds with N, C and O atoms. Group 15 elements follow the general trend showing an increasing covalent radius when descending within the group.

Phosphorus has several allotropes, with white, red and black phosphorus being the main ones.

Allotropes are defined as the two or more physical forms of one element. Allotropes of carbon are graphite, carbon and diamond. These allotropes are all based on carbon atoms but exhibit different physical prop-erties, especially with regard to hardness.

White phosphorus is a solid consisting of tetrahedral P4 molecules with single bonds. White phospho-rus is the standard state of the element, but it is metastable, potentially due to the strained 60 bond angles (Figure 6.2).


Heating of white phosphorus in an inert gas atmosphere results in red phosphorus, which is an amorphous solid (several crystalline forms are known) with an extended covalent structure (Figure 6.3).

Black phosphorus is the most stable allotrope of phosphorus and can be obtained by heating white phospho-rus under high pressure. In contrast to white phosphorus, black phosphorus does not ignite spontaneously in air. The reactivity of red phosphorus lies between those of the white and black allotropes. White phosphorus is insoluble in water and is therefore stored under water to prevent oxidation.

Arsenic and antimony vapour consists of As4 or Sb4 molecules, respectively. In the solid state, arsenic, antimony and bismuth are grey solids with a lattice structure similar to that of black phosphorus.

 

Oxidation states and ionisation energy

The general electron configuration for group 15 elements is ns2np3 and all elements form the oxidation states of +3 and +5. Nitrogen is more versatile and shows a range of oxidation states ranging from −3 to +5.

The ionisation energies relating to the removal of the first five electrons (two s and three p electrons) are relatively low. There is a significant increase in the ionisation energy necessary for the removal of a sixth electron, as this will be removed from an inner complete quantum shell (Table 6.1).


 

Chemical properties

Nitrogen is relatively unreactive because the bond enthalpy of the nitrogen–nitrogen triple bond is very high (944 kJ/mol). N2(g) is usually used as an inert atmosphere for reactions that cannot be carried out in oxy-gen. Only lithium reacts directly with nitrogen with the formation of Li3N. Nitrogen fixation is an important mechanism developed by some microorganism in order to directly incorporate nitrogen gas into proteins. This process is an important step in the early food chain.

There are five oxides of nitrogen known – N2O, NO, N2O3, NO2 and N2O5 (oxidation numbers ranging from +1 to +5, respectively). Nitric(III) acid (nitrous acid) HONO and nitric(V) acid (nitric acid) HNO3 are the most important oxoacids of nitrogen. HNO3 is a highly reactive oxidising and nitrating agent.

In general, phosphorus is more reactive than nitrogen. White phosphorus ignites spontaneously in air and forms phosphorus(V) oxide. Phosphoric acid (H3PO4) is the most important oxoacid of phosphorus and its main use is in the manufacture of fertilisers (Figure 6.4).


Hydrogen-containing compounds of nitrogen and phosphorus, namely NH3 and PH3, both act as a Lewis base because of their lone pair. Phosphine (PH3) is less water soluble than NH3 as it does not form hydrogen bonds. NH3 (ammonia) is produced in the so-called Haber Bosch process. This industrial process uses finely divided iron as catalyst and a reaction temperature of around 450 C at a pressure of 50 atm. Ammonia is used to produce fertilisers, nitric acid, nylon and many more products important to our modern life style.

N2(g) + 3H2(g) 2NH3(g)

For clinical applications, nitrogen and phosphorus compounds are mostly used as heteroatoms in organic compounds or counter-ions in inorganic salts with no specific therapeutic effect. Arsenic differs because it exhibits its own typical therapeutic and toxic properties, which has resulted in its long-standing use in clinical applications and in the invention of chemotherapy. Therefore, the following clinical discussion will concentrate on arsenic-based drugs.

 

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