Oxygen allotropy

The existence of an elementary substance in two forms is called allotropy (Greek alldtropia, variety, from alios, other, and tropos, direction). Ordinary oxygen and ozone are the allotropes of oxygen. Allotropy is shown by many elements it is due either to the existence of two or more kinds of molecules (containing different numbers of atoms) or to the existence of two or more different crystalline forms that is, of different arrangements of the atoms or molecules in a crystalline array. 

Allotropy is a very common phenomenon shown by metals, metalloids, and nonmetals. In the gaseous or liquid state, allotropes most often differ from one another in molecular formula. Consider, for example, the two allotropes of gaseous oxygen, 02, and ozone, 03. 

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

Many elements including sulphur, carbon and oxygen can exist in two or more forms with different physical, and often chemical, properties such elements are said to exhibit allotropy and the different forms are known as allotropes or allotropic forms. 

An allotropic form of oxygen (03) (see Allotropy) of considerably greater degradative effect on rubber than oxygen itself. The ozone content of the atmosphere normally varies from 0.5 to 5.0 parts per 100 million (pphm) of air, but in certain areas (notably Los Angeles, USA) it may reach as much as 40 pphm of air. 

Allotropy.—All of the dements exhibit allotropy. Oxygen yields two gaseous allotropcs, namely, ordinary oxygen and ozone. These can co-exist at room temperatures for indefinite periods wilhout manifesting any tendency to reach a stage of chemical equilibrium. 

The allotropy of carbon, oxygen, phosphorus, and sulfur results from the versatility of their covalent bonding. Carbon occurs as diamond and as graphite (Fig. 21.1). Diamond is extremely hard, in consequence of its stable network covalent structure, which is entirely o--bonded. Graphite is relatively soft, in part because of the ease with which its TT-bonded atomic layers can slip past one another. At ordinary temperatures and pressures, both forms are quite unreactive, and graphite is the form with lower free energy (more stable) by about 0.7 kcal/mole. 

Another phenomenon occasionally encountered is the existence of an element in more than one molecular form, a situation referred to as allotropy. Thus, oxygen exists in two allotropic forms, the predominantly abundant diatomic oxygen, O2, and the much less abundant allotrope oTume, O3. The molar mass of ordinary dioxygen is 31.998 g 02/mol O2, and that of ozone is 47.997 g Os/mol O3. 

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