Allotropes are different structural modifications of an element whereas isomers are chemical compounds that share the same molecular formula but have different structural formulae.

Certain elements can exist in two or more different forms. These forms are called allotropes in which the element's atoms are bonded together in a different manner. For example, dioxygen (O2), ozone (O3), tetraoxygen (O4) and octaoxygen (O8) are allotropes of oxygen. Another example is carbon whose allotropes include graphite and diamond. In short, allotropes contain the same element (the same atoms) that bond together in different ways to produce different molecular structures.

In contrast, isomers are compounds (see Elements vs. Compounds) that share the same molecular formula but have different structural formulas. Isomers do not share their chemical properties unless they belong to the same functional group. For example, propanol has the formula C3H8O (or C3H7OH) and occurs as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II). The difference between the two isomers lies in the position of the oxygen atom: it is attached to an end carbon in propan-1-ol, and to the center carbon in propan-2-ol. There is a third isomer of C3H8O whose properties are so different that it's not an alcohol (like propanol) but an ether. Called methoxyethane (methyl-ethyl-ether; III), this isomer has an oxygen connected to two carbons rather than to one carbon and one hydrogen.

Comparison chart

Allotropes versus Isomers comparison chart
Edit this comparison chartAllotropesIsomers
Definition Allotropes are different structural modifications of an element. For example O and O2 Isomers are chemical compounds that share the same molecular formula but have different structural formulae.
Examples Diamond, Graphite etc. 2-methylpropan-1-ol and 2-methylpropan-2-ol.

History of allotropes and isomers

Both allotropy and isomerism were concepts proposed by the Swedish scientist Jöns Jakob Berzelius. He proposed the concept of allotropy in 1841. After the acceptance of Avogadro's hypothesis in 1860 it was understood that elements could exist as polyatomic molecules, and the two allotropes of oxygen were recognized as O2 and O3. In the early 20th century it was recognized that other cases such as carbon were due to differences in crystal structure.

Isomerism was first noticed in 1827, when Friedrich Woehler prepared cyanic acid and noted that although its elemental composition was identical to fulminic acid (prepared by Justus von Liebig the previous year), its properties were quite different. This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be different only when they had different elemental compositions. After additional discoveries of the same sort were made, such as Woehler's 1828 discovery that urea had the same atomic composition as the chemically distinct ammonium cyanate, Jöns Jakob Berzelius introduced the term isomerism to describe the phenomenon.

Types of Isomers

Different classes of isomers include stereoisomers, enantiomers and geometrical isomers.

Types of Isomers
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Types of Isomers

References

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