Difference between Allotropes and Isomers
Allotropes refer to the different physical forms of an element which are in the same physical state (solid, liquid or gas). These physical forms differ from each other in the bonding arrangement of atoms within the molecule; also differ in physical properties and sometimes in chemical properties. Allotropes consist of only one type of atoms as they are the different forms the same element. Allotropes generally occur only with certain elements of certain groups of the periodic table.
- Diamond, graphite and fullerenes are allotropes of carbon.
- (O2) dioxide, (O3) ozone are allotropes of oxygen.
- White phosphorus, red phosphorus, violet phosphorus and black phosphorus are allotropes of phosphorus.
- Plastic (amorphous) sulphur, rhombic sulphur and monoclinic sulphur are allotropes of sulphur.
- Amorphous boron and crystalline boron are allotropes of boron.
- Amorphous silicon and crystalline silicone are allotropes of silicone.
Isomers refer to organic compounds which have same molecular formula but different structural formula or different structures. They contain the same number of atoms of each element but have different arrangements of their atoms. In other words, the organic compounds having the same molecular formula but a different arrangement of atoms are called isomers. This phenomenon is known as isomerism and it is of two types: structural isomerism and stereoisomerism.
Structural Isomerism: In structural isomerism or isomers, the compounds have the same molecular formula but differ in structural formula. It is classified into 6 types:
- Chain Isomerism: The isomers have same molecular formula but differ in bonding arrangements of carbon atoms, e.g. Butane and 2-Methylpropane
- Positional Isomerism: The isomers have same carbon skeleton or molecular formula but differ in the position of the functional group, e.g. isomers of butane: But-1-ene and But-2-ene.
- Functional Isomerism: The isomers have the same molecular formula but differ in the nature of functional group, e.g. ethanol and dimethyl ether.
- Metamerism Isomerism: The isomers have same the molecular formula but differ in the nature of alkyl groups attached to the same functional group, e.g. diethyl ether and methyl propyl ether.
- Tautomerism Isomerism: The isomers have the same molecular formula and exist in dynamic equilibrium with each other, e.g. nitro-ethane and isonitroethane.
Stereoisomerism: In this type of isomerism, the compounds have same structural and molecular formula but they differ in the spatial arrangement of atoms or groups in space. It is of two types:
- Geometrical or Cis-Trans Isomerism: The isomers have the same molecular formula but differ in the spatial arrangement of atoms or groups around the carbon atoms (C=C). It is further of two types: cis isomers (same groups are on the same side), and trans isomers (same groups are on opposite sides. E.g. cis-2-butene and trans-2-butene are cis and trans isomers respectively.
- Optical Isomerism: The isomers have the same molecular formula and are nonsuperimposable mirror images of each other. Such isomers are known as enantiomers, e.g. D-alanine and L-alaline.
Based on the above information, some of the key differences between allotropes and isomers are as follows:
|The different physical forms of the same element in the same physical state with different bonding arrangements between atoms and have physical properties.
||The compounds have the same molecular formula but different structural formula.
|They have different number of atoms, e.g. o2, o3
||They have the same number of atoms, e.g. but-1-ene (C4H8) and but-2-ene (C4H8)
|They are composed of same element, e.g. o2, o3
||Composed of different elements, e.g. but-1-ene (C4H8) and but-2-ene (C4H8).
|The same element with different chemical formulas.
||Different elements with same chemical formula.
|Always have different structures.
||May have similar or different structures.
|Observed in metals, non-metals, and metalloids
||Isomerism is observed in organic (hydrocarbon) and inorganic molecules.
|Types: metal allotropes, non-metal allotropes and metalloid allotropes
||Types: structural isomers and stereoisomers