5 differences between Allotropes and Isomers

Allotropes can be considered as different arrangements of an element that may not have the same number of atoms. Diamond and graphite are made up of carbon but have very different properties due to their structural differences. Isomers have the same chemical formula but have a different structural formula. L-cysteine, and D-cysteine have the same chemical formula, but their structures is different. In this article, we discuss the 5 differences between allotropes and isomers.

Differences between allotropes and isomers

                    Allotropes                    Isomers
1.Allotropes are made of a single element(like carbon, sulfur, phosphorus, etc) but they have an arrangement of atoms.1. Allotropes are made of a single element(like carbon, sulfur, phosphorus, etc) but they have an arrangement of atoms.
2. It is the same element with a different chemical formula.2. Isomers have the same chemical formula
3. Allotropes have different numbers of atoms. 3. Isomers have the same number of atoms.
4. They differ in the atomic arrangement.4. They differ in the molecular structure
5. Allotropes have different chemical and physical properties.5. Isomers may share some similar chemical properties but usually differ in chemical and physical properties.
Example: Diamond and graphite.Example: Butane and isobutene  

Allotropes are composed of a single element like carbon or sulfur, but the arrangement of the atoms of the element is different. For example, fullerenes are allotropes of carbon, but they have 60 atoms in a bucky ball-like shape. Graphene on the other hand is a 2-D allotrope of carbon.

Isomers are composed of the same chemical formula but have different molecular arrangements. There are different types of isomers structural isomers, chain isomers, position isomers, functional isomers, stereoisomers, optical isomers, etc.

Allotropes have different chemical formulas. C60, C70, C80 are all allotropes of carbon with different formulas and a different number of atoms. Isomers, on the other hand, have the same chemical formula as L-glucose, and D-glucose has the same chemical formula as C6H12O6.

Below is shown the electron microscopy images of newer allotropes of carbon like graphene and carbon nanotubes. Carbon nanotubes have a tube-like structure composed of carbon atoms. Graphene on the other hand is a 2D material composed of a few layers of graphite.

carbon nanotubes- allotrope of carbon
Structure of carbon nanotubes (Fang H, Cui Y, Wang Z, Wang S (2018) Toxicological assessment of multi-walled carbon nanotubes combined with nonylphenol in male mice. PLoS ONE 13(7): e0200238. https://doi.org/10.1371/journal.pone.0200238)
graphene allotrope of carbon
Structure of graphene (Zhang X, Cao H, Wang H, Zhang R, Jia H, Huang J, et al. (2021) Effects of graphene on morphology, microstructure and transcriptomic profiling of Pinus tabuliformis Carr. roots. PLoS ONE 16(7): e0253812. https://doi.org/10.1371/journal.pone.0253812)

Below shown is the difference in the chemical structure of the two isomers. Though the formula is the same the wedge bonds are different.

difference in structures of isomers
Chemical structure and X-ray crystal structure of (-)-OIDV and (+)-OIDV. (Uno I, Kozaka T, Miwa D, Kitamura Y, Azim MA-u, Ogawa K, et al. (2016) In Vivo Differences between Two Optical Isomers of Radioiodinated o-iodo-trans-decalinvesamicol for Use as a Radioligand for the Vesicular Acetylcholine Transporter. PLoS ONE 11(1): e0146719. https://doi.org/10.1371/journal.pone.0146719)

See also

Benzyl alcohol
12 Difference between methanol and ethanol
10 Difference between corrosion and rusting
Benzene