Difference Between Isotopes and Isomers

Isotopes vs Isomers
 

There are variations between different atoms. Also, there are variations within the same elements. Isotopes are examples for differences within a single element.

A molecule or ion having the same molecular formula can exist in different ways depending on the bonding orders, charge distribution differences, the way they arrange themselves in the space etc; these are known as isomers.

Isotopes

Atoms of the same element can be different. These different atoms of the same element are called isotopes. They are different from each other by having different number of neutrons. Since the neutron number is different, their mass number also differs. However, the isotopes of the same element have the same number of protons and neutrons. Different isotopes present in varying quantities, and this is given as a percentage value called relative abundance. For example, hydrogen has three isotopes as protium, deuterium and tritium. Their number of neutrons and relative abundances are as follows.

¹H – no neutrons, relative abundance is 99.985%

²H- one neutron, relative abundance is 0.015%

³H- two neutrons, relative abundance is 0%

The number of neutrons a nucleus can hold differs from element to element. Among these isotopes, only some are stable. For instance, oxygen has three stable isotopes, and tin has ten stable isotopes. Most of the time, simple elements have the same neutron number as the proton number but, in heavy elements, more neutrons are there than the protons. The number of neutrons is important to balance the stability of the nuclei. When the nuclei are too heavy, they become unstable and, therefore, those isotopes are becoming radioactive. For example, ²³⁸ U emits radiation and decays to much smaller nuclei. Isotopes may have different properties because of their different masses. For example, they may have different spins, thus their NMR spectra differs. However, their electron number is similar giving rise to a similar chemical behavior.

A mass spectrometer can be used to get information about isotopes. It gives the number of isotopes which an element has, their relative abundances and masses.

Isomers

Isomers are different compounds with the same molecular formula. There are various types of isomers. Isomers can be mainly divided into two groups as constitutional isomers and stereo isomers. Constitutional isomers are isomers where the connectivity of atoms differs in molecules. Butane is the simplest alkane to show constitutional isomerism. Butane has two constitutional isomers, butane itself and isobutene.

In stereo-isomers atoms are connected in the same sequence, unlike constitutional isomers. Stereoisomers differ only in the arrangement of their atoms in space. Stereoisomers can be of two types, enantiomers and diastereomers. Diastereomers are stereoisomers whose molecules are not the mirror images of each other. The cis trans isomers of 1,2-dichloroethene are diastereomers. Enantiomers are stereoisomers whose molecules are non-superposable mirror images of each other. Enantiomers occur only with chiral molecules. A chiral molecule is defined as one that is not identical with its mirror image. Therefore, the chiral molecule and its mirror image are enantiomers of each other. For example, 2-butanol molecule is chiral, and it and its mirror images are enantiomers.