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Isotopes are variants of a chemical element which have the same atomic mass, but different atomic number (also called mass number). An isotope can also be defined as being variants of a chemical element which have the same number of protons, but different number of neutrons. These two definitions are connected by the fact that the atomic number describes, among other things, the number of neutrons in a chemical element. Whereas the atomic mass describes, among other things, the combined number of electrons and neutrons in a chemical element. 

This idea can be seen below for the isotopes of Hydrogen;

	Hydrogen-1	Hydrogen-2 (deuterium)	Hydrogen-3 (tritium)
Atomic mass	1	1	1
Atomic number (mass number)	1	2	3
Protons	1	1	1
Neutrons	0	1	2
Electrons	1	1	1

Isotopes can be utilised by researchers through using isotopic labelling. This is a technique whereby an isotope of an atom replaces an atom already in a molecule. Commonly used replacements are replacing Hydrogen-1 with Hydrogen-2 (deuterium) or Carbon-12 with Carbon-13. This can be useful to monitor the progress of a reaction which can be analysed with techniques such as mass spectrometry or NMR. The use of deuterated solvents such as deuterated chloroform (CDCl₃) is also important for NMR, as this minimises the solvent peak in an NMR spectra. 

Page Author; Thomas Squire

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