Chemistry - Can isotopes of a given element be represented by different symbols?
Solution 1:
IR-3.3.1 Isotopes of an element
The isotopes of an element all bear the same name (but see Section IR-3.3.2) and are designated by mass numbers (see Section IR-3.2). For example, the atom of atomic number 8 and mass number 18 is named oxygen-18 and has the symbol $\ce{^{18}_{}O}$.
IR-3.3.2 Isotopes of hydrogen
Hydrogen is an exception to the rule in Section IR-3.3.1 in that the three isotopes $\ce{^{1}_{}H}$, $\ce{^{2}_{}H}$ and $\ce{^{3}_{}H}$ can have the alternative names protium, deuterium and tritium, respectively. The symbols D and T may be used for deuterium and tritium but $\ce{^{2}_{}H}$ and $\ce{^{3}_{}H}$ are preferred because D and T can disturb the alphabetical ordering in formulae (see Section IR-4.5). The combination of a muon and an electron behaves like a light isotope of hydrogen and is named muonium, symbol $\ce{Mu}$.⁵ These names give rise to the names proton, deuteron, triton and muon for the cations $\ce{^{1}_{}H+}$, $\ce{^{2}_{}H+}$, $\ce{^{3}_{}H+}$ and $\ce{Mu+}$, respectively. Because the name proton is often used in contradictory senses, i.e. for isotopically pure $\ce{^{1}_{}H+}$ ions on the one hand, and for the naturally occurring undifferentiated isotope mixture on the other, it is recommended that the undifferentiated mixture be designated generally by the name hydron, derived from hydrogen.
Source:
N.G. Connelly, T. Damhus, R.M. Hartshorn, A.T. Hutton (eds) (2005). Nomenclature of Inorganic Chemistry (PDF). RSC–IUPAC. ISBN 0-85404-438-8.
Addendum:
The small subscript ⁵ present in the source is a reference to Names for Muonium and Hydrogen Atoms and Their Ions, W.H. Koppenol, Pure Appl. Chem., 73, 377–379 (2001) which can be viewed over at:
https://www.iupac.org/publications/pac/pdf/2001/pdf/7302x0377.pdf
$\cdots$ A particle consisting of a positive muon and an electron ($\pu{\mu^+ e^–}$) is named “muonium” and has the symbol $\ce{Mu}$. Examples: “muonium chloride,” $\ce{MuCl}$, is the equivalent of deuterium chloride $\cdots$
Solution 2:
While it is read like "carbon 12", and "carbon 14", etc. the atomic notation of the nuclides is the other way around, i.e. the mass number precedes the element symbol. Hence it is not $\ce{C^{12}}$, but $\ce{^{12}C}$.
The special names and symbols seen for $\ce{^1H}, \ce{^2H}, \ce{^3H}$ (but not the other isotopes of neither hydrogen, nor the ones of other elements) follow a recommendation by IUPAC, published in Pure and Applied Chemistry in 1988 (doi 10.1351/pac198860071115, open access). It is still in power, as the corresponding recommendation by 2001 in the same journal shows (doi 10.1351/pac200173020377).