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A chemical symbol is the standardized letter label for a chemical element, such as $\mathrm{C}$ for carbon or $\mathrm{Co}$ for cobalt. Capitalization is meaningful: $\mathrm{Co}$ names one element, whereas $\mathrm{CO}$ is a formula containing carbon and oxygen.

Nuclide notation adds information to the element symbol: in ${}_Z^{A}X$, $Z$ is the atomic number and $A$ the mass number. An isotope changes $A$ without changing the element identity fixed by $Z$. Ionic charge is written separately, as in ${\mathrm{Fe}}^{3+}$. A formula's subscripts count atoms per formula unit; they are neither atomic numbers nor charges.

Jöns Jacob Berzelius developed an alphabetic system of chemical symbols for expressing composition. Combining element symbols with numerical proportions made a chemical formula compact enough to write and print while retaining information about a compound's constituents. His original notation placed the numbers differently from the subscripts familiar in modern formulas.^[ref]

Lutetium is the chemical element with atomic number $71$ and chemical symbol $\mathrm{Lu}$. It is a dense rare-earth element whose ordinary chemistry is dominated by the $+3$ oxidation state. The neutral atom has a filled $4f$ shell with outer $5d6s^2$ occupancy.^[ref]

Its close chemical similarity to neighbouring rare earths makes separation difficult. This illustrates why identifying a new element and obtaining a pure metallic specimen are different experimental achievements. Element identity is fixed by nuclear charge, whereas similar outer-electron chemistry controls the practical separation.

Rhodium is a platinum-group transition metal with chemical symbol $\mathrm{Rh}$ and atomic number $45$. Both its metallic surfaces and its coordination complexes are important in catalysis; automotive exhaust treatment is a major application.^[ref]

Different ligand environments can change a rhodium centre's accessible oxidation states and reaction pathways. Thus the relatively unreactive bulk metal and a reactive molecular rhodium catalyst are compatible descriptions of different chemical environments, rather than contradictory properties of the element.