Final answer:
Isotopic symbols for the most abundant isotopes of carbon and chlorine are 126C and 3517Cl respectively.
Step-by-step explanation:
To write the isotopic symbol for an isotope, you start with the element's chemical symbol, add the atomic number as a subscript, and the mass number as a superscript. The atomic number is the number of protons in an atom's nucleus, and the mass number is the sum of protons and neutrons. For example, the most abundant isotope of carbon has 6 protons and 6 neutrons, leading to a mass number of 12. Its isotopic symbol would be 126C. Likewise, the most abundant isotope of chlorine has 17 protons and 18 neutrons, giving it a mass number of 35. Its symbol would be 3517Cl.