Final answer:
The superscript in an isotope's nuclear symbol represents the mass number, comprising both protons and neutrons, while the atomic number, signifying protons, is often provided as a subscript but can be omitted due to its implicit presence through the element symbol.
Step-by-step explanation:
The superscript in the nuclear symbol of an isotope indicates the mass number, not the atomic number. The mass number is the total number of protons and neutrons in the nucleus. The atomic number, which is the number of protons and defines the identity of the element, is represented as a subscript to the left of the element's symbol. For instance, the nuclear symbol for carbon-12 (¹²C) has a mass number of 12 as the superscript, which means there are 6 neutrons since carbon's atomic number is 6. The atomic number, often omitted in isotopic symbols since the element symbol implies it, would be a subscript.
Each isotope of an element has the same atomic number but different mass numbers due to variations in the number of neutrons. This leads to different isotopes such as deuterium (²H or D) and tritium (³H or T), which are isotopes of hydrogen. Isotopes can be simply represented by the element name followed by the mass number, like magnesium-24 or Mg-24.