a) What is an isotope?
Atoms with nuclei of the same atomic number differing in the number of neutrons are termed isotopes. One isotope differs from another isotope of the same element in that it possesses a different number of neutrons, i.e., by the mass number or nucleon number.
b) Are mass numbers given in subscript or superscript? Do they precede or follow the elemental symbol?
The mass number is the sum of the total number of protons and neutrons in an atom, molecule, or ion. The mass number of an isotope is given as superscript preceding the elemental symbol, e.g., 12C.
c) Which are the common ways to normalize isotopic abundances?
Isotopic abundances are either normalized relative to the sum of all isotopic abundances or to the most abundant isotope of an element (or isotopic compositions of an ion).