The subscript giving the atomic number of an atom is often left off when writing an isotope symbol. For example, 6 often written simply as 13C. Why is this allowed?

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This results in different atomic masses for the isotopes. For example, carbon-12 and carbon-14 are isotopes of carbon, with 6 protons and 6 or 8 neutrons, respectively. The notation for isotopes typically includes the mass number (total protons and neutrons) and the element symbol.

The atomic number of an element is the number of protons found in the nucleus of an atom, which uniquely identifies the element. It is denoted by the subscript in isotope notation. For carbon, the atomic number is 6, meaning all carbon atoms have 6 protons. When writing isotope symbols, the atomic number can be omitted if the element is clear from the context, as each element has a unique atomic number.

Nuclear notation is a shorthand way of representing isotopes, where the mass number is written as a superscript and the atomic number as a subscript next to the element symbol. For example, ¹³C represents carbon with a mass number of 13. Omitting the atomic number in this notation is permissible because the element symbol inherently conveys the atomic number, allowing for a more concise representation without losing clarity.