What is the correct symbol for an isotope of iodine with $53$ protons and $78$ neutrons?

Hint: We have to know that isotopes are different species of particular compound that could show difference in neutron number, and so, there would be difference in nucleon number. We have to know that all isotopes of a given compound would have the same number of protons whereas the number of neutrons could be varied.

Complete answer:
We have to know that isotopes are atoms of a particular compound that could contain a number of neutrons in the nucleus of an atom. All atoms of the same component contain a similar amount of protons. We have to know that the amount of protons is nothing but the atomic number of that element. We have to know that the different isotopes contain several numbers of neutrons which differ in mass number. We have to know that the mass number is the sum of the number of nucleons found in the nucleus.
We can give the general notation of an isotope as,
\[_Z^AX\]
Here,
We can represent the chemical symbol for the element as X.
We can represent the atomic number of the element as Z.
We can represent the mass number of the element as A.
So, for the given isotope of iodine, the number of protons is $53$ and the number of neutrons is $78$. So, when we add the number of protons and number of neutrons, we get the mass number. Therefore, the calculated mass number is $131$.
We can write the isotopic notation of iodine with $53$ protons and $78$ neutrons as,
\[_{53}^{131}I\]

Note:
We have to know that isotopes of similar elements exhibit identical physical properties and chemical properties. We have to know that elements could contain stable isotopes as well as radioactive isotopes. We have to know that an isotope is radioactive if its nucleus contains a chance of spontaneously changing at some course of time.