Explanation :
Isotope : It is defined as the element that have the same number of protons but have the different number of neutrons of each of the atom.
Isotope notation : It is also called as nuclear notation. In isotopic notation we can easily determine an isotope's mass number, atomic number, the number of neutrons and protons in the nucleus.
The general isotopic representation of an atom is:

where,
Z = Atomic number of the atom
A = Mass number of the atom
X = Symbol of the atom
For example :
atom.
From this we conclude that,
Atomic number of Ni atom = 28
Mass number of Ni atom = 64
The symbol of atom = Ni