The number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral (non-ionized) atom.
Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons.
In most cases, for obvious reasons, if an element has stable isotopes, those isotopes predominate in the elemental abundance found on Earth and in the Solar System.
The fact that each isotope has one proton makes them all variants of hydrogen: the identity of the isotope is given by the number of neutrons.
Curie · Skłodowska-Curie · Davisson · Fermi · Hahn · Jensen · Lawrence · Mayer · Meitner · Oliphant · Oppenheimer · Proca · Purcell · Rabi · Rutherford · Soddy · Strassmann · Szilárd · Teller · Thomson · Walton · Wigner The three naturally-occurring isotopes of hydrogen.
All isotopes of a given element have the same number of protons in each atom.
From left to right, the isotopes are protium ( Isotopes are variants of a particular chemical element which differ in neutron number.