What is the isotope of an element

Atoms with the same atomic number (proton number) but different numbers of neutrons are called Isotopes designated.
An example to understand this: The two uranium isotopes uranium-235 and uranium-238 each have 92 protons in their atomic nucleus. The number of protons in the atomic nucleus always determines which chemical element the atom is. In the periodic table, the number of protons of any element can easily be read from the atomic number. This is identical to the atomic number, i.e. the number of protons.
235 or 238 indicates the mass number or nucleon number. This number tells you how many protons and neutrons there are in total in the atomic nucleus. Uranium-235 has 143 neutrons (trial calculation: 92 + 143 = 235), uranium-238 has 146 neutrons (trial calculation: 92 + 146 = 238).

The whole thing again at a glance:
Uranium-235 92 protons + 143 neutrons = 235 (mass number)
Uranium-238 92 protons + 146 neutrons = 238 (mass number)

What should be evident from this example: Isotopes from a series of elements (in this case uranium) are always a single element. The distinguishing feature is the different number of neutrons! So if you know the isotope, you can easily calculate the corresponding neutron number:

Mass number = atomic number + neutron number

Of course, not only the chemical element uranium has isotopes. A total of over 3000 different isotopes are known. Every element has a more or less large number of isotopes.