State, in terms of the number of subatomic particles, one similarity and one difference between the atoms of these isotopes of element X.

The atomic number is determined by the number of protons in the atom.

Neutrons can vary the mass of atoms of the same element; atoms of the same element with the different atomic masses are called isotopes.

Electrons do not contribute a meaningful amount of mass to an atom, and only vary the charge of the atom. A charge atom is an ion.

Explanation:

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the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.