Question

Why are the isotopes of an element chemically similar?

Answer 1

Isotopes are two or more types of atoms with the same atomic number (number of protons in their nuclei) and periodic table position (and so belong to the same chemical element) but distinct nucleon numbers (mass numbers) owing to varying numbers of neutrons in their nuclei

Complete answer:
While all isotopes of the same element have almost identical chemical characteristics, their atomic weights and physical attributes differ.
The number of electrons in a neutral atom is the same as the number of protons. As a result, various isotopes of the same element have the same amount of electrons and an electrical structure that is identical. Because an atom's electrical structure determines so much of its chemical activity, various isotopes have essentially similar chemical behaviour.
The number of neutrons in the nuclei of various isotopes varies. The arrangement of electrons, particularly the outermost electrons, determines the majority of chemical characteristics. Some chemical characteristics are also influenced by the size of an atom. Because the number of neutrons in an element has no effect on any of these characteristics, isotopes of the same element will behave (chemically) the same. A heavier isotope's larger mass, on the other hand, provides certain important distinctions. The difference in masses can be used to distinguish Uranium isotopes (235 and 238), for example. This was formerly done in lengthy diffusion chambers, but it is currently done with centrifuges.
Isotopes are elements that have the same atomic number but differ in their atomic mass. They both have the same amount of electrons and, as a result, have the same electrical structure. Because an element's chemical characteristics are determined by its electrical arrangement, these elements are chemically similar.

Note:
Similarly, two molecules with similar electronic structures and almost identical physical and chemical characteristics differ solely in the isotopes of their atoms (isotopologues) (again with deuterium and tritium being the primary exceptions). Because a molecule's structure and the masses of its constituent atoms influence its vibrational modes, various isotopologues have distinct sets of vibrational modes. Isotopologues have distinct optical characteristics in the infrared region because vibrational modes allow a molecule to absorb photons of matching energies.