Question

Does the isotopic composition of an element affect the law of definite proportions?

Answer 1

No.

Explanation:

Any element in any compound consists of the different isotopes of that element. The fact that an element's relative atomic mass (atomic weight) on the periodic table is never a whole number, is because it is the weighted average of the isotopic masses for the element.

The different isotopes of an element have the same numbers of protons and electrons, so they react the same when forming a compound.