Isotopes have the same atomic number hence the same number of electrons which causes them to have the same chemical properties. But isotopes have different number of neutrons which causes them to have different mass numbers. The difference in mass numbers causes isotopes to have different physical properties. Heavier isotopes have a higher mass numbers than lighter isotopes because of which they have a higher melting point, boiling point etc.
The answer is A. Isotopes only differ in the number of neutrons in the nucleus of their atoms. Otherwise, all other subatomic particles are the same. The isotope with the more neutrons in its nucleus is therefore, heavier (have higher mass number). They share the same chemical properties but have slight physical differences such as boiling points with the heavier element having a slightly higher boiling point that the lighter element.
