Explain why isotopes of the same element have the same chemical properties but different physical properties.

The chemical property of an element is dependent on the number of protons and electrons it has. All isotopes of the same element (assuming that they are not ions) have the same number of protons and electrons. Therefore, they are chemically the same.

Isotopes, on the other hand, have varying numbers of neutrons. This changes their mass, and mass affects physical properties.