Why do isotopes have different physical but similar chemical properties?

Isotopes have different physical properties due to varying numbers of neutrons, but similar chemical properties due to identical electron configurations.

Isotopes are variants of a particular chemical element that differ in neutron number, while all isotopes of a given element have the same number of protons. The number of protons in an atom determines the element's identity and its place in the periodic table, and it also determines the configuration of electrons around the nucleus. Since chemical reactions primarily involve the interactions of these outer electrons, all isotopes of a given element exhibit similar chemical behaviour.

However, the number of neutrons in an atom does not affect the electron configuration, but it does influence the atom's mass. This is why isotopes of the same element have different physical properties. For instance, the three isotopes of hydrogen - protium, deuterium, and tritium - all behave chemically like hydrogen, but they have different physical properties. Protium, the most common isotope, has no neutrons; deuterium has one, and tritium has two. This difference in neutron number results in different atomic masses, which in turn affects the isotopes' physical properties such as density, boiling point, and melting point.

Furthermore, the number of neutrons can also affect the stability of the nucleus, leading to differences in radioactive behaviour among isotopes. Some isotopes are stable, while others are radioactive and decay over time, releasing radiation. This is another physical property that can vary among isotopes of the same element.

In summary, while the number of protons and the resulting electron configuration determine an element's chemical properties, the number of neutrons influences its physical properties, leading to differences among isotopes.