Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Explain why these two isotopes have the same chemical properties but different relative atomic masses.

Chlorine‑35 and chlorine‑37 have the same number of protons (17) and therefore the same atomic number, so they have the same number of electrons and the same electronic structure. Chemical properties depend on the arrangement of electrons, so both isotopes behave chemically the same.

The two isotopes differ in the number of neutrons: chlorine‑35 has 18 neutrons, chlorine‑37 has 20 neutrons. The extra neutrons increase the mass of the nucleus, giving different mass numbers (35 and 37). Consequently the relative atomic masses of the two isotopes are different.