Which statement best describes the properties of isotopes?

• A. Chemical properties are identical; physical properties never differ.
• B. Chemical properties differ; physical properties can differ.
• C. Chemical properties are different; physical properties are identical.
• D. Chemical properties are identical; physical properties can differ.

Answer: (D)

Isotopes have the same number of protons and electrons, so they form the same bonds and participate in chemical reactions in essentially the same way. That makes their chemical properties identical. What changes with different numbers of neutrons is the mass of the atom, which affects properties that depend on mass such as density, as well as subtle effects on reaction rates and vibrational energies. So the best description is that chemical properties are identical while physical properties can differ. For example, two forms of hydrogen (one with the usual proton and one with a neutron) will make the same compounds, but the heavier form diffuses differently and has slightly different vibrational characteristics.