Why Do Isotopes of the Same Element Behave Differently Physically but Not Chemically?

At SL and HL, isotopes of the same element have essentially the same chemical properties because they have the same electron configuration. Their physical properties can differ because they contain different numbers of neutrons and therefore have different masses.

The Reasoning

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers.

For neutral atoms, the number of electrons equals the number of protons. Isotopes consequently have the same number and arrangement of electrons, including the same number of valence electrons. Because chemical reactions involve the rearrangement or sharing of valence electrons, isotopes normally form the same ions, bonds, and compounds.

For example, chlorine-35 and chlorine-37 both contain 17 protons and 17 electrons. Both have the same valence-electron arrangement and form chloride ions with a charge of $-1$.

Their neutron numbers and masses are different, however. This affects properties that depend on particle mass or nuclear composition.

A common misconception is that isotopes are completely identical chemically. Their chemical behavior is nearly, not absolutely, identical: isotopic mass can cause small differences in reaction rate, especially for hydrogen isotopes. For IB Structure 1.2, the key explanation remains that chemical properties depend mainly on electron configuration, while many physical properties depend on mass.

Exam Technique

For an explain question, link each conclusion to its cause: same proton number $\rightarrow$ same electron configuration $\rightarrow$ similar chemical properties; different neutron number $\rightarrow$ different mass $\rightarrow$ different physical properties. Do not say that neutrons directly determine chemical bonding.