Paramagnetism And Diamagnetism

Whether an ion is paramagnetic or diamagnetic depends on the presence or absence of unpaired electrons. The copper(I) ion, Cu^+, has the electronic configuration 3d^10, a completely filled d-subshell with all electrons paired, so it has no unpaired electrons and is diamagnetic. The copper(II) ion, Cu^2+, has the configuration 3d^9, which contains one unpaired electron, making it paramagnetic and also giving its solutions their characteristic blue colour. Thus the difference in magnetic behaviour follows directly from their electronic configurations. The option that Cu^+ has one unpaired electron is incorrect, since its 3d^10 shell is full. The option that Cu^2+ has no d-electrons is false, as it has nine. The option that both have the same configuration is wrong, since they differ by one electron. This link between configuration and magnetism is a standard NCERT d-block concept. This concept also bridges to Atomic Structure and Coordination Compounds, so mastering it strengthens performance on linked questions from those topics as well. A common JEE pitfall is to ignore the role of paramagnetism, yet it is exactly this factor that distinguishes the correct answer from the tempting alternatives. Plausibility check: the colourless diamagnetic Cu^+ compounds versus the blue paramagnetic Cu^2+ compounds confirm the configurations and their magnetic consequences.