Step 1: Contrast their origin at the atomic level.
The whole difference comes from electron arrangement. In a paramagnetic atom some electrons are unpaired, so the individual electronic magnetic moments do not cancel and the atom carries a net moment. In a diamagnetic atom every electron is paired, so the moments cancel and the atom has no net moment.
Step 2: Explain their response to an applied field.
When a field is switched on, a paramagnetic sample lets its existing atomic magnets partly line up with the field, so it is drawn into the field (attraction). A diamagnetic sample develops tiny induced moments that oppose the field (Lenz-law style), so it is pushed out of the field (repulsion).
Step 3: Tabulate the measurable differences.
• Susceptibility \(\chi\): paramagnetic \(\to\) small and positive; diamagnetic \(\to\) small and negative.
• Permeability \(\mu_{r}\): paramagnetic \(\to\) just above 1; diamagnetic \(\to\) just below 1.
• Temperature effect: paramagnetism weakens on heating (follows Curie's law \(\chi\propto 1/T\)); diamagnetism is essentially independent of temperature.
• Examples: paramagnetic \(\to\) aluminium, platinum; diamagnetic \(\to\) bismuth, copper.
Result: Paramagnetics are feebly attracted with positive \(\chi\) and permanent atomic moments; diamagnetics are feebly repelled with negative \(\chi\) and no permanent moment.