Question

Why can a magnet separate iron atoms from a mixture but not from a compound?

Answer 1

Because the iron atoms are chemically bonded to other atoms.

Answer 2

A magnet can separate iron atoms from a mixture because the iron atoms retain their magnetic properties in the mixture. However, in a compound, the iron atoms are chemically bonded to other atoms and the attachments to sulfur atoms are stronger than the pull of the magnet.

Compounds are pure substances that require energy to break. The energy required to break the bonds of 1 mole of the compound is called the lattice energy. These values are usually really high, in the thousands of kilojoules per mol of compound.