How are the particles in a block of iron affected when the block is melted?
They gain more energy and lose their lattice structure.
Normally a block of iron has a lattice structure. Positive metal nuclei are attracted to negative electrons from the d-orbitals, which forms malleable rows and columns of a metal lattice.
When more energy is added through heat (e.g. melting), atoms gain more kinetic energy as well, which moves them around and overrides the electromagnetic attractions which hold the lattice together.
Without these, the atoms lose a lot of their organisation and form a molten mess.