The mobile electrons in a metal can move from one end of the block to the other, and conduct electricity. Salts do not conduct electricity when crystalline, because the ions then are locked in place. If a salt is either melted or dissolved in water, then it can conduct electricity through the migration of entire ions, not electrons.

Heat is simply motion at the atomic or molecular level. If a match is touched to one end of a solid, the molecules or atoms at that end begin to vibrate more rapidly, and as each rapidly vibrating atom induces a slower neighbor to vibrate faster, heat flows along the solid object. The mobile electrons in a metal are very good carriers of this vibrational energy, so metals are good conductors of heat as well as electricity. Metals feel cold to the touch because they conduct heat away from your fingertips so fast. In salts each ion is held in place by the pull from neighboring ions of opposite charge, thereby forming a rigid framework. The vibrations of ions are damped down quickly by the attraction of neighboring ions, and vibrations are passed down the crystal lattice inefficiently. Salts therefore are poor conductors of heat.