Interstitial compounds are formed by a wide of metals with hydrogen, boron, carbon, range nitrogen and other elements. The invasion of interstitial sites does not significantly alter the metal structure. It still looks like a metal, and still conducts heat and electricity. However filling some of the holes has a considerable effect on the physical properties, particularly the hardness, malleability and ductility of the metal. Interstitial compounds are extremely inert chemically, have high melting points and are extremely hard.