Molecular crystals are made of molecules held together by weak forces like van der Waals, dipole–dipole, or hydrogen bonds, making them soft, with low melting points, and poor conductors of heat and electricity. Ionic crystals are made of positive and negative ions held by strong ionic bonds; they are hard and brittle, have high melting and boiling points, and can conduct electricity only when melted or dissolved, like in salt (NaCl). Covalent or atomic crystals are built from atoms joined by strong covalent bonds throughout the structure, making them very hard, with high melting points, and usually poor conductors (except graphite). Metallic crystals consist of positive metal ions surrounded by a “sea” of free-moving electrons; they are good conductors of heat and electricity, can be bent or stretched (malleable and ductile), and their hardness and melting points vary depending on the metal.