For ionic solids and covalent network solids to melt, chemical bonds must be broken. Melting points of these solids are relatively high. They shatter when hit with a hammer. Ionic crystals are held together with ionic bonds and are hard and brittle. Covalent network crystals like diamond and silicon carbide are among the hardest substances known.
Metallic crystals are malleable and can be shaped by hammering.
The electrons are localized to particular ions or bonds. X rays indicate that most metallic crystals can be considered as pile of similar spheres which when packed together as many spheres as possible touch. The second layer is placed in alternate holes of the first layer. The two types of holes alternate so that in forming the third layer all spheres go in one kind of depression. As soon as one atom chooses a hole, all other atoms go into the same kind of hole.