Physically, they are soft (Mohs harness 1.5-2.75) and malleable (or brittle in the case of Ga and Bi), have poor mechanical strength, and usually have melting points lower than those of the transition metals. Their crystalline structures tend to show covalent or directional bonding effects, having generally greater complexity or fewer nearest neighbours than other metallic elements. All the p-block metals have the electronic structures of metals. They are all used, or have been trialled, as soft metal lubricants. Their low melting points make them suitable for soldering materials. When mixed with metals they form alloys.