The most important classification of elements is that of non-metallic versus metallic. Metallic elements form solids that are good conductors of electricity, and where bonding is not strongly directional and have structures with many near neighbors. Non-metallic elements form molecules or covalent solids, which are usually poor conductors of electricity and where bonding is markedly directional in character. This distinction on the basis of physical properties. All elements of the d, s and f blocks are metallic (except hydrogen), non-metallic ones being confined to hydrogen and to the upper right-hand part of the p block.