Foil (metal)
A foil is a very thin sheet of metal, usually made by hammering or rolling. Foils are most easily made with malleable metals, such as aluminium, copper, tin, and gold. Foils usually bend under their own weight and can be torn easily. The more malleable a metal, the thinner foil can be made with it. For example, aluminium foil is usually about 1/1000 inch (0.03 mm), whereas gold (more malleable than aluminium) can be made into foil only a few atoms thick, called gold leaf. Extremely thin foil is called metal leaf. Leaf tears very easily and must be picked up with special brushes.
Post-transition metal
Bismuth is a soft metal (MH 2.5) that is too brittle for any structural use. It has an open-packed crystalline structure (BCN 3+3) with bonding that is intermediate between metallic and covalent. For a metal, it has exceptionally low electrical and thermal conductivity. Most of the ordinary compounds of bismuth are covalent in nature. The oxide, BiO is predominantly basic but will act as a weak acid in warm, very concentrated KOH. It can also be fused with potassium hydroxide in air, resulting in a brown mass of potassium bismuthate. The solution chemistry of bismuth is characterised by the formation of oxyanions; it forms anionic bismuthates in strongly basic solutions. Bismuth forms Zintl phases such as NaBi, RbInBi and BaCdBi. Bailar et al. refer to bismuth as being, 'the least "metallic" metal in its physical properties' given its brittle nature (and possibly) 'the lowest electrical conductivity of all metals.'