Metal is a substance with properties such as luster (i.e., strong reflection of visible light), malleability, and good conductivity and thermal conductivity. These characteristics of metals are related to the presence of free electrons within the metal crystal. In nature, the vast majority of metals exist in a combined state, while a few metals such as gold, platinum, silver, and bismuth exist in a free state. Metal minerals are mostly oxides and sulfides. Other forms include chlorides, sulfates, carbonates, and silicates. The bonding between metals is metallic, allowing for the re-establishment of connections when the positions are changed, which is why metals are highly malleable. Metal elements typically only exhibit positive valence in compounds. Metals with a relatively high atomic mass are known as heavy metals.