Metal is a substance with luster (i.e., strong reflection of visible light), malleability, and conductivity and thermal conductivity, among other properties. These characteristics of metals are related to the presence of free electrons within the metal crystals. In nature, the vast majority of metals exist in a combined state, with only a few metals such as gold, platinum, silver, and bismuth existing in a free state. Metal minerals are mainly oxides and sulfides. Other forms include chlorides, sulfates, carbonates, and silicates. The bonding between metals is metallic, allowing for reconnection when positions are changed, which is why metals are highly malleable. Metal elements usually only show positive valence in compounds. Metals with a higher relative atomic mass are referred to as heavy metals.