Chemical element of an element to exist in one of many structural forms is known as 'allotropy'. The standard state, also known as the reference state, of an element is defined as its thermodynamically most stable state at a pressure of 1 bar and a given temperature (typically at 298.15 K). In thermochemistry, an element is defined to have an enthalpy of formation of zero in its standard state. For example, the reference state for carbon is graphite, because the structure of graphite is more stable than that of the other allotropes. Several kinds of descriptive categorizations can be applied broadly to
Chemical nomenclature the element name. For example, NH is called "azane". This naming method has been developed principally for coordination compounds although it can be more widely applied. An example of its application is [CoCl(NH)]Cl pentaamminechloridocobalt(III) chloride. Ligands, too, have a special naming convention. Whereas "chloride" becomes the prefix "chloro-" in substitutive naming, in a ligand it becomes "chlorido-". Chemical nomenclature A chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC's rules for