The most stable form (reference state) of an element is its standard reference form under standard conditions, such as graphite for carbon or O₂ gas for oxygen. By convention, the standard enthalpy of formation of an element in this form is defined as zero. This reference is what makes a consistent set of enthalpies of formation possible.

元素的最稳定形态(参考态)是它在标准条件下的标准参考形式,如碳的石墨或氧的 O₂ 气体。按约定,元素以这种形态存在时的标准生成焓定义为零。正是这一参考使一套自洽的生成焓成为可能。