化合价是元素的原子相互化合时的数目,主要由该元素原子的最外层电子数决定。化合价和化学式之间有着密切的联系,根据元素的化合价可以写出物质的化学式;反过来,根据物质的化学式,也可以求出某种元素的化合价。 The valence is the number at which the atoms of the elements are combined with each other and is mainly determined by the number of the outermost electrons of the elemental atom. There is a close relationship between valence and chemical formulae. According to the valence of the element, the chemical formula of the substance can be written; in turn, based on the chemical formula of the substance, the valence of an element can also be found.