If we examine a number of symbols of binary compounds (compounds made of two elements) of hydrogen, such as HC1, H1O, NH3, CH4, we find that the first compound contains one atom, the second two atoms, the third three atoms, and the fourth four atoms of hydrogen. This means that the different elements are combined with the same element (hydrogen), or other elements equivalent to it in combining power, in different amounts as expressed by the symbols. This power of an element to combine with different amounts of hydrogen or its equivalent is called a valence. The combining power of hydrogen, which is one, is selected as the unit.