For a long time, people thought that this number was a fixed property of the element in question. They thought that carbon always has four bonds, oxygen always has two, and hydrogen always has one. The problem was seen only later. For example, phosphorous sometimes behaves as if it had three bonds, a valence of three. At other times, though, it seems to have five bonds.
IUPAC saw this problem, and proposed oxidation numbers. This means there is one number per chemical element. The problem with this approach is that it leaves aside most[source?] chemical properties of the elements in question.
