text: Before Werner, chemists defined the valence of an element as the number of its bonds without distinguishing different types of bonds. However, in complexes such as [Co(NH)]Cl for example, Werner considered that the Co-Cl bonds correspond to a "primary" valence of 3 at long distance, while the Co-NH bonds which correspond to a "secondary" or weaker valence of 6 at shorter length. This secondary valence of 6 he referred to as the coordination number which he defined as the number of molecules (here of NH) directly linked to the central metal atom. In other complexes, he found coordination numbers of 4 or 8.
  On these views, and other similar views, in 1904 Richard Abegg formulated what is now known as Abegg's rule which states that the difference between the maximum positive and negative valence of an element is frequently eight. This rule was used later in 1916 when Gilbert N. Lewis formulated the "octet rule" in his cubical atom theory.
  In modern terminology, Werner's primary valence corresponds to the oxidation state, and his secondary valence is called coordination number. The Co-Cl bonds (in the above example) are now classed as ionic, and each Co-N bond is a coordinate covalent bond between the Lewis acid Co and the Lewis base NH.