The "d" sub-shell is the next-to-last sub-shell and is denoted as sub-shell. The number of s electrons in the outermost s sub-shell is generally one or two except palladium (Pd), with no electron in that "s" sub shell in its ground state. The "s" sub-shell in the valence shell is represented as the "ns" sub-shell, e.g. 4s. In the periodic table, the transition metals are present in eight groups (4 to 11), with some authors including some elements in groups 3 or 12.