J.R. S. answered • 03/10/19
Ph.D. University Professor with 10+ years Tutoring Experience
They have variable valences, and it also depends on how you define "valence electrons". For example, if you define valence electrons as the "outermost" electrons, then Sc (1s2 2s2 2p6 3s2 3p6 3d1 4s2) would have 2 valence electrons (those in the 4d subshell). However if you define valence electrons as those outside the noble gas configuration, then Sc has 3 valence electrons. But you are asking for the "valencies", and Sc can be 1+, 2+ or 3+. Another example is Mn which has 7 valence electrons but the normal valence state is usually 2+ because it easily loses the 2 electrons in the 4s subshell. See why it is variable and very difficult to answer your question?
Most transition metals have electron configurations of ns2(n−1)d, so these ns2 electrons are the valence electrons (outermost). Copper and chromium have one valence electron (they are exceptions), because they have one 4s electron because having a half filled 3d (Cr) or a filled 3d and half filled 4s(Cu) is more stable.
