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I understand that the term valency refers to the number of bonds an element would be able to form with either chlorine or hydrogen. If this is directly related to the number of valence electrons an atom has, why do transition metals have variable valencies?

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  • $\begingroup$ Only maximum valency is, no one says there can't be unpaired electrons or lone pairs. Also valency is rather primitive concept, coordination number is better, but still not perfect. $\endgroup$ – Mithoron Aug 21 '16 at 0:28
  • $\begingroup$ The concept of valence electrons doesn't really work for transition metals. $\endgroup$ – f'' Aug 21 '16 at 1:06
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I think it's kind of a poor use of terms here. When talking about transition metals, the valency generally refers to the oxidation state of the metal atom. E.g: hexavalent chromium is $\ce{Cr^6+}$, not necessarily a chromium bonded to six other things.

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    $\begingroup$ You can (and should!) use mhchem to format chemical expressions: typing $\ce{Cr^6+}$ renders as $\ce{Cr^6+}$. Nitpicking, I would say that hexavalent chromium is Cr(VI), not $\ce{Cr^6+}$ as the latter implies a naked ion with a 6+ charge. $\endgroup$ – orthocresol Aug 21 '16 at 8:33
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    $\begingroup$ I don't think it's just a poor use of terms. It's the overuse of a concept that was ill-defined in the first place. According to the IUPAC gold book there is only one valence per element. Hence the statement of "variable valencies" is self-contradictory already. As well as "maximum valence" being redundant. Any making sense of that is diving into muddy waters. One should switch to different concepts, like oxidation numbers, and coordination numbers, as they do not only contain more information, but also less ambiguity. $\endgroup$ – Martin - マーチン Aug 31 '16 at 5:11
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Transition metals have variable valencies because the energies of the 3d orbital and 4s orbitals (or similar orbital comparisons in lanthanides and actinides, etc.) are similar, so electrons are able to bond from the d-shell as well. Kind of similar to how some high atomic number elements are able to violate the octet rule, such as Xenon in xenon tetrafluoride or Sulfur in sulfur hexafluoride, because their 3d orbitals are able to participate in the valence bonding.

This effect also increases because the elements in the transition series also increases in the number of unpaired electrons, which increases the effect of the d-orbital.

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  • $\begingroup$ Someone good can check over my work... it might be primitive cause I'm not a rocket scientist $\endgroup$ – phi2k Aug 21 '16 at 4:24

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