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Transition elements

Increase in number
of d-electrons entering
the core; I.e. only
s-electrons are formally
removed during bonding

Lower valences
become more stable
from Fe to Cu, and 2+
becomes dominant
oxidation number
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My prof wrote this on a slideshow. I'm not sure what he is trying to say. Can someone clarify?

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Second transition row

-larger electron shell

-less attraction of
d-electrons to core

-higher stability of

-higher oxidation number

Lower oxidation number
more stable at the end
of transition row

Oxidation number second row:
Y: 3, Zr: 4, Nb: 3, 5, Mo: 3, 6,
Pd: 0, 2, Ag: 0, 1, 2,

Third transition row

-filled 4f orbitals

-more protons in core than
in second row

-similar size of
electron shell

-similar stability of
oxidation number than
in second row

-Lower oxidation number
more stable at the end
of transition row

Oxidation number second row:
La: 3, Hf: 4, Ta: 3, 5, W: 3, 6,
Pt: 0, 2, Au: 0, 1, 3, Hg: 0, 1, 2
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Why is it that when you move from left to right in a transition element row the elements are more stable in a lower oxidation state? I tried looking at the electron configuration but I couldn't derive anything from them.