Does Zeff increase, decrease or stay the same for transition metals in a row?
The effective nuclear charge (
This is because more protons are added in the nucleus, whereas the same number of electrons is added in the (n-1)d subshell, (where n is the number of the period and the quantum number of the s type outer electrons).
If the d subshell were completely enclosed in the atom's core, the core positive charge "felt" by the valence electrons, that is, the effective nuclear charge, would remain exacly consfant for a transition series.
Technically speaking, this would be expressed by saying that the added (n-1)d electrons would be entirely capable of shielding the positive charge of the protons.
Since the (n-1)d orbitals are excedingly elongated, their electrons reach the outer n shell, provoking a non-negligible repulsion on the valence electrons.
Hence the (n-1)d electrons are only partially capable of shielding the increasing nuclear charge, and, consequently, the effective nuclear charge or overall positive core charge, increases as the atomic number increases for a transition series.