In a standard book it is given:

"Atomic radius of Gallium is less than that of Aluminium. This can be understood from the variation in the inner core of the electronic configuration. The presence of additional 10 d-electrons offer only poor screening effect for the outer electrons from the increased nuclear charge in gallium. Consequently, the atomic radius of gallium (135 pm) is less than that of aluminium (143 pm)."

In another chapter, it is given:

"As we move along the period in 3d series, we see that nuclear charge increases from scandium to zinc but electrons are added to the orbital of inner sub shell, i.e., 3d orbitals. These 3d electrons shield the 4s electrons from the increasing nuclear charge somewhat more effectively than the outer shell electrons can shield one another. Therefore, the atomic radii decrease less rapidly."

In the first para, it is given that the d-electrons shield poorly, but the opposite is given in the second but it is unlikely that the book is wrong. So what am I missing?

  • $\begingroup$ See Slater's rules for an empirical evaluation of shielding. $\endgroup$
    – MaxW
    Jun 21 '21 at 18:51

Both sections do not contradict themselves, as they address different shielding aspects.

  • 3d electrons give worse shielding of 4s/4p electrons than 1-3s and 2-3p electrons.
  • 3d electrons give better shielding of 4s/4p electrons than 4s/4p electrons themselves (mutually).

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