Final answer:
D orbitals have poor shielding because they are complex in shape, less penetrating, and electrons in them spend more time farther from the nucleus, hence they are less effective at shielding other electrons from the nuclear charge.
Step-by-step explanation:
The d orbitals have poor shielding because they have complex shapes and penetrate less effectively compared to s and p orbitals. Electrons in the same principal shell, such as d orbitals, do not shield each other effectively from the nuclear charge. Additionally, as the principal quantum number (n) increases, the size of the orbital also increases, and electrons are located farther from the nucleus, which further reduces their shielding effect. For example, 4d orbitals are less penetrating than 4s orbitals. This reduced penetration means that there is less electron density close to the nucleus, which reduces the ability of d electrons to shield each other from the nuclear charge.
Moreover, the energy level of electrons is influenced by both the principal quantum number (n) and the azimuthal quantum number (l). For larger orbitals, the increases in energy due to n and l are similar, which affects their shielding abilities. Hence, d orbitals, with their higher energy and complex shapes, exhibit weaker shielding compared to orbitals with lower values of l, like s and p orbitals.