What causes d lag on the periodic tabel?

The "d lag" in the Periodic Table, often referred to as the d-block contraction, is primarily caused by the filling of the d-orbitals in transition metals. As we move across a period, the increasing nuclear charge pulls electrons closer to the nucleus, which leads to a decrease in atomic radius. This effect is more pronounced in d-block elements due to the shielding effect of the inner s and p electrons, resulting in a more compact structure than expected based solely on the number of protons. Consequently, this creates a lag in the expected trends of atomic size and other properties in the d-block elements.