Although d subshell has poor screening effect and result in increase of zeff but in case of aluminium and gallium, gallium has a full new shell added to it, still it is smaller than aluminium. Why? And, we know that down the group, atomic size is the main property to study.so why do we consider zeff?

Dear Student,
The reason d-orbitals have any kind of effect is that electrons in d-orbitals don't screen atomic charge as viably as those in s and p orbitals. This is a direct result of something many refer to as entrance .. basically the scientific states of d-orbitals keep them from enabling electrons to enter intently to the core, contrasted and electrons in s or p-orbitals. In gallium, you have 10 electrons in the filled 3d-subshell, and each of these electrons is making a marginally more regrettable showing with regards to (generally) of screening the atomic charge than the electrons in the s and p orbitals. Accordingly, the compelling atomic charge in gallium is marginally higher than that in aluminum, so the expansion in the span is a significantly littler than would be normal in view of the contrast amongst boron and aluminum, or gallium and indium.