Why d and s orbitals overlap even when there is considerable difference in their energies?

In my grade 11 chemistry NCERT textbook, its written that hybridization happens when orbitals of almost same energy overlaps. But S orbital and D orbital have significant difference in their energies. But still they overlap .I would be grateful to know why it happens then.

• 4s and 3d are particularly close in energy values this is a major factor in transition metals which frequently (In the context of VBT) undergo hybridisation with these orbitals. What you might be mistaking about must be the energy levels of s and d of same shells which do have significant energy gaps. Jun 13, 2021 at 18:46

In multielectronic atoms we have a relatively large difference between $$s$$ and $$d$$ orbitals when they have the same $$n$$ quantum number, or in terms of the actual quantum mechanics when they have the same total number of nodes in the wavefunction (this being what we label as $$n-1$$). But in the case of transition metals the $$s$$ orbital that mixes in with the $$d$$ orbitals has one higher $$n$$ or equivalently one more total node -- for instance, with iron it would be $$\color{blue}{4}s$$ (three total nodes) mixing with $$\color{blue}{3}d$$ (two total nodes). The opposing differences between angular momentum and between total nodal count leave the relevant orbitals close together in energy.