So, for example, looking at HF and HCl, we see that the 1s orbital of the H overlap with the 2p orbital of F more effectively. Which argument can i use to explain that? Density?
As the quantum number increases, the size of the orbital, or more precisely, the area in which electrons are found, increases. The same 1s orbital of H combines with greater overlap with 2p of F, while the larger 3p of Cl can’t be approached to that extent because of its size. For the sake of demonstration, consider a child shaking hand with the other child, which is a full overlap of palms, while a child shaking hand with an adult is not!