What is the relationship between concentration gradient and diffusion?

A concentration gradient happens when there is a change of the concentration in solution for period of time. Diffusion is the movement of molecules or ions. The relationship between the two of them is molecules diffuse down from an area with a higher concentration to a lower one. An example of this is putting dye in water.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.