Question

why electrons are promoted to 3d instead of 4s during excited state formation

Answer 1

Electrons are promoted according to the energy level in ascending order it means orbital with less energy is filled first, now the energy level is calculated by the formula (n+l) where n is orbit no. and l is azimuthal quantom no. So let's try it,for 3d energy=3+2=5(d,l=2),For 4s energy=4+0=4(s,l=0), thus energy of 4s is less than that of 3d therefore it's filled first.