In a multi-electron atom, the electrons are distributed in different energy levels or shells, and each energy level is further divided into subshells. The energy of the subshells is not the same, and various subshells have different energies. The 3d subshell has a higher energy than the 4s subshell in a multi-electron atom. This phenomenon is known as the Aufbau principle.


The Aufbau principle states that the electrons fill the lowest energy subshell first before filling the higher energy subshells. The 4s subshell is filled before the 3d subshell because it has a lower energy level. However, in some cases, the energy levels of the subshells are close to each other, and the order of filling the subshells is not the same as predicted by the Aufbau principle.


The 4s subshell has a lower energy than the 3d subshell in the isolated atom. However, in a multi-electron atom, the electrons in the 4s subshell experience the repulsion from the electrons in the 3d subshell. This repulsion reduces the energy of the 4s subshell and increases the energy of the 3d subshell.

The 3d subshell has a higher energy than the 4s subshell because of the following reasons:

1. Shielding Effect: The electrons in the 3d subshell are shielded from the nucleus by the inner electrons. However, the electrons in the 4s subshell are not shielded from the nucleus by any other electrons. Therefore, the 4s subshell experiences a greater attraction towards the nucleus, which lowers its energy level.

2. Effective Nuclear Charge: The electrons in the 3d subshell experience a higher effective nuclear charge than the electrons in the 4s subshell. The effective nuclear charge is the net positive charge experienced by the electrons. The effective nuclear charge is higher for the 3d subshell because it is closer to the nucleus than the 4s subshell. This higher effective nuclear charge increases the energy of the 3d subshell.


In conclusion, the 3d subshell has a higher energy than the 4s subshell in a multi-electron atom due to the shielding effect and the effective nuclear charge. The Aufbau principle predicts the order of filling the subshells, but it does not take into account the repulsion between the electrons in different subshells.

U can compare the energy of orbitals by comparing the sum of their principal and azimuthal quantum number.
which in this case 3d is great than 4s