Question

why should electron enter in the 4s instead of 3D after 3p​

Answer 1

Electrons enter the 4s orbital before the 3d orbital after the 3p orbital because of Aufbau's Principle.

• Aufbau's principle states that the electrons fill the atomic orbitals with lower energy levels before the orbitals with higher levels of energy.
• The energy level of an atomic orbital is decided by observing the n + l value of the atomic orbital.

        (where n is the principal quantum number and

                     l is the Azimuthal quantum number.)

• So, according to this rule, the orbital with a lower n + l value will be filled first. In other words, the filling of the orbitals is in the increasing order of the n + l value of the orbitals.

∴ Electrons enter 4s instead of 3d after 3p because its n+l value is lower.

Answer 2

1. The Aufbau principle governs how electrons are filled in an atom's atomic orbitals when it is in its ground state.

2. It claims that electrons fill atomic orbitals in the sequence of increasing orbital energy levels.

3. According to the Aufbau principle, the lowest energy atomic orbitals are occupied first, followed by the higher energy levels.

As a result, electrons fill in the orbitals with increasing energies. Due to the lower (n+l) value, after 3p, the energy of 4s(n+l=4) and 3d(n+l=5) is lower, which means electrons tend to fill in the 4s orbital first.