Question

State Aufbau principle.

Answer 1

Hint: A Aufbau principle dictates the manner in which the electrons are filled in the atomic orbitals of an atom in ground state.It states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels.

Complete step by step answer:
The Aufbau principle states that the electrons are filled into an atomic orbital in the increasing order of orbital energy level, according to the Aufbau principle, the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels.
Following is the chart to understand energy levels –

The electrons are to be filled in the order.
$ 1s\, 2s\, 2p\,3s \,3p\, 4s \,3d\, 4p\, 5s\, 4d \,5p\, 6s \, 4f \,5d\, 6p \, 7s\, 5f \,6d\, 7p\, 8s\,.......$
We can understand it in few points:
-According to the Aufbau principle, electrons first occupy those orbitals whose energy is the lowest. This implies that the electrons enter the orbitals having higher energies only when orbitals with lower energies have been completely filled.

-The order in which the energy of orbitals increases can be determined with the help of the \[\left( {n + l} \right)\]rule, where the sum of the principal and azimuthal quantum numbers determines the energy level of orbitals.

-Lower \[\left( {n + l} \right)\] values corresponds to lower orbital energies, if two orbitals share equal \[\left( {n + l} \right)\] values, the orbital with the lower value of $n$ is said to have lower energy associated with it.

Note:
 Ruthenium, Rhodium, silver, platinum, chromium and copper are all exceptions of Aufbau’s principle because of half-filled or fully filled subshells. In the lower atomic number, the difference between the energy levels for the normal sequence of electron shells is larger and exceptions are not as common.