“ The orbitals with lower energies are filled before those with higher energies”. ![]() In other words, each orbital in the subshell is filled up by electrons one by one before the pairing up with opposite spins. In simple terms, it means that every time, electrons are first filled up in an orbital singly and only when needed, do the electrons with opposite spin pair up. “ For a given electron configuration, the lowest energy term is the one with the greatest value of spin multiplicity”. This also limits the number of electrons occupying an orbital to two. It essentially means that if electrons must occupy an atomic orbital, they must have opposite spin. “ No two electrons have the same 4 quantum numbers n, l, m, s”. Read more about Electron Structure The Pauli Exclusion Principle: There are 3 rules which need to be followed for the accurate prediction of the electronic configuration. Filling up of the electrons and the resulting electronic configuration of an element is governed by some rules which are pivotal to the understanding of the chemical processes. The arrangement of electrons within an atom is called the electronic configuration and the electrons are filled up according to the energy of the levels as: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f. The order of increasing energy of orbitals as shown below, is backed by experimental data. The electrons in the 3d orbitals are not able do this and hence are higher in energy. It is expected that the 3p subshell will be followed by 3d, but it is really the 4s which follows. Although their energies are close, 4s is lower in energy because of electrons in this orbital being able to move close to the nucleus due to the symmetrical spherical shape of the s orbital. Its observed that everything looks correct until the 3p subshell is passed. The inter-electronic repulsion inside the atoms is the cause of the chaotic order of the orbital energies. As a replacement, we would consider a diagrammatic representation along with the physical view of the arrangement of electrons. Herein starts the anomalous behaviour which is a result of the complex mathematics which are avoided here.
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