Answer to Question #94238 in Organic Chemistry for ibrahim jibril

Question

What principles or rules are violated in the following electronic configuration \\(1s^{2}2s^{3}\\)?

Answer 1

There are a set of general rules that are used to figure out the electron configuration of an atomic species: Aufbau's Principle, Hund's Rule and the Pauli-Exclusion Principle. 

 The Aufbau principle states that electron's occupy orbitals in order of increasing energy. The order of occupation is as follows:

1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p

This rule is not violated in the electron configuration given:

$$1S^2 2S^3$$

Hund's Rule states that when electrons occupy degenerate orbitals (i.e. same n and l quantum numbers), they must first occupy the empty orbitals before double occupying them. Furthermore, the most stable configuration results when the spins are parallel :

This rule is not violated in the electron configuration given as we do not have here degenerate orbitals:

$$1S^2 2S^3$$

The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins. In other words maximum of 2 electrons in an orbital but only if they have apposite spins

Here we see that there are three electrons on 2S orbital while there should be only 2. The Pauli Exclusion Principle is violated.