I'm to find the violation of a certain rule or principle in the following electronic configuration:

$$\ce{1s^2 2s^2 2p^1_x 2p^0_y 2p^0_z}$$

  1. $n+l$ rule is not violated because: for $\ce{1s^2}$, $n+l=1+0=1$ so it is filled first, for $\ce{2s^2}$, $n+l=2+0=2$ so it is filled second, and so on.

  2. Aufbau Principle is not violated because: the orbitals with minimum energy are filled first with electrons and $\ce{1s < 2s < 2p}$ which is the sequence of increasing energy orbital judged by the $n+l$ rule.

I'm quoting here a paragraph from my chemistry textbook of the chapter Atomic Structure:

The orbitals given by a particular value of $l$ if $n$ is same, have the same energy and such orbitals are called degenerate. Like $l=1$ gives three $p$ orbitals specified as $\ce{p_x, p_y, p_z}$ and the p orbitals are said to be three fold degenerate. These orbitals are filled according to Hund's Rule.

The numerical value of the principle quantum number $n$ for $\ce{2s}$ and $\ce{2p}$ is the same; however, they don't have a particular value of the azimuthal quantum number $l$. For $\ce{2s}$, $l=0$, and for $\ce{2p}$, $l=1$, so $\ce{2s}$ and $\ce{2p}$ are not degenerate. So none of the three principles or rules whether Aufbau's, $n+l$ or Hund's is violated in this electronic configuration.

However, my friend says that there is a violation of the Hund's Rule in this electronic configuration, and according to him the right configuration is: $\ce{1s^2 2s^1 2p^1_x 2p^1_y 2p^0_z}$, which in turn violates the Aufbau Principle, because the orbitals of lower energy level have to filled first completely and $\ce{2s < 2p}$.

So, which one of the electronic configurations is right, $\ce{1s^2 2s^2 2p^1_x 2p^0_y 2p^0_z}$ or $\ce{1s^2 2s^1 2p^1_x 2p^1_y 2p^0_z}$?

Note: The required electronic configuration is for the electrons in their ground state.

  • $\begingroup$ When you write electronic configurations, you should distinguish your orbitals by $n$ and $l$ only, but not by $m_l$ (or $m_s$), for two reasons. Firstly $m_l$ is not well defined in an atom unless you have an external field; secondly because the electronic configuration is a rough approximation that does not distinguish the energies of different states depending on how you distribute the electrons for different values of both $m_s$ and $m_l$. Indeed the distribution of the $m_s$ has a stronger effect than that of $m_l$ in general. $\endgroup$
    – perplexity
    Commented Oct 17, 2013 at 10:21
  • $\begingroup$ Both are correct although of different energy as has been pointed out. However, you cannot add a label to decide if the electron is in $p_x$ or in $p_y$ or $p_z$ as they are equivalent unless the symmetry is broken by an external magnetic or electric field. $\endgroup$
    – porphyrin
    Commented Dec 3, 2016 at 14:51

4 Answers 4


If we talk only about electronic configurations and rules violation then you are correct. The 2nd one (excited state) is referred for hybridization that is sp2. An excited state configuration is meant only when the particular atom gets ready or excited for forming a bond. Like boron does it with 3 fluorine atoms to form $\ce{BF3}$. So, if we talk about electronic config and rule violations we must go for ground states exceptional cases are $\ce{Cu}$ , $\ce{Cr}$ and $\ce{Mo}$.


No that's wronge. the first one is right which is called ground state for an atom. $\mathrm{(2s)^2}$ must be filled first becouse from hybridization concept we know that S-sub shell is more closer to the nucleus then thw P-sub shell and thus will have lower energy, so $\mathrm{(1s)^2 (2s)^2 (2p)^1}$ is the right configuration for Boron, in this case neither Aufbau rules are voilated nor hund's rule becouse of lower energy of S-sub shell.


It is violating Hunds Rule.. " If degenerated orbitals are available then electrons want to live separately with same spin rather than mutually in same orbital with opposite spin."


The upper one is correct because it is according to the rules of electronic configuration and it does not violate any rule, while the lower one is not in line with the Afbau principle which states that the orbitals are filled with electrons in increasing order of the orbital energies.


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