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Sulfur trioxide violates the octet rule. Upon drawing the Lewis dot structure for sulfur trioxide, we see that the central sulfur atom is bonded to three other oxygen atoms by double covalent bonds. Therefore, sulfur is in fact surrouded by 12 electrons and not 8 as per the octet rule. This is explained by the fact that the excited state of sulfur has the following electronic configuration: $1s^22s^22p^63s^13p^34s^03d^2$. As there are 3 oxygen atoms, $sp^2$ hybridised orbitals take part in single $\sigma$ covalent bonds with the unpaired $p_y$ orbitals of the oxygen atoms. In sulfur, there still exist one unpaired $p$ orbital and 2 unpaired $d$ orbitals. These remaining unpaired orbitals take part in $\pi$ covalent bonds with the unpaired $p_z$ orbitals of the oxygen atoms. My question is, why do the electrons in sulfur from the $3s$ and $3p$ orbitals in the ground state promote themselves to the $d$ orbitals rather than one electron going into the intermediate $4s$ orbital and the next electron going into the first $3d$ orbital? After all, according to the Aufbau Principle, $4s$ is less energetic when compared to $3d$.

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I feel you are confused on atomic orbital and molecular orbital.

In the excited state of a single sulfur atom, electron do fill 4s first. You are right here. However, it is a different story in molecules.

In sulfur trioxide the bonding electrons are filling the molecular orbital, not the atomic orbital such as 4s or 3d. It is forbidden that both 3s and 4s participate any hybridization or formation of molecular orbital. So no matter how low is the energy of 4s, because 3s is already participating, 4s can not be part of the bonding.

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  • $\begingroup$ I am repeatedly amazed at the somersaults theorists did to validate d orbital participation in main group chemistry. $\endgroup$
    – Jan
    Dec 6, 2018 at 17:40
  • $\begingroup$ @Jan can I get the reference for this claim "It is forbidden that both 3s and 4s participate in the formation of molecular orbitals? Is it true. $\endgroup$ Jul 2, 2021 at 6:49
  • $\begingroup$ @DeepakArya Why are you pinging me? I did not make the claim nor is it a claim I would put my name under. $\endgroup$
    – Jan
    Jul 2, 2021 at 15:09
  • $\begingroup$ @Jan Don't take it anything more, I tried @ Coconut and I think he/she is not here anymore. It is just that I have this doubt for over a week and searched this site and books. So I thought maybe a better knowledgeable person can at least point me to a better source. If you disagree with my behavior, I will delete both of my comments. (P.S. I asked a similar question but ppl took that down) $\endgroup$ Jul 3, 2021 at 3:37
  • $\begingroup$ @DeepakArya in the help centre you will find that the post owner (here: Coconut) always automatically receives a ping for a comment under their question or answer. One additional user can be pinged by @-ing their username if they edited the post or commented previously. $\endgroup$
    – Jan
    Jul 5, 2021 at 7:41

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