For example, this carbon has an incomplete octet:

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Can we say this is because C has configuration $\mathrm{1s^2 2s^2 2p^2}$ and hence here its just forming 3 bonds whereas it should form 4 to make it $\mathrm{2p^6}$ and complete its octet? Or do we say that it's forming 3 bonds and hence has 6 electrons and therefore it's octet isn't complete?

  • $\begingroup$ Even though this structure "exists" on paper or computer screen, it is meaningless in the real world. In other words, this structure does not exist in the real world as drawn and has only come into existence as part of the human effort to describe the chemistry of this world using simple and straightforward symbolism. As you probably know, the true structure is a hybrid of all resonance structures, and the octet rule is satisfied for the central carbon. Therefore, I think this question has no actual answer. $\endgroup$ – Don_S Jul 30 '17 at 8:49
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    $\begingroup$ Ok I agree with that. But my main concern is that how do we see the octet rule in atoms? Like I just want you explain this with respect to this particular structure. If you want I can present another example. I'm actually very confused how to check the octet rule in structures like these. $\endgroup$ – user50191 Jul 30 '17 at 8:54
  • $\begingroup$ Like how can we say the octet isn't complete? By seeing the configuration and seeing the vacancy available or check how many bonds and lone pair an atom has? What're we supposed to do normally? $\endgroup$ – user50191 Jul 30 '17 at 9:02
  • $\begingroup$ The octet rule is a measure that helps you to draw a correct and plausible Lewis structure. The rules for drawing correct Lewis structure are found in every basic chemistry book and should be followed in order. See section 5 in this link: If the octets are incomplete, and more electrons remain to be shared, move one electron per bond per atom to make another bond... - if the octets are incomplete - if you follow the rules in sequence, you should not be left with an incomplete octet for carbon. $\endgroup$ – Don_S Jul 30 '17 at 9:10
  • $\begingroup$ Also, traditionally, when you move (again, on paper...) the electrons from the double bond to the carbonyl oxygen, you should simultaneously move another pair of non-bonding electrons from the other oxygen in order to complete carbon's octet. Even on paper, you should never leave carbon with less than 4 bonds if they can be formed. This 3-bonded, positively-charged carbon is merely a methodological transition state for education purposes. Octet rule does not really apply in these circumstances. $\endgroup$ – Don_S Jul 30 '17 at 9:15

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