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Why are some structures considered isomers but others are not? Take C3H9N for instance, according to https://www.chegg.com/homework-help/questions-and-answers/four-constitutional-isomers-molecular-formula-c3h9n-q17859870 , only the four checked boxes are isomers, but why are the other 4 not? The other four seem to also satisfy all Lewis rules and do have same formula but with different arrangements. Also, how do we determine which structure is the "original" while which are the isomer(s)? Thanks for all help

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  • $\begingroup$ Count the electrons and that should show the difference. Focus on whether bonds are single or double. $\endgroup$ – Tyberius Jul 11 '18 at 2:35
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    $\begingroup$ All of the (valid) structures are a group of isomers. Among, other example, pentanes, you could say n-pentane is the most basic compared to the two branched isomers, but that's just an arbitrary definition. The isomers of C3H9N don't even have a common name, except the subgroub of the two propylamines. $\endgroup$ – Karl Jul 11 '18 at 6:14
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The supposed answers are wrong. The Lewis dot structures are based on forming an octet of electrons around each atom using the s and p orbitals, typically hybridized to sp3. The four isomers are:

  • Ethylmethylamine

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  • Isopropylamine

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  • Propylamine (which isn't even shown on shown key...)

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  • Trimethylamine

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  • Error 1 - This structure isn't valid since the double bond shown in red box gives carbon and nitrogen both 10 electrons instead of 8.

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  • Error 2 - This structure isn't valid since the double bond shown in red box gives both carbon atoms 10 electrons instead of 8.

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  • Error 3 - This structure isn't valid since the double bond shown in red box gives carbon and nitrogen both 10 electrons instead of 8.

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  • Error 4 - This structure isn't valid since the double bond shown in red box gives carbon and nitrogen both 10 electrons instead of 8.

enter image description here

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