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Given was the fact there are 3 isomers of pentane, two were given: pentane and 2-methyl butane. Then I was asked to draw to other one. I drew up the star like structure and checked it to be ok (coincidental I might add).

Then I drew some more and came up with amore turnstile shaped molecule.

     H
     | 
   H-C-H   H   H
     |     |   |
   H-C-----C---C-H
     |     |   |
   H-C-H   H   H
     |
     H

To me this also seems like $\ce{C5H12}$ molecule and all valences seem ok. Why isn't this a valid isomer?

Edit: this is 2-methyl butane according to the book:

        H
        | 
   H  H-C-H   H   H
   |    |     |   |
H--C----C-----C---C-H
   |    |     |   |
   H    H     H   H
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    $\begingroup$ It is surely valid, and is known as 2-methyl butane. $\endgroup$ – Ivan Neretin Aug 18 '16 at 18:59
  • $\begingroup$ That one has 4Cs adjecent to each other, doesn't look like this according to the book.. (see edit) and if they are the same, then why? Can you just rotate the methyl group and the hydrogen? Am I thinking too much in 2 dimensions? $\endgroup$ – Apeiron Aug 18 '16 at 19:02
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    $\begingroup$ A man with arms outstretched and with arms down does not look quite the same, but still we will recognize him instantly. Same thing with molecules. Yes, the real molecule is very much 3D. Yes, you kinda can rotate (unless there is a double bond, but that's another story). $\endgroup$ – Ivan Neretin Aug 18 '16 at 19:06
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    $\begingroup$ Right, try using sticks and balls, bonds are 109.5° . $\endgroup$ – Ben Welborn Aug 18 '16 at 19:09
  • $\begingroup$ Ok, clear now! If you change it to/add an answer I can approve it :) $\endgroup$ – Apeiron Aug 18 '16 at 19:10
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To clarify Apeiron's answer. You have drawn 2-methyl butane, the same isomer that was given in the question. The rotation of a bond is probably the source of any confusion. A 3D model of your two structures would be the same.

To answer your question:

Neopentane is the third isomer of the pentane series.

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