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Organic molecules such as molecules containing only C and H, form bonds by sharing electrons in covalent bonds. The number of bonds an atom makes depends on how many electrons it needs to reach a noble gas configuration.

Carbon makes 4 bonds to get the noble gas configuration of neon. H makes 1 bond to get the Nobel gas configuration of helium. C and H cannot exceed these numbers of bonds because their valence shells can fit only 8 and 2 electrons, respectively.

There is no way to arrange 2 C atoms and 7 H atoms while following these rules. Below are some representations of C₂H₇, all of which are impossible

The underline atoms showing the exceed in valency number

The atoms shown make more than the allowed number of bonds.

An organic molecule cannot have the formula because that would require a C atom to have more than 4 bonds or an H atom to have more than 1 bond.