Same Molecular Formula, Different Shape

(Polar molecules, Non-polar molecules, etc.)

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Alison Trinh 1E
Posts: 104
Joined: Thu Jul 25, 2019 12:16 am

Same Molecular Formula, Different Shape

Postby Alison Trinh 1E » Sun Nov 17, 2019 8:30 pm

What causes a molecule to have a different molecular shape than another molecule with the same molecular formula such as in the case of pentane and 2, 2 dimethylpropane?

Midori Hupfeld 3D
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Joined: Fri Aug 30, 2019 12:17 am

Re: Same Molecular Formula, Different Shape

Postby Midori Hupfeld 3D » Sun Nov 17, 2019 8:35 pm

They have their dipoles in different areas, so dimethylpropane is spherical, which makes the bonds less strong compared to pentane which has rod-shaped dipoles. The rod shaped dipoles are closer together which makes the bonds stronger.

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Re: Same Molecular Formula, Different Shape

Postby Chem_Mod » Sun Nov 17, 2019 8:41 pm

The molecular formula does not always give you the exact molecular shape, as shape depends on the location of bonding and lone pairs of electrons. This is especially true with organic compounds, where multiple molecules have the same molecular formula (number of each atom). This is why in problems, if the structure of the molecule is not obviously apparent they give you the structural formula. This gives you a better idea of how atoms are bonded to each other in a molecule. In this case, pentane and 2,2-dimethylpropane have the same molecular formula but differ in the arrangement of the atoms (we call these constitutional isomers), hence their different shapes.

Also, to clarify the reply above, note that the intermolecular interactions between the two types of molecules are London dispersion forces, or induced dipole-induced dipole interactions (they are nonpolar and have no permanent dipole moment).

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