Chemistry, atoms, ions, bonds / principles and exploration
Molecular shape and polarity — principles and investigation
Molecules with polar bonds are not always polar molecules. Rotate the three-dimensional shapes of water, ammonia, carbon dioxide, and methane and compare the directions of bond dipoles. This is an activity that distinguishes between the polarity of a single bond and the polarity of the entire molecule by the sum of vectors and symmetry.
How does it work?
H₂O·NH₃ is polar, and CO₂·CH₄ is nonpolar. Even if there is a polar bond, if it is symmetrical, the sum can be 0.
Molecular dipole = vector sum of bond dipoles
Try this investigation
- Drag the molecule to rotate it. You can also use the left/right/up/down rotation slider. When you run it, check the bond dipole and sum.
- Run and record H₂O and CO₂ separately and compare bond angles and polarities.
- Record NH₃ and CH₄ to compare symmetry and vector cancellation. Rotation only changes the observation point.
Interpreting your results
Why is H₂O polar and CO₂ nonpolar?
Molecular polarity is determined by the vector sum of three-dimensional bond dipoles. Even if you turn the viewpoint, the polarity of the molecule itself is the same.
Assumptions and limitations
It is a fixed ideal structure of H₂O 104.5°, CO₂ 180°, NH₃ 107°, and CH₄ approximately 109.47°. The size of each bond dipole in the same molecule is set to 1 relative unit. It is not a size comparison between molecules or Debye actual measurements. It does not calculate new stable structures, electron densities, or vibrations. The arrow points toward the negative charge. The bond angle is a three-dimensional angle, not the projected screen angle.