Dipole molecule
Dipole molecule is an uncommon term for an externally electrically neutral molecule that has a permanent electrical dipole moment because the centers of gravity of its positive and negative charges do not coincide locally. If this property is in the foreground, such a molecule is also called a dipole for short .
Cause of dipole molecules
Polar atomic bonds can lead to a dipole moment of the entire molecule. That depends on the structure of the molecule, because the dipole moments of different bonds in the molecule add up depending on the direction ( vectorial ) and can therefore cancel or reinforce each other. As a diatomic, heteronuclear compound, hydrogen fluoride has a dipole moment. For reasons of symmetry, carbon dioxide has no permanent dipole moment, the atoms are arranged linearly and the oppositely aligned bond dipoles cancel each other out. However, bending vibrations of the molecule break the symmetry and lead to dipole moments and the effect of CO 2 as a greenhouse gas . Water has a larger total dipole moment than hydrogen fluoride, although the polarity of the bond between hydrogen and oxygen atom (H – O) is smaller than that of the bond between hydrogen and fluorine atom (H – F). The cause lies in the addition of the two H – O bond dipoles, which are at a bond angle of approx. 105 ° to one another, and the smaller angle of the free electron pairs to the axis of symmetry .
| connection | Hydrogen fluoride (HF) | Water (H 2 O) | Carbon dioxide (CO 2 ) |
| Structural structure |
|
|
|
| Electronegativity difference | 1.9 | 1.4 | 1.0 |
| Dipole moment μ in Debye | 1.82 | 1.85 | 0 |
As a rule of thumb it can be said that molecules with an asymmetrical structure and a difference in electronegativities (ΔEN) according to Pauling smaller than 1.7 but larger than 0.5 appear as dipole molecules, i.e. In other words, they are electrically neutral towards the outside, but have a (measurable) dipole moment. If the ΔEN is greater than 1.7, the character of an ionic bond is assumed. However, the limit ΔEN <1.7 is to be regarded as a guide value, as shown in the examples of hydrogen fluoride and aluminum chloride (ΔEN = 1.5). If ΔEN is less than 0.5, non-polar molecules are assumed.
Individual evidence
- ↑ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, pp. 304-305.
- ↑ a b c Entry on dipole moment. In: Römpp Online . Georg Thieme Verlag, accessed on April 26, 2012.