The positive and negative charge centers in the molecule do not coincide. From the perspective of the whole molecule, the charge distribution is uneven and asymmetric. Such molecules are polar moleculesPolar bondCombinedDiatomic moleculeMust be polar molecule, polar bond boundPolyatomic moleculeDepending on the structure, such as CHfourIs not a polar molecule.
If the molecular configuration is asymmetric, the molecule is polar molecules.Such as ammonia molecule, HCl molecule, etc.
Judgment of non-polar molecules
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Valence method
Composed of ABnIf the valence of the central atom A is equal to the ordinal number of the family, the compound is non-polar.E.g. CHfour、CClfour、SOthree、PClfiveEtc.
Stress analysis method
If the bond angle (or spatial structure) is known, the force analysis can be carried out. If the resultant force is 0, it is a non-polar molecule.E.g. COtwo、CtwoHfour、BFthreeEtc.
Diatomic molecules composed of the same atom are non-polar molecules。
Those who are not non-polar molecules are polar molecules.
Bond and molecule
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The polarity of bonds and the polarity of molecules are two different concepts. There are both connections and differences between polar bonds and polar molecules.Polar molecules must contain polar bonds, that is, polar bonds are necessary to form polar molecules, and may also contain non-polar bonds.Molecules containing polar bonds are not necessarily polar molecules, that is, polar molecules not only contain polar bonds, but also have asymmetric molecular structures.If all the bonds in the molecule are non-polar, such molecule is generally non-polar.Common types are as follows:
1. Non polar molecules containing polar bonds, such as COtwo、CStwo、CHfourEtc.
2. Non polar molecules containing non-polar bonds, such as Htwo、Cltwo、Ntwo、OtwoEtc.
3. Polar molecules containing both polar and non-polar bonds, such as H-O-O-H, etc.
4. Non polar molecules containing both polar and non-polar bonds, such as H-C ∨ C-H, etc.
Generally, it can be judged from the structure and solubility: exposed hydroxyl, exposed carboxyl, exposedaminoThe material polarity of may be very large;Then, in the solubility experiment, the polarity of the solvent is generally consistent with the polarity of the substance.Look at the diffusion of the point. If the point is very condensed, the polarity of the substance may be higher than that of the solvent. With the diffusion of the solvent, the polarity may be lower.
Influence of molecular polarity on properties
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Solubility
The polarity of molecules has a great influence on the solubility of substances.Polar molecules are easily soluble in polar solvents, and non-polar molecules are easily soluble in non-polar solvents, that is, "the same kind is mutually soluble".
Melting and boiling points
With the same molecular weight, polar molecules have higher boiling points than non-polar molecules.This is because the orientation force between polar molecules is greater than the dispersion force between non-polar molecules.
