Are ion bonds polar or non-polar?

Polarity (chemistry)

polarity In chemistry, denotes the formation of separate centers of charge caused by a charge shift in atomic groups, which means that an atomic group is no longer electrically neutral (see also Pol). The electric dipole moment is a measure of the “polarity” of the molecule.

Polar fabrics

A more polar Substance consists of polar molecules, which are characterized by a permanent electrical dipole moment.

Polar fabrics dissolve well in polar solvents (such as salts in water). The more similar the interaction forces between the particles of the solvent and between those of the solute, the better the solubility.

If the difference in electronegativity (ΔEN) exceeds a certain limit value (approx. 1.7), the binding electrons completely pass from one binding partner to the other. Two ions remain, which only attract each other due to the undirected electrostatic Coulomb force. As charge carriers, ions are basically "polar", i. H. Salts are always polar substances.

The polarity of an entire molecule is caused by polar atomic bonds, or in extreme cases by ionic bonds. Polar bonds are characterized by the uneven distribution of binding electrons between the binding partners. If atoms with different electronegativity connect, this results in such a polarization of the bond. If there are only polarized atomic bonds in a molecule, the individual dipole moments of the bonds add vectorially to form a total dipole moment. If this is zero due to symmetry, the substance is non-polar (example: carbon dioxide, CO2). However, if there is a permanent total dipole moment other than zero, the molecule is polar (example: water molecule). Depending on the size of this total dipole moment, a substance is more or less polar. The difference is therefore flowing from extremely polar to completely non-polar. Solvents are arranged in an elutropic series based on their polarity.

Polarity reversal: polarity of chloromethane (left) and the Grignard compound methyl magnesium chloride produced from it

In organic chemistry, polar atomic bonds play an important role in the qualitative assessment of the reactivity of a molecule. In a haloalkane (example: chloromethane) z. B. assigned the partial charge δ− to the chlorine atom covalently bound to the carbon atom and the partial charge δ + to the carbon atom of the methyl group. If chloromethane is combined with magnesium to give the corresponding Grignard compound CH3MgCl around, umpolung occurs: The carbon atom of the methyl group now has the partial charge δ−. Considering the polarity of organic substances has significant consequences for their reactivity.

Examples:

Non-polar substances

A non-polar or apolar Molecules, on the other hand, have no permanent dipole moment.

Non-polar substances dissolve well in non-polar solvents (organic substances in benzene or ether). The more similar the interaction forces between the particles of the solvent and between those of the solute, the better the solubility.

Examples:

The dipole moment of a substance determines its solubility or its ability to act as a solvent. The rule of thumb here is that polar substances dissolve well in polar solvents, but poorly soluble in non-polar solvents. Non-polar substances, on the other hand, are good in non-polar solvents (e.g. gasoline, hexane), but poorly soluble in polar solvents. Similar to the doctrine of the medieval alchemists "similia similibus solvuntur" (lat: "similar is solved by similar").

Many salts are also readily soluble in the polar solvent water due to their ionic structure, whereas non-polar substances such as fat or wax are not.

Many flavorings or fragrances, for example, are not soluble in water and are therefore dissolved in an oil or in ethanol, which is why alcohol is also listed as an ingredient in many foods.

A simple experiment that can be used to determine that water has a permanent electrical dipole moment is the following:
For example, you can charge a plastic comb by combing dry hair or rubbing it on a wool sweater. Now you let a very thin stream flow from a tap, just so that it does not tear off and drip. If you now carefully approach the comb to the water jet, it makes an arc and approaches the comb. The water jet must not touch the comb, otherwise the comb will be discharged.

The explanation for this behavior is simple: In the inhomogeneous electric field, the dipoles of the water molecule align themselves in such a way that they point towards the ridge. Due to the inhomogeneous field, a greater attractive force acts on the end of the molecule closer to the comb than does the repulsive force on the other end of the molecule. All in all, a small attractive force remains for each water molecule, which then deflects the water jet.

Determination of polarity

To determine whether a compound is non-polar, polar or even an ionic bond, one can use the electronegativity difference $ \ Delta EN $. It is the difference in the electronegativity values ​​of the atoms involved. Guide values ​​for this classification can be seen in the table below.

However, it must be taken into account that charge-separated mesomeric limit formulas can have a weight that cannot be neglected. Despite an electronegativity difference of about 1, carbon monoxide is an almost non-polar gas that can only be liquefied by pressure below −140 ° C.

$ \ Delta EN $ Binding type Characteristics of the bond
0,0 non-polar bond Electron pairs are equally stressed by all atoms, so that no centers of charge arise.
0,1…0,4 weak polar bond One atom demands electron pairs a little more than the other.
0,4…1,7 strongly polar bond One atom puts a much greater strain on electron pairs than the other.
> 1,7 Ionic bond There are no shared electron pairs; That is, ions are formed

See also