Chemical polarity

Chemical polarity, also known as bond polarity or just polarity, is a concept in chemistry which describes how equally bonding electrons are shared between atoms. It is a physical property of compounds and affects other physical properties such as solubility, melting points and boiling points. Polarity also affects intermolecular forces, leading to some compounds or molecules within compounds being labelled as polar or non-polar.

A commonly-used example of a polar compound is water (H2O). The electrons of water's hydrogen atoms are strongly attracted to the oxygen atom, and are actually closer to oxygen's nucleus than to the hydrogen nuclei; thus, water has a relatively strong negative charge in the middle (red shade), and a positive charge at the ends (blue shade).


Electrons are not always shared equally between two bonding atoms: one atom might exert more of a "pull" on the electron cloud than the other. This "pull" is termed electronegativity and measures the attraction for electrons a particular atom has. The unequal sharing of electrons within a bond leads to the formation of an electric dipole: a separation of positive and negative electric charge.

Atoms with high electronegativities - such as fluorine, oxygen, and nitrogen - exert a greater pull on electrons than atoms with lower electronegativities. In a bonding situation this can lead to unequal sharing of electrons between atoms as electrons will spend more time closer to the atom with the higher electronegativity.

Bonds can fall between one of two extremes - being completely non-polar or completely polar. A completely non-polar bond occurs when the electronegativities are identicle and therefore possess a difference of zero. A completely polar bond is more correctly termed ionic bonding and occurs when the difference between electronegativities is large enough that one atom takes an electron from the other. The terms "polar" and "non-polar" bonds usually refer to covalent bonds. To determine the polarity of a covalent bond using numerical means, the difference between the electronegativity of the atoms is taken, if the result is below 1.7 and above 0.5 then the bond is polar.

Polarity of molecules

A compound is comprised of one or more chemical bonds between atoms. The polarity of each bond within the compound determines the overall polarity of the compound: how polar or non-polar it is. A polar molecule contains polar bonds - bonds which have unequal sharing of electrons between the two atoms involved in bonding. A non-polar compound contains non-polar bonds - bonds which have identical or similar sharing of electrons.

However, a compound's symmetricity and net polarity must also be considered when determining the polarity of the overall molecule . Even if a compound contains only polar bonds, it may be non-polar overall as the direction of the polarities cancel each other out, giving the molecule a net polarity of zero. This occurs in boron trifluoride, which contains three identicle polar bonds all cancelling each other out due to their symmetrical arrangement. Trigonal planar, tetrahedral and linear bonding arrangements often lead to symmetrical, non-polar molecules which contain polar bonds.

Properties and examples

Whilst molecules can be described as "polar" or "non-polar" it must be noted that this is often a relative term, with one molecule simply being more polar or more non-polar than another. As such, there are no ultimate properties which can be ascribed to polar or non-polar molecules. However, the following properties are typical of such molecules.

Polar molecules
Examples of household polar molecules include table salt, ammonia and sugar (glucose). Polar molecules are generally able to dissolve in water (hydrophilic) due to the polar nature of water - like dissolves like.

Non-polar molecules
Examples of household non-polar compounds include fats, oil and petrol. Most non-polar molecules are water insoluable (hydrophobic) at room temperature. However many non-polar organic solvents, such as turpentine, are able to dissolve non-polar substances - like dissolves like.

When comparing a polar and nonpolar molecule with similar molar mass, the polar one generally has a higher boiling point.

Predicting Polar and Nonpolar Molecules


Formula: AB | Description: Linear Molecules | Example: CO
Formula: HAx| Description: Molecules with a single H | Example: HCl
Formula: AxOH | Description: Molecules with an OH at one end | Example: C2H5OH
Formula: OxAy | Description: Molecules with an O at one end | Example: H2O
Formula: NxAy | Description: Molecules with an N at one end | Example: NH3


Formula: Ax | Description: All elements | Example: O2
Formula: CxAy | Description: Most carbon compounds | Example: CO2


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