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.

Polarity refers to the dipole-dipole intermolecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, water is thought of to be polar because of the uneven sharing of its electrons. However, methane is considered non-polar because the carbon shares the hydrogen molecules uniformally.

Theory

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 identical 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 identical 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. Polar molecules have slightly positive and slightly negatively charged ends.

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	Description	Example
Polar	AB	Linear Molecules	CO
	HA_x	Molecules with a single H	HCl
	A_xOH	Molecules with an OH at one end	C₂H₅OH
	O_xA_y	Molecules with an O at one end	H₂O
	N_xA_y	Molecules with an N at one end	NH₃
Nonpolar	A_x	All elements	O₂
Nonpolar	C_xA_y	Most carbon compounds	CO₂