NONPOLAR COVALENT BOND: CHARACTERISTICS, FORMATION, TYPES - CHEMISTRY - 2024

A nonpolar covalent bond is a type of chemical bond in which two atoms that possess similar electronegativities share electrons to form a molecule.

This type of bond is found in a large number of compounds that have different characteristics, being found between the two nitrogen atoms that form the gaseous species (N ₂), and between the carbon and hydrogen atoms that hold the methane gas molecule together. (CH ₄), for example.

Nonpolar covalent bond of methane. By CNX OpenStax, via Wikimedia Commons

It is known as electronegativity to the property that chemical elements possess that refers to how great or small is the ability of these atomic species to attract electron density towards each other.

The polarity of the nonpolar covalent bonds differs in the electronegativity of the atoms by less than 0.4 (as indicated by the Pauling scale). If it were greater than 0.4 and less than 1.7 it would be a polar covalent bond, while if it were greater than 1.7 it would be an ionic bond.

It should be noted that the electronegativity of atoms describes only those that are involved in a chemical bond, that is, when they are part of a molecule.

General characteristics of the nonpolar covalent bond

The term "nonpolar" characterizes molecules or bonds that do not exhibit any polarity. When a molecule is nonpolar it can mean two things:

-Its atoms are not linked by polar bonds.

-It does have polar type bonds, but these have been oriented in such a symmetrical way that each one cancels the dipole moment of the other.

By Jacek FH, from Wikimedia Commons

Similarly, there are a large number of substances in which their molecules remain linked together in the structure of the compound, whether in the liquid, gas or solid phase.

When this happens it is due, in large part, to the so-called van der Waals forces or interactions, in addition to the temperature and pressure conditions at which the chemical reaction takes place.

These types of interactions, which also occur in polar molecules, occur due to the movement of subatomic particles, mainly electrons when they move between molecules.

Due to this phenomenon, in a matter of instants, electrons can accumulate at one end of the chemical species, concentrating in specific areas of the molecule and giving it a kind of partial charge, generating certain dipoles and making the molecules stay quite close to each other. at each other.

Polarity and symmetry

However, this small dipole is not formed in compounds linked by nonpolar covalent bonds, because the difference between their electronegativities is practically zero or completely zero.

In the case of molecules or bonds made up of two equal atoms, that is, when their electronegativities are identical, the difference between them is zero.

In this sense, bonds are classified as nonpolar covalent when the difference in electronegativities between the two atoms that make up the bond is less than 0.5.

On the contrary, when this subtraction results in a value between 0.5 and 1.9, it is characterized as polar covalent. Whereas, when this difference results in a number greater than 1.9, it is definitely considered a bond or compound of a polar nature.

So, this type of covalent bonds is formed thanks to the sharing of electrons between two atoms that give up their electron density equally.

For this reason, in addition to the nature of the atoms involved in this interaction, the molecular species that are linked by this type of bond tend to be quite symmetrical and, therefore, these bonds are usually quite strong.

How is the nonpolar covalent bond formed?

In general, covalent bonds originate when a pair of atoms participate in the sharing of pairs of electrons, or when the distribution of electron density is equally between both atomic species.

The Lewis model describes these unions as interactions that have a dual purpose: the two electrons are shared between the pair of atoms involved and, at the same time, fill the outermost energy level (valence shell) of each of them, granting them greater stability.

As this type of bond is based on the difference in electronegativities between the atoms that make it up, it is important to know that the elements with the highest electronegativity (or more electronegative) are the ones that attract electrons the most strongly towards each other.

This property has the tendency to increase in the periodic table in the left-right direction and in an ascending direction (bottom-up), so that the element considered as the least electronegative of the periodic table is francium (approximately 0.7) and the one with the highest electronegativity is fluorine (approximately 4.0).

These bonds most commonly occur between two atoms belonging to nonmetals or between a nonmetal and an atom of a metalloid nature.

Ordering and energy

From a more internal point of view, in terms of energy interactions, it can be said that a pair of atoms attract each other and form a bond if this process results in a decrease in the energy of the system.

Likewise, when the given conditions favor that the atoms that are interacting attract each other, they get closer and that is when the bond is produced or formed; as long as this approach and subsequent union involve a configuration that has less energy than the initial arrangement, in which the atoms were separated.

The way in which atomic species combine to form molecules is described by the octet rule, which was proposed by the American-born physicochemist Gilbert Newton Lewis.

This famous rule mainly states that an atom other than hydrogen has a tendency to bond until it is surrounded by eight electrons in its valence shell.

This means that the covalent bond originates when each atom lacks enough electrons to fill its octet, that is when they share their electrons.

To achieve stability in the CO2 structure, the carbon atom is required to form two double bonds with each oxygen atom, thus fulfilling the octet rule.

This rule has its exceptions, but in general terms it depends on the nature of the elements involved in the link.

Types of elements that form the nonpolar covalent bond

When a nonpolar covalent bond is formed, two atoms of the same element or different elements can be joined by sharing electrons from their outermost energy levels, which are what are available to form bonds.

When this chemical union occurs, each atom tends to acquire the most stable electronic configuration, which is the one that corresponds to the noble gases. So each atom generally "seeks" to acquire the closest noble gas configuration on the periodic table, either with fewer or more electrons than its original configuration.

So, when two atoms of the same element join to form a nonpolar covalent bond, it is because this bond gives them a less energetic and therefore more stable configuration.

The simplest example of this type is that of gaseous hydrogen (H ₂), although other examples are the gases oxygen (O ₂) and nitrogen (N ₂).

Two identical hydrogen atoms in which the pair of electrons attract in the same way, which results in there being no polarity in the bond.

Nonpolar covalent bonds of different atoms

A non-polar bond can also be formed between two non-metallic elements or a metalloid and a non-metallic element.

In the first case, the non-metallic elements are made up of those that belong to a select group in the periodic table, among which are halogens (iodine, bromine, chlorine, fluorine), noble gases (radon, xenon, krypton, argon, neon, helium) and a few others such as sulfur, phosphorus, nitrogen, oxygen, carbon, among others.

An example of these is the union of carbon and hydrogen atoms, the basis for most organic compounds.

In the second case, metalloids are those that have intermediate characteristics between non-metals and the species belonging to metals in the periodic table. Among these are: germanium, boron, antimony, tellurium, silicon, among others.

Examples

It can be said that there are two types of covalent bonds. Although in practice these do not have any difference between them, these are:

-When identical atoms form a bond.

-When two different atoms come together to form a molecule.

Between identical atoms

In the case of nonpolar covalent bonds that occur between two identical atoms, the electronegativity of each does not really matter, because they will always be exactly the same, so the difference in electronegativities will always be zero.

This is the case of gaseous molecules such as hydrogen, oxygen, nitrogen, fluorine, chlorine, bromine, iodine.

Nonpolar covalent bond of two identical oxygen atoms.

Between different atoms

On the contrary, when they are unions between different atoms, their electronegativities must be taken into account to classify them as nonpolar.

This is the case of the methane molecule, where the dipole moment formed in each carbon-hydrogen bond is canceled out for reasons of symmetry. This means the lack of separation of charges, so they cannot interact with polar molecules such as water, making these molecules and other polar hydrocarbons hydrophobic.

Other nonpolar molecules are: carbon tetrachloride (CCl ₄), pentane (C ₅ H ₁₂), ethylene (C ₂ H ₄), carbon dioxide (CO ₂), benzene (C ₆ H ₆) and toluene (C ₇ H ₈).

Nonpolar covalent bond of carbon dioxide.

References

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