Covalent bond

A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These shared electron pairs help atoms become more stable. When atoms share electrons, they can each attain a full outer layer of electrons, which makes them happier and more balanced.

Covalent bonding is very common in organic chemistry, where molecules are formed by sharing electrons instead of simply sticking together through electrical forces. This type of bonding can take many forms, including different ways atoms connect and share their electrons.

One simple example of a covalent bond is in a molecule of hydrogen, H₂. The two hydrogen atoms share their electrons through covalent bonding. Covalent bonds are strongest when the atoms involved have similar abilities to attract electrons, making the sharing fair and balanced.

History

The idea of covalent bonding started with a scientist named Gilbert N. Lewis in 1916. He described how atoms share pairs of electrons to stay together. He used dots around symbols to show these electrons, called Lewis notation.

Later, in 1919, Irving Langmuir used the term covalence to describe how many pairs of electrons an atom shares with others.

In 1927, Walter Heitler and Fritz London used advanced science to explain how these bonds work in simple molecules.

Types of covalent bonds

Atoms can form different kinds of connections called covalent bonds by sharing tiny particles called electrons. The strongest type is called a sigma (σ) bond, which happens when electron areas overlap straight on. Most single bonds between two atoms are sigma bonds. Another type is a pi (π) bond, which is weaker and happens when electron areas overlap sideways. Double bonds have one sigma and one pi bond, while triple bonds have one sigma and two pi bonds.

The way atoms share electrons can also change based on how much each atom wants the electrons. If two atoms want electrons the same, they form a nonpolar bond, like in hydrogen gas (H–H). But if one atom wants the electrons more, it creates a polar bond, like in hydrogen chloride (H−Cl). Whether a bond is polar also depends on the shape of the molecule.

Covalent structures

Covalent substances can form different kinds of structures. One type is individual molecules, where atoms are held together by strong bonds, but the molecules themselves don’t stick to each other much. These are often gases, like HCl, SO₂, CO₂, and CH₄.

Another type is molecular structures, where the molecules are held together by weaker forces. These are usually liquids with low boiling points, like ethanol, or solids with low melting points, like iodine and solid CO₂.

Macromolecular structures have many atoms linked together in long chains by covalent bonds. Examples include synthetic materials like polyethylene and nylon, as well as natural materials like proteins and starch.

Finally, there are network covalent structures, where atoms are linked in sheets or 3-dimensional patterns. Examples are graphite, diamond, and quartz. These substances usually have high melting and boiling points and can be brittle.

One- and three-electron bonds

Bonds with one or three electrons are found in special types of atoms called radicals, which have an odd number of electrons. The simplest example of a 1-electron bond is in the dihydrogen cation, H⁺₂. These bonds often have about half the strength of regular bonds but can sometimes be stronger, like in dilithium.

The simplest example of a three-electron bond is in the helium dimer cation, He⁺₂. This bond is called a “half bond” because it shares only one electron. Another example is nitric oxide, NO, which has a three-electron bond along with two regular bonds. The oxygen molecule, O₂, can also be thought of as having two three-electron bonds and one regular bond. This explains some of its special properties. Molecules with these unusual bonds are often very reactive and only stay stable between atoms that pull electrons similarly.

Resonance

Main article: Resonance (chemistry)

Sometimes, one picture of how atoms are connected in a molecule isn't enough to explain what we see. We need to think of a mix of different pictures. For example, in a molecule like nitrate, the bonds between nitrogen and oxygen can look different in each picture. When we look at the real molecule, the bonds are kind of in-between, not just single or double bonds.

Aromaticity

Main article: Aromaticity

In organic chemistry, some ring-shaped molecules are extra stable when they follow a special rule. This rule says how many kinds of electrons they need to have. Benzene is a good example — it has a ring of six atoms that share electrons in a special way, making it very stable.

Hypervalence

Main article: Hypervalent molecule

Some molecules, like xenon difluoride and sulfur hexafluoride, have more bonds than we might expect. This happens because their electrons are shared in a more complex way, following special rules that help explain their structure.

Electron deficiency

Main article: Electron deficiency

Some molecules, like diborane, don’t have enough electrons to make the usual kinds of bonds. Instead, they share electrons in groups of three atoms, which helps them stay connected. These bonds give the molecules a special shape and let all the atoms fit together properly.

Quantum mechanical description

After quantum mechanics was developed, two main theories were suggested to explain chemical bonding: valence bond (VB) theory and molecular orbital (MO) theory. Both help us understand how atoms connect by sharing tiny particles called electrons.

Valence bond theory focuses on how atoms share electrons to form strong bonds, while molecular orbital theory looks at how electron areas spread out over the whole molecule. Scientists use these ideas to study things like how much energy a bond has or how molecules change during reactions. Modern computers often use molecular orbital theory because it works well with calculations.

Analogous effect in nuclear systems

Scientists think something similar to covalent bonding might happen in tiny parts of atoms called nuclei. Instead of sharing electrons, certain tiny particles called quarks might share themselves. This sharing of quarks seems to be very important for holding nuclei together, especially when the particles involved have quarks in common.