Covalent Bond

Just four simple elements make up over 96 percent of your body: oxygen, carbon, hydrogen, and nitrogen. Life without these elements would look completely different. Carbon is the basis of most organic molecules, oxygen and hydrogen combine to make water, and nitrogen is a fundamental part of all of the proteins we are made of, such as the collagen that makes up our bones and the enzymes that catalyse cellular reactions. Scientists have proposed life forms based on other molecules - for example, ammonia-based life - but even these would require some of the above elements.

You’ll notice that these elements are all non-metals. They don’t tend to exist as single atoms. Even in their most basic form, oxygen atoms go round in pairs whilst carbon can form long, branched chains. It is fair to say that life revolves around these elements and the bonds between their atoms, but what exactly are these bonds?

Covalent bonds form between two non-metal atoms through the overlap of some of their outer shell electrons. It usually results in the atoms having full outer shells. This gives them the electron configuration of a noble gas, which is a more stable electron arrangement. Strong electrostatic attraction between the positive nuclei and the shared pair of electrons, also known as the bonded pair, holds the atoms together.

Covalent bond diagrams

We can show covalent bonds using dot and cross diagrams. Let’s work through an example together.

A chlorine atom has seven electrons in its outer shell. Two chlorine atoms can come together to form a chlorine molecule held together by a covalent bond. Remember, a covalent bond is just a shared pair of electrons, and so to create this bond, each chlorine atom shares one of its valence electrons. These electrons, one from each atom, form a shared or bonded pair. The pair sits in the outer shell of both chlorine atoms, forming a covalent bond. Because each chlorine atom now has an extra electron in its outer shell, they both have a noble gas electron configuration and are more stable. This is shown in the diagram below, known as a dot and cross diagram:

A dot and cross diagram showing a chlorine molecule.StudySmarter Originals

You should follow these steps when drawing dot and cross diagrams:

• Represent the electrons from one atom as dots and the other as crosses.
• Draw the electrons in pairs.
• Only show the outer shell electrons.
• Draw the outer shells of the atoms involved with an overlap and place the shared pair in the overlapping section.

You can also show covalent bonds in molecules using a line drawn between the atoms, as in the example below:

A methane molecule. The carbon is bonded to four hydrogen atoms using covalent bonds.commons.wikimedia.org

Types of covalent bond

In the above example, chlorine atoms share just one pair of electrons to achieve full outer shells. However, sometimes atoms need more than that if they want to get a stable electron configuration. We’ll first go over single bonds and then explore examples of double covalent bonds in molecules.

Single bonds

In a single covalent bond, each atom shares just one of its electrons. An example is the hydrogen molecule, H₂. Hydrogen atoms have one electron in their outer shell, but if two hydrogen atoms share their electrons with each other they’ll both have two, completing the outer electron shell:

Two hydrogen atoms, joined by a single covalent bond, form a hydrogen molecule.StudySmarter Originals

Double bonds

In a double covalent bond, each atom shares two of its own electrons. An example is oxygen, O₂. In total there are four electrons shared between the two atoms.

An oxygen molecule. Notice how each atom shares two of its electrons. Both atoms now have a full outer shell.StudySmarter Originals

Dative covalent bonds

In all of our above examples, two atoms both provide shared electrons to form a covalent bond. But what if both electrons came from the same atom? This is a special type of covalent bond known as a dative covalent bond or a coordinate bond.

To form a dative bond, you need a species with a lone pair of electrons and a species with an empty electron orbital. We represent the bond using an arrow drawn from the donor species towards the receiving species. However, dative bonds are exactly the same as regular covalent bonds in all other regards - they are the same length and have the same properties.

An example is ammonia, NH₃. Ammonia already has three covalent bonds between the nitrogen and hydrogen atoms, but the nitrogen has a remaining lone pair of electrons. If it encounters a hydrogen ion, H⁺, the nitrogen atom can donate this lone pair to form a dative bond, as shown below:

An ammonium ion. The nitrogen atom donates its lone pair of electrons to the hydrogen ion to form a dative covalent bond, shown by the arrow.StudySmarter Originals

Covalent structures

The two most abundant elements that make up the Earth, oxygen and silicon, both contain covalent bonds, but in their elemental forms they are structured in very different ways. Whilst oxygen atoms go around in pairs, silicon atoms make up huge crystal structures of indeterminate size. Their contrasting structures give them both different properties.

Simple covalent molecules

Simple covalent molecules are made up of a small number of atoms covalently bonded together. They are generally neutral. Although the covalent bonds themselves are strong, the forces between the individual molecules are weak and don’t require much energy to overcome. This gives simple covalent molecules low melting and boiling points. They don’t tend to dissolve in water and are poor conductors - because they are neutral, they can’t carry a charge. Examples include carbon dioxide (CO₂), methane (CH₄), bromine (Br₂) and of course oxygen (O₂).

Oxygen molecules. Although the covalent bond within each molecule is very strong, the intermolecular forces between the molecules are weak. These are represented by the grey dashed line.StudySmarter Originals

Macromolecules

Macromolecules, also known as giant covalent structures, are lattices of atoms joined together by multiple covalent bonds in all directions.

Macromolecules have high melting and boiling points as all of their covalent bonds are extremely strong and require a lot of energy to overcome. For this same reason they are hard and strong. However, like simple covalent molecules they are insoluble and can’t conduct electricity.

Silicon forms a giant lattice structure much like diamond, another macromolecule. If we compare silicon with oxygen, it is easy to see just how strong covalent bonds are - to melt oxygen you only need to overcome weak intermolecular forces, and so oxygen melts at -218℃. On the other hand, melting silicon requires breaking lots of strong covalent bonds, and so silicon has a melting point of 1410℃!

The structure of silicon. The lattice stretches in all directions and contains many covalent bonds.commons.wikimedia.org