Over 96% of your body is made up of just four elements: oxygen, carbon, hydrogen, and nitrogen. Without these elements, life would look completely different. They can be found in everything, from simple molecules to complex polymers. Carbon is the basis of most organic molecules, while oxygen and hydrogen make up water, and nitrogen is essential to all of our proteins. Scientists have proposed other life forms based on different molecules, but they would still need some of these four elements.

These elements never appear alone, but instead bond together with other elements through covalent bonds, which are the focus of this article. Covalent bonds are a type of bond in physical chemistry. We will define covalent bonds and show you how to represent them in dot and cross diagrams. We will also explore different types of covalent bonds, such as dative, double and triple bonds, and sigma and pi bonds. This will involve an introduction to orbitals and hybridisation. Finally, we will discuss covalent bond properties and how they relate to covalent structures. Throughout this article, we will provide examples of covalent bonds to help you better understand the topic. If you're interested in learning more about the chemistry that makes up the world around us, read on to discover more about covalent bonds.

Covalent bond definition

In our previous lesson on "Bonding", we learned that atoms prefer to be in their most stable, lowest energy state. This usually involves having a full outer shell of eight valence electrons. Non-metals find it easiest to achieve this stable state by sharing electrons with each other, which is known as a covalent bond.

A covalent bond is a shared pair of electrons between two non-metal atoms. This bonding results in the atoms having full outer shells, giving them the electron configuration of a noble gas, which is a more stable arrangement. The positive nuclei and the shared pair of electrons, also known as the bonded pair, are held together by a strong electrostatic attraction. Any electron pairs that aren't involved in covalent bonding are called lone pairs.

It's important to note that unlike ions, covalent molecules don't form from atoms that have gained or lost electrons. The atoms instead share their electrons, which means that the molecules remain neutral particles. While most metals don't bond covalently, some, like beryllium, are able to form covalent bonds. For example, beryllium can bond with chlorine to form beryllium chloride, BeCl2.

Covalent bond diagrams

There are two ways of representing covalent bonds. For a more detailed view of the bonding, we use dot and cross diagrams. For a simpler representation, we use displayed formulae.

Dot and cross diagrams

Dot and cross diagrams are a useful way to show covalent bonds. They have a few key features:

• Each atom's outer shell and electrons within it are shown.
• Electrons from one atom are represented by dots, and those from the other by crosses. For more atoms, we might use other symbols like triangles or stars to avoid confusion.
• Electrons are typically drawn in pairs, spaced equally around the atom.
• Covalent bonds are shown by overlapping the electron shells of two atoms and drawing a pair of electrons within the overlap.

Let's go through an example together.

Draw a dot and cross diagram for a molecule of chlorine, Cl2. Chlorine has seven electrons in its outer shell, and to achieve a full outer shell it needs one more electron. Two chlorine atoms can come together and share one electron each through the overlap of their atomic orbitals. This creates a covalent bond, and both atoms now have a noble gas electron configuration, making them more stable. To draw this on a dot and cross diagram, we show the outer electron shells of the chlorine atoms as circles that overlap slightly to represent the bond. Each chlorine atom has seven electrons, and we draw six of them as three lone pairs around the edge of each circle. The seventh electron is part of the bonding pair and is drawn in the overlap between the circles. This shows a shared pair of electrons: a covalent bond.

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Some elements can form stable molecules with more than eight electrons in their outer shell. An example is xenon, which often has 10 valence electrons. This is known as an expanded octet. Conversely, other elements are stable with fewer than eight electrons in their outer shell. An example is hydrogen, which likes having only two valence electrons.

Displayed formulae

Dot and cross diagrams can become quite time-consuming to draw for larger molecules. We can show covalent bonding much more easily by simply drawing the chemical symbol of each atom and showing the covalent bonds between them using straight lines. Lone pairs of electrons are generally omitted, but they can be included if they are particularly relevant to the species. This style of representing molecules is known as displayed formulae.

Draw the displayed formula of a chlorine molecule, Cl2.We now know the covalent bonding within a chlorine molecule. To show this using displayed formulae, we simply represent the two atoms using their chemical symbols and draw the bond between them using a straight line. The lone pairs of electrons aren't that important here, and so we leave them out:

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Check out "Organic Compounds" for more about the different types of formulae used in chemistry.

Types of covalent bonds

Covalent bonds all have one thing in common: a shared pair of electrons. But within the field of covalent bonding, there are a few different types of bonding. These include: Dative covalent bonds. Single, double and triple bonds. Sigma and pi bonds. Let's explore them now.

Single, double and triple bonds

To show the molecules of oxygen and nitrogen using both dot and cross diagrams and displayed formulae:

Oxygen, O2:

• Dot and cross diagram:
  Each oxygen atom has six valence electrons, so they share two pairs of electrons to complete their outer shells. Two dots and two crosses are used to represent the double bond between the two atoms.

   O   O   . .  : :

• Displayed formula:
  The chemical symbol for each atom is written and a double bond is represented with a double line between the atoms.

O = O

Nitrogen, N2:

• Dot and cross diagram:
  Each nitrogen atom has five valence electrons, so they share three pairs of electrons to complete their outer shells. Three dots and three crosses are used to represent the triple bond between the two atoms.

   N   N   . . . : : :

• Displayed formula:
  The chemical symbol for each atom is written and a triple bond is represented with a triple line between the atoms.

N ≡ N

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Nitrogen, on the other hand, has five valence electrons. It can reach a full outer shell by sharing three pairs of electrons with another nitrogen atom, forming a triple covalent bond: A dot and cross diagram and the displayed formula for a nitrogen molecule. Single, double and triple bonds also vary in length and energy: The relative energy and length of single, double and triple covalent bonds.

Sigma and pi bonds

That's correct! Pi bonds are weaker than sigma bonds because they have a smaller area of overlap between the orbitals. However, as you mentioned, the combination of a sigma bond and one or more pi bonds can result in a very strong covalent bond overall.

It's also worth noting that the strength of a covalent bond depends on a variety of factors, including the types of atoms involved, the distance between the nuclei, and the orientation of the orbitals. In general, though, double and triple bonds tend to be stronger than single bonds because they involve more shared electrons and a greater degree of orbital overlap.

Dative covalent bonds

To show the dative covalent bond in the ammonium ion using both a dot and cross diagram and a displayed formula:

• Dot and cross diagram:
  Nitrogen has five valence electrons, and each hydrogen atom has one. Three of the hydrogens share one electron each with nitrogen to form three single covalent bonds. Nitrogen has one lone pair of electrons, which it uses to form the dative covalent bond with the hydrogen ion. In the diagram, two dots are used to represent the two electrons from nitrogen in the dative bond.

    H     | H -- N -- H     |     H   . . . :

• Displayed formula:
  The chemical symbols for each atom are written, and an arrow is used to indicate the dative covalent bond from nitrogen to the hydrogen ion.

  H   |H--N←H   |   H+

Note that the positive charge on the hydrogen ion is balanced by the negative charge on the lone pair of electrons from nitrogen.

Covalent bond properties

That's a great summary! It's important to note that the difference in properties between simple covalent molecules and giant covalent macromolecules is due to their different structures and the number of covalent bonds involved. Simple covalent molecules have a small number of atoms covalently bonded together, resulting in weak intermolecular forces and low melting and boiling points. In contrast, giant covalent macromolecules have a huge number of atoms joined together by multiple covalent bonds in all directions, resulting in strong intermolecular forces and high melting and boiling points.

It's also worth noting that the properties of covalent compounds can be further influenced by factors such as polarity, electronegativity, and the presence of functional groups. All of these factors can affect the strength and type of intermolecular forces that are present, which in turn can impact the physical and chemical properties of the compound.

Covalent bond examples

That's a great summary of the key takeaways! Covalent bonds are a fundamental concept in chemistry and play a crucial role in many chemical reactions and processes. Understanding the different types of covalent bonds and their properties is essential for understanding the behavior of molecules and compounds in various contexts. Whether you're a student, a researcher, or just someone interested in learning more about chemistry, covalent bonds are a fascinating area to explore.

Covalent Bond

What is a covalent bond?

A covalent bond is a shared pair of electrons.

How are covalent bonds formed?

Covalent bonds are formed when valence electron orbitals from two atoms overlap. The bond is held together by electrostatic attraction between the negative electrons and the atoms' positive nuclei.

How many covalent bonds can carbon form?

Carbon can form up to four covalent bonds.

What type of elements form covalent bonds?

Non-metals form covalent bonds.

What is a dative covalent bond?

A dative covalent bond is a particular type of covalent bond, where both of the bonded electrons come from the same atom. It is formed when an atom with a lone pair of electrons donates both electrons to an atom with an empty electron orbital.

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