Covalent Bond Formation

A covalent bond occurs when two atoms share a pair of electrons to complete their outermost shells. This contrasts with an ionic bond, where one atom donates electrons, and the other receives them.

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A diagram illustrating the covalent bonding process between two atoms. On the left and right are atoms represented by large circles, each with one smaller circle labelled 'Single Electron'. In the centre, the two atoms are shown with their electron circles overlapping, labelled 'Shared Electrons,' with a pair of electrons depicted in the overlap area, one marked with an 'x' and the other with a dot, signifying the electron pair that make the covalent bond.

Let’s look at an example.

The Covalent Bonding of Water

The chemical formula of water is H2O, which tells us that one water molecule is made up of two hydrogen atoms and one oxygen atom. These atoms form water through covalent bonding.

First let’s remember the maximum capacity of electrons that elements can have in their outermost shell:

ShellMaximum Number of Electrons
First2
Second8
Third8

Hydrogen has one electron in its only shell, which is also its outermost shell. This shell can hold a maximum of two electrons. By sharing an electron with another atom, hydrogen can fill this shell.

It’s important to remember that covalent bonds can also form between atoms of the same element, unlike ionic bonds.

Let’s look at the covalent bonding between two hydrogen atoms to form a hydrogen molecule:

A sequence of diagrams showing the formation of a covalent bond between two hydrogen atoms. The first two diagrams depict separate hydrogen atoms, each with a blue circle representing an electron and a red plus sign indicating the proton in the nucleus. Below each atom is labelled 'Hydrogen Atom H.' The third diagram shows a blue arrow labelled 'Bonding' pointing towards the fourth diagram. This final diagram illustrates the two hydrogen atoms with their electron circles overlapping, forming a 'Hydrogen Molecule H2' with a 'Single Covalent Bond H-H' indicated by the pairing of the two electrons between the nuclei.

Oxygen, with six electrons in its outermost shell, seeks two additional electrons to achieve a full shell of eight electrons.

Let’s look at the covalent bonding between two oxygen atoms to form an oxygen molecule:

A series of diagrams depicting the formation of a double covalent bond in an oxygen molecule. Two separate oxygen atoms are shown, each with eight electrons in outer shells, arranged in pairs. In the final image, two oxygen atoms are bonded, sharing two pairs of electrons, indicated by the overlapping shells and four shared electrons. This results in an oxygen molecule, O2, with a double covalent bond, as annotated by the equal sign between two O letters.
  • You can see in this case that each oxygen atom shares two of its electrons, rather than one.

Below, oxygen shares two electrons with each of two hydrogen atoms, allowing all involved atoms to complete their outermost shells.

A diagram illustrating the formation of water (H2O) from hydrogen and oxygen molecules. On the left, two hydrogen molecules (H2), each with a single covalent bond, are shown above and below an oxygen molecule (O2) with a double covalent bond. Each hydrogen molecule is composed of two hydrogen atoms with one shared pair of electrons, and the oxygen molecule consists of two oxygen atoms sharing two pairs of electrons. An arrow indicates their reaction to form two water molecules, depicted on the right. In the water molecules, oxygen and hydrogen atoms are connected by polar covalent bonds, with oxygen having a partial negative charge and hydrogen having a partial positive charge.

Comparing Covalent and Ionic Bonds

Covalent BondsIonic Bonds
Involve the sharing of a pair of electrons by two atomsInvolves the loss or gain of electrons by each atom involved
Can occur between atoms of the same element or elements closer on the periodic tableOccur between a non-metal and a metal, typically positioned on opposite sides of the periodic table
There are similar levels of polarity between the involved atomsThere are high differences in polarity between the involved atoms
Result in the formation of moleculesResults in the formation of giant ionic lattices
Molecules have weak intermolecular forces between them, leading to low melting and boiling pointsIonic lattices have strong forces of attraction between ions, leading to high melting and boiling points
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