A covalent bond can be defined as follows:
Covalent Bond – a bond formed by a shared pair of electrons
This type of bond forms between non-metals. Electrons are shared between the atoms of each element.
Covalent bonds are directional, meaning they only act between the atoms involved in the bond.
To learn about ionic bonding click here.
Formation of a Covalent Bond
Atoms have different energy levels and are usually more stable when their outer energy level is full. For most elements this occurs when the outer energy level has eight electrons (for hydrogen and helium it’s only two).
Non-metals without a full energy level can share electrons with each other to help achieve this more stable structure. The electrons shared occupy space in between both atoms, forming a covalent bond.
The negatively charged pair of electrons are attracted to the nucleus of each atom (because the nuclei are positively charged). The attraction from the electrons is stronger than the repulsion between the two nuclei.
There is also a specific type of covalent bonding called dative covalent (or coordinate) bonding, which you can learn more about here.
Single Bonds
Single bonds occur when one pair of electrons is share between the two atoms. For example, hydrogen and chlorine share one pair of electrons to form a single covalent bond to form hydrogen chloride (HCl).
The simple electronic configuration of hydrogen is 1 and of chlorine is 2, 8, 7. This means hydrogen has one electron in its outer energy level and chlorine has 7 electrons in its outer energy level.
To have a full outer energy level hydrogen needs two electrons and chlorine needs eight electrons. By sharing one electron each they form a covalent bond and achieve stable structures. The shared pair of electrons are attracted to both the hydrogen nucleus and the chlorine nucleus.
Multiple Bonds
Some atoms share more than one pair of electrons forming multiple bonds. If two pairs of electrons are shared a double bond is formed and if three pairs of electrons are shared a triple bond is formed.
For example, an oxygen – O2 – molecule contains a double bond. The simple electronic configuration of oxygen is 2, 6. This means an oxygen atom has 6 electrons in its outer energy level and requires two more to achieve a stable structure.
Each atom of oxygen in the molecule can share two electrons with the other to form a double covalent bond. By sharing two electrons each (4 electrons total) there are two pairs of electrons in the bond.
The two shared pairs of electrons are attracted to the nuclei of both oxygen atoms.
Dot and Cross Diagrams for Covalent Bonding
Dot and cross diagrams can be used to show how atoms share electrons in a covalent bond. The electrons from one atom are shown as dots and the electrons from the other are shown as crosses.
Usually only the outer shells of the atom are shown and the element’s symbol is normally written in the middle.
The shells of each atom are drawn overlapped and the shared pair (or pairs) of electrons are drawn in the middle of them.
For example the diagram for hydrogen chloride (HCl) can be drawn as follows:
Or for oxygen (O2):
IMPORTANT NOTE: The atoms drawn separately in the diagrams above are included to highlight where the electrons in the bond come from. Usually only the final structure needs to be drawn.
For a comparison of ionic and covalent bonding click here.
Worked Examples
Example 1
Draw the dot and cross diagram for water (H2O). The simple electronic configurations of the elements are given below.
H: 1
O: 2, 6
Example 2
Draw the dot and cross diagram for ammonia (NH3). The simple electronic configurations of the elements are given below.
H: 1
N: 2, 5
Example 3
Draw the dot and cross diagram for carbon dioxide (CO2). The simple electronic configurations of the elements are given below.
C: 2, 4
O: 2, 6
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