Covalent bonding
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Explainer Video
Summary
Covalent bonding
In a nutshell
One type of bonding between atoms is covalent bonding. This summary will review covalent bonding, common examples and properties of covalent substances.
Covalent bonding
Definition
Covalent bonding is the sharing of a pair of electrons between two non-metal atoms in order to gain a full outer shell.
Important points:
- The bond is maintained by the strong electrostatic forces between the positively charged nuclei and negatively charged electrons.
- Outer-shell electrons are the electrons that are shared.
- Each atom involved in the covalent bond will share its own electrons.
Dot and cross diagrams
These diagrams are the best way to represent a covalent bond between atoms.
Example 1
Here is the dot and cross diagram for hydrogen chloride ().
How is this drawn?
- Hydrogen has one electron in its outer shell so needs one more to have a complete shell.
- Chlorine has seven electrons in its outer shell so needs one more to have a complete shell.
- Hydrogen and chlorine will therefore share their outer electrons.
- Each atom is drawn overlapping each other, one of which holds electrons as crosses, and the other as dots.
- Hydrogen now has a full outer shell (two) and so does chlorine (chlorine).
Example 2
What bonding occurs between the two non-metal atoms, hydrogen and oxygen, to form ? Support your answer with a diagram.
Covalent bonding occurs which is the sharing of electrons to form a full outer shell.
Other diagrams
Dot and cross diagrams do not show the relative sizes or arrangement of atoms in space, so alternative diagrams are sometimes used.
diagram | Benefits | drawbacks |
| Displayed formula  | Shows bonding between large molecules very well. | Does not show the 3D structure or which atoms the shared electrons originated from. |
| models 
| Shows atoms, the bonds and their arrangement in space. | Does not show which atoms the shared electrons originated from. |
Simple molecular substances
Definition
These are substances made up of molecules containing a few atoms joined by covalent bonds.
Example | dot-cross diagram | Type of bond |
| Hydrogen, |  | Single covalent |
| Oxygen, |  | Double covalent |
| Nitrogen, |  | Triple covalent |
| Methane, |  | Four single covalent |
| Water, | | Two single covalent |
| Hydrogen chloride, | | One single covalent |
Properties of simple molecular substances
Property | explanation |
| Low melting and boiling points | Atoms are held together by strong bonds, however the molecules are held by weak electrostatic forces of attraction. Low amounts of energy is required to overcome these weak forces so they are often gases/liquids at room temperature. |
| Cannot conduct electricity | There are no delocalised (free) electrons to carry a charge. |
FAQs - Frequently Asked Questions
Why can't covalent substances conduct electricity?
They can't conduct electricity as there are no delocalised (free) electrons to carry a charge.
Why do covalent substances have low melting points, despite the covalent bonds between atoms being strong?
The molecules are held by weak electrostatic forces of attraction. Low amounts of energy is required to overcome these weak forces.
How do you define a covalent bond?
Covalent bonding is the sharing of a pair of electrons between two non-metal atoms in order to gain a full outer shell.