Representing Covalent Bonds
Applied Science: BTEC Level 3 National Extended Diploma, topic 1.1.6
Dot-and-Cross Diagrams
Dot-and-cross diagrams can be used to represent covalent bonds and to show the sharing of electron pairs.
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Chlorine (Cl2)
- Each chlorine (Cl) atom begins with 7 electrons in its outer shell.
- By sharing 1 pair of electrons in a single covalent bond, each Cl atom obtains a full outer shell with 8 electrons.
Water (H2O)
- Because oxygen has 6 electrons in its outer shell, it needs to have 2 extra electrons shared with it to get a full outer electron shell.
- To do this, it forms a single covalent bond with 2 hydrogen atoms.
Double and triple bonds
- The earlier examples show single covalent bonds (1 shared pair of electrons between atoms), but it is also possible to form double (2 shared pairs) and even triple covalent bonds (3 shared pairs).
- Double and triple bonds are normally stronger and require more energy to break.
Oxygen (O2)
- Each oxygen needs 2 electrons to get a full outer shell.
- So, each oxygen atom shares 2 electrons with the other, leaving 2 pairs of electrons in the shared space between the atoms.
- This is an example of a double covalent bond.
Representing Covalent Bonds
There are many ways of representing covalent bonds in a molecule. 4 ways of representing ammonia are shown below:
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Dot-and-cross with shells
- Dots represent the outer shell electrons of atoms of 1 element.
- Crosses represent the outer shell electrons of atoms of the other element.
- A limitation of this method is that it shows electrons differently for each atom, when they are actually exactly the same.
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Dot-and-cross without shells
- Dots represent the outer shell electrons of atoms of 1 element.
- Crosses represent the outer shell electrons of atoms of the other element.
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Ball-and-stick
- Balls represent atoms.
- Sticks represent bonds between the atoms.
- A limitation of this method is that atoms are much closer together than the diagram shows.
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Lines
- The atomic symbol represents the atom.
- Lines between the atoms represent a covalent bond.
Test yourself on Representing Covalent Bonds
Nitrogen atoms have 5 electrons in their outer shell. What type of bond forms between 2 nitrogen atoms?
When compared with single covalent bonds, double and triple covalent bonds...
What is not a way of representing covalent bonds in chemistry?