Lewis Diagrams
Introduced| English |
|---|
| Lewis diagram/ˈluːɪs ˈdaɪəɡræm/ |
| octet/ɒkˈtet/ |
Drawing the electrons
- Chemists sketch molecules with dots and dashes.
- Each dot is a valence electron; each dash is a shared pair.
- From the sketch you can read a molecule's shape and behaviour.
- A few simple rules build the picture every time.
Dots and lines
- A Lewis diagram 路易斯结构式 shows the valence electrons around each atom.
- A shared pair (a bond) is drawn as a line; unshared pairs as dots.
- Atoms aim for a full octet 八隅体 of 8 electrons.
Most atoms are stable with a full outer shell of ____ electrons (an octet).
A full octet is 8 valence electrons.
In a Lewis diagram, a line between two atoms represents...
One line is one bonding pair (2 shared electrons).
Building the diagram
- Count the total valence electrons from all the atoms.
- Connect the atoms with single bonds, then complete each octet.
- Add double or triple bonds if the electrons run short.
What is the first step in drawing a Lewis diagram?
You must know the total electron count before placing anything.
If single bonds leave a central atom short of an octet, you should...
Multiple bonds share more electrons to complete the octet.
When the octet bends
- Hydrogen is happy with just 2 electrons, not 8.
- Some atoms (like P and S) can hold an expanded octet of more than 8.
- Always start from the correct total electron count.
Draw a Lewis structure
Follow the steps to draw a valid Lewis diagram.
Hydrogen should be given a full octet of 8 electrons in a Lewis diagram.
Hydrogen holds only 2 electrons, never 8.
Draw $\text{CO}_2$.
- Valence electrons: C(4) + 2 x O(6) = 16.
- Two C=O double bonds give carbon and both oxygens a full octet.
How many valence electrons does $\text{CO}_2$ have? (C = 4, O = 6 each)
$4 + 2\times6 = 16$ valence electrons.
Count the total valence electrons first and make your diagram use exactly that many -- no more, no less. Hydrogen never takes more than 2 electrons, so never give it an octet. When single bonds leave an atom short, add a double or triple bond rather than extra lone pairs.
A Lewis diagram draws valence electrons as dots and bonds as lines, with atoms reaching a full octet. Count all valence electrons, join atoms with single bonds, complete the octets, and add multiple bonds if electrons are short. Watch the exceptions: hydrogen stops at 2, and some atoms expand past 8.