2. Purpose
• The properties of substances are directly influenced by their
structure. Change the shape of a wheel and it will not
function the same way. Change the shape of a water
molecule, and it will behave very differently – though we
cannot change the shape of a water molecule. In order to
understand and predict properties, we have to be able to
understand and predict the shapes of molecules. The first
step to doing this is writing Lewis dot diagrams.
3. What are Lewis Dot Diagrams
• Lewis dot diagrams show the localization of valence electrons – to which
atoms the electrons belong.
• Lewis Dot Diagrams are two dimensional structures. They do not show
the shape of the molecule! We will use them to predict the shape.
4. Lewis Dot Diagrams of Atoms
• To draw the dot diagram of an atom
– Count the valence electrons
– Write the symbol of the atom
– Place dots around the atom one at a time – top, bottom, left, and right
– Once you have four dots – one on each side, then you can pair up
• Example: Draw the dot diagram of oxygen
– O has 6 valence electrons
• Note: Notice how there are four positions – top, bottom, left, and right.
Also notice how we kept the electrons unpaired if possible
5. Dot Diagrams of Period 2 Atoms
• Li – 1 valence electron
• Be – 2 valence electrons
• B – 3 valence electrons
• C – 4 valence electrons
• N – 5 valence electrons
• O – 6 valence electrons
• F – 7 valence electrons
• Ne – 8 valence electrons
7. Pause and Practice
• Draw the dot diagrams of the following:
– Mg
– S
– P
– Br
Note: It does not matter if you choose
to put the dots on the top, bottom, left,
or right. For example, bromine could
have had its single dot in any of the
positions.
8. Dot diagrams of molecules
• Now that we understand the basics of dot diagrams, we are ready to draw
them for molecules. There are a few more rules for drawing dot diagrams
for molecules. Follow the rules and the dot diagrams should not be too
difficult.
9. The process
• Following the process to make it easy:
1. Count the total number of valence electrons for all of the atoms in
the molecule
2. Place the least electronegative atom (left most on the periodic
table) in the center. Place other atoms on the top, bottom, left and
right. If more than four surrounding atoms are present, place them
between the four sides.
3. Draw single bonds between the central atom and the surrounding
atoms.
4. Place left over electrons on surrounding atom in pairs until each has
its octet (or duet for H) complete including the bonding electrons.
5. If you still have electrons, place them on the central atom.
6. If octet on the central atom is incomplete, pull in a pair of electrons
from a surrounding atom to make a double bond.
10. The Octet Rule
• Octet Rule – an octet is 8 valence electrons for an atom. When drawing
dot diagrams most of the elements we encounter will want to achieve an
octet of electrons.
– Exceptions to the octet rule
• Less than an octet:
– Hydrogen – hydrogen gets at most 2 valence electrons (duet).
You never want to have a situation where you have more than
two electrons with hydrogen.
– Boron and Aluminum – boron and aluminum often have only
6 valence electrons associated with them in bonding
situations.
• More than an octet:
– While some atoms can have more than an octet, it will not be
covered in this lesson.
11. Bonds and Electrons
• A single bond between atoms is made up of two electrons.
• A double bond between atoms consists of 4 electrons.
• A triple bond between atoms consists of 6 electrons.
12. Example: Draw the dot diagram of OCl2
1. Count the number of valence electrons
• Each chlorine has 7 valence electron for a total of 14.
• The oxygen has 6 valence electrons.
• Total: 14 + 6 = 20 total valence electrons
2. Place the left most atom on the periodic table in the center and the other
atoms, top, bottom, left, or right.
3. Place single bonds between the central atom and the outer atoms
Note: It does not matter if you draw the bonds to the top bottom left or right.
You could have drawn it as or any other combination of top, bottom, left,
and right.
13. Example: Draw the dot diagram of OCl2
4. Place left over electrons on outer atoms until they have 8 total around
them (including the 2 electrons in the bond)
Used: 4
Remain: 16
Used: 6
Remain: 14
Used: 8
Remain: 12
Used: 10
Remain: 10
Used: 12
Remain: 8
Used: 14
Remain: 6
Used: 16
Remain: 4
14. Example: Draw the dot diagram of OCl2
5. Place left over electrons on the central atom
– We have used 16 electrons and have 4 remaining. This allows us to
place two non-bonding pairs (lone pairs) on the central atom.
6. Check for octet on central atom – if incomplete, pull in pairs of electrons
– In this case the oxygen has 8 electrons around it – 2 bonds (4 e- ) and 2
non-bonding pairs (4e-) so the dot diagram is done.
Used: 16
Remain: 4
Used: 18
Remain: 2
Used: 20
Remain: 0
15. Another example – CO2
1. Total valence electrons = 16
2. Draw skeletal structure with single bonds:
3. Fill up octets of surrounding atoms with remaining electrons:
4. Place any left over electrons on central atom – in this case, there are no
left over electrons.
5. If central atom does not have a full octet, pull in non-bonding pairs from
surrounding atoms to form multiple bonds.
16. A few rules of thumb
• Hydrogen will never make more than one bond and it will not have any
nonbonding electrons on it – only a single bond and that is it.
• Halogens usually only make one bond
• Oxygen usually makes two bonds
17. Pause and Practice – follow the rules
• Draw the dot diagrams of the following:
– NF3
– CHCl3 (Carbon is central)
– COCl2 (carbon is central, double bond (4e-) between carbon and
oxygen)
18. Pause and Practice – follow the rules
• Draw the dot diagrams of the following:
– NF3
– CHCl3 (Carbon is central)
– COCl2 (carbon is central, double bond (4e-) between carbon and
oxygen)
