Lewis Dot Structures of Covalent
Compounds
Atoms are made up of protons, neutrons, and electrons. The
protons and neutrons are located at the center of the atom,
the nucleus. These electrons can be divided into core
electrons and valence electrons. The valence electrons are
the outermost electrons and are the ones involved in
chemical reactions
Rules for writing Lewis Dot structures
• Rule 1
Add together the number of valence electrons for each atom
in the molecule. For example, CF4
Carbon has four valence electrons and each fluorine has
seven valence electrons = 4 + 4(7)
= 32
Rule 2
Write out the elements of the molecule so that the least
electronegative elements is in the center surrounded by the
other elements. For example, CF4
F
F C F
F
Rule 3
Place a covalent bond between the central atom and the
outside atoms. Remember each covalent bond contains two
electrons.
F
F C F
F
The four covalent bonds use eight of the 32 valence
electrons in CF4
F
F
C F
• This uses 24 electrons.
There Are no electrons
left, so this is The Lewis
dot structure for CF4
F
• Rule 4
There are 24 valence electrons remaining. Add electrons to
the outer atoms as lose pairs to satisfy the Octet Rule.
Rule 5 for example, NH3
• First apply Rules 1-4 to the molecule
• Rule 1: Count the valence electrons
• Rule 2: Place the least electronegative element at the
centre, except for H which is always an outer atom
• Rule 3: Add covalent bonds between the centre atom and
the outer atoms
• Rule 4: Add lone pairs to the outer atoms
• Rule 5: Add lone pairs to the centre atom
Rule 1
Nitrogen has 5 valence electrons and each hydrogen has 1
valence electron
The total number of valence electrons = 5 + 3 (1) = 8
Rule 2
Hydrogen is always an outer atom and is never at the centre
of a molecule
H NH
H
Rule 3
Add the bonding electrons. This uses 6 of the 8 valence
electrons.
Rule 4
The 2 remaining valence electrons are not added to the outer
atoms, because each H has its maximum of 2 valence
electrons.
• Rule 3
Add the bonding electrons. This uses 6 of the 8
valence electrons.
• Rule 4
The 2 remaining valence electrons are not added to
the outer atoms, because each H has its maximum
of 2 valence electrons.
Rule 5
H NH
H
This is the
Lewis structure
For NH3
Place the remaining 2 Valence
electrons on the central
nitrogen atom
Rule 6
Check all atoms in the molecule to ensure that each has 8
electrons(2 for hydrogen). If an atom has fewer than 8
electrons, create double or triple bonds. (Note: Double
bonds only exist between C,N,O and S atoms)
Apply rule 6 to the following; CH4, CF4,
H
H C H
• Hydrogen : 1 bond = 2
electrons (stable)
• Carbon
: 4 bonds = 8
electrons (stable)
H
F
F
C F
F
• Fluorine : 1 bond + 3
lone pairs = 2 + 3 (2)
= 8 electrons (stable)
• Carbon : 4 bonds = 8
electrons (stable)
Example; CH2O
Apply Rules 1-5 to the molecule
Rule 1: Count the valency electrons
Rule 2: Place the least electronegative element at the
centre, except for H, which is always an outer atom
Rule 3: Add covalent bonds between the centre and the
outer atoms
Rule 4: Add lone pairs to the outer atoms
Rule 5: Add lone pairs to the centre atom
Rule 1
Carbon has 4 valence electrons, each hydrogen has 1 valence
electron, and oxygen has 6 valence electrons.
Total number of valence electrons : 4 + 2(1) + 6 = 12
Rule 2
Carbon is at the centre of the molecule because it is less
electronegative than oxygen. Hydrogen is always an outer
atom and is never at the centre of the molecule.
H
C
O
H
• Rule 3
Add the bonding electrons.
This uses 6 of the 12 valence
electrons
H
C
O
H
H
C
O
H
• Rule 4
Add the remaining 6 lectrons to
the outer atom. Hydrogen does
not need any more electrons, but
Oxygen needs 6 to complete its
octet.
Rule 5 There are no valence electrons left to add to the centre
H
C
H
O
• Rule 6
Oxygen shares one of its
lone pairs with C and O
and give the desired 8
electron total
H
C
H
O
H
This is the
Lewis dot
Structure for
CH2O
C
O
H
Exceptions to the Octet Rule
The Octet Rule applies to Groups IVA through VIIA in the
second row of the Periodic Table, but there are exceptions
to the rule among some other elements. The following two
cases are an example
Example BF3
Rule 1
Boron has 3 valence electrons and each Fluorine has 7
valence electrons
Total number of electrons = 3 + 3 (7) = 24
F
B
F
F
Rule 3
Add the bonding electrons.
This uses 6 of the 24 valence
electrons
Rule 2
Boron is at the centre of
the molecule because it is
less electronegative than
fluorine
F
B
F
F
Rule 4
Add the remaining electrons
to the outer atoms. Each
Fluorine has the required 8
electrons
F
B
F
F
Rule 5
This uses the remaining
electrons leaving none to add
to the Boron central atom
F
B
F
F
Rule 6
Check the number of electrons around each atom. Each
Fluorine atom has 8 electrons, but the Boron Atom has only
6. This is an exception to the Octet Rule. A B=F bond is not
an option, because double bonds exist only between C,N,O,
and S atoms
F
B
F
F
This is the Lewis
dot structure BF3
Example PF5
Rule 1
Phosphorus has 5 valence
electrons and each fluorine
has 7 valence electrons
Rule 2
Phosporus is at the centre
because it is less
electronegative than fluorine
Total number of electrons
• = 5 + 5(7) = 40
F
F
F
P
F
F
Rule 3
Add the bonding electrons.
P
This uses 6 of the 24
valence
electrons.
F
F
F
F
F
Rule 4
Add the remaining electrons to
P
the outer atoms. Each
Fluorine requires 6 more
electrons
F
F
F
F
F
Rule 5
This uses the remaining
electrons leaving none to
the central P atom
P
F
Rule 6
Check the number of electrons
around each atom. Each
Fluorine atom has 8 electrons,
but the phoshorus atom has 10.
This is an exception to the
Octet Rule.
F
F
F
F
F
F
P
F
F
F
Rule 6
Check the number of electrons
around each atom. Each fluorine
atom has 8 electrons, but the phoshorus
atom has 10 . This is an exception to the
Octet Rule.