Lewis Dot Diagrams
for
Covalent Substances
Representations of Molecules
Lewis Structures of Reaction
H2 = Molecular Formula
H-H = Structural Formula.
- Represents 1 pair of electrons.
Beyond H2: Other diatomic
elements
• Hydrogen (H2), Nitrogen (N2), Oxygen
(O2), Fluorine (F2), Chlorine (Cl2),
Bromine (Br2), and Iodine (I2)
• These molecules are more stable
than the individual atoms.
Halogens
Share 1 pair or 2 electrons, forming a
single covalent bond.
Rules for drawing Lewis Diagrams
1. Arrange the symbols on the paper the way
you think the atoms are arranged.
- Diatomics are easy – next to each other!
- Hydrogens are always terminal.
- Atom with least attraction for shared electrons is
the central atom.
2. Add up the valence electrons on all the atoms.
3. Distribute the electrons starting with single
bonds (electron pairs) between all the atoms.
4. Test validity (2 tests)
Tests for Lewis Structures
Must Pass Both!
1. Number of dots = number of valence
electrons found in step 2
2. Every atom except H has an octet of
electrons around it. H only wants 2.
Bonding electrons get counted 2
times - for the atom on each side.
Assessing Lewis Diagrams
• If the diagram passes both tests, you are
finished.
• If the diagram fails one or both tests, you
must try again.
• If single bonds don’t work, move on to
multiple bonds.
– Single bond = 2 electrons
– Double bond = 4 electrons
– Triple bond = 6 electrons
Try some examples.
• H2, F2, HF, O2, N2
• Step 1: Draw the symbols the way
you think the atoms are arranged.
–Diatomics are easy – the atoms are
right next to each other!
Drawing Lewis Diagrams
H:H
Step 2: Count up the
valence electrons.
Each H has 1 valence
electron so the total = 2.
Step 3: Distribute the valence electrons,
starting with single bonds between all
atoms.
Testing Lewis Diagrams
Test 1:
2 dots in
diagram & a
total of 2
valence
electrons
H :H
Test 2:
Each H has 2
valence
electrons. Rem
H only wants 2.
Testing Lewis Diagrams
Step 1
Step 2
Step 3
F F
2 X 7 = 14 valence electrons
.. ..
: ..F : F
:
..
Try HF
Step 2
Step 3
HF
7 + 1 = 8 valence electrons
..
H ..
F:
:
Step 1
Types of Covalent Bonds
• Single: 2 atoms share 1 pair of
electrons. (That’s 2 electrons!)
• Double: 2 atoms share 2 pairs of
electrons. (4 electrons!)
• Triple: 2 atoms share 3 pairs of
electrons. (6 electrons!)
Try O2
Step 1:
OO
Step 2:
2 X 6 = 12 valence electrons
Step 3:
.. ..
: O: O :
..
Step 4: Test
No Good!
Try O2 Again!
Step 1:
Step 2:
2 X 6 =12
.. ..
::
O
O
.. ..
Step 3: Distribute electrons, but a single
bond between the atoms did not work, so
try a double bond.
Step 4: Test!
Try N2
Step 1:
..
:N
:
:
N
..
Step 2:
2 X 5 = 10 electrons
No Good!
Step 3: Distribute electrons starting with a
single bond between the nitrogens.
Step 4: Test
Try N2 with a double bond
: N :: N :
..
No Good!
Try N2 with a triple bond
: N ::: N :
Step 4: Tests