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Chemical Bonding 3: Writing Lewis Diagram
(a)
The Lewis Structures of Simple Ionic Compounds
Extend from the previous class
Draw Lewis Structure for MgCl2
(b)
The Lewis Structure of Covalent Compound that obey the Octet Rule
Lewis Structure show how the VALENCE electrons are distributed in a molecule. The
octet rule states that most atoms, other than hydrogen, ten to attain an octet of
electrons as a result forming covalent bonds
For example: H2O
The “Rule of the Game”
a. Count up the total number of valence electrons in the molecule. Adjust number by
subtracting one electron for every positive charge and adding one electron by
every negative charge
b. Determine which atomes are bonded together and put 2 electrons into each bonds.
(NOTE: you will always be shown which atoms are connected to which other atoms)
c. Use the remaining valence electrons to complete the octets of the atoms
surrounding central atom(s). Then place any remaining electrons, in pairs, on the
central atom(s).
d. If a central atom has less than an octet of electrons, have a neighbour share
electrons with the deficient atom by putting an extra pair (or pairs) of electrons
into the shared bond
e. Tidy up: replace each pair of electron engaged in a bond with a dash “-”
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Example:
NH4+
CHO2-
HOPO
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(c)
Extension: The Lewis Structure of Covalent Compound that Violates the Octet
Rule
1) Electron deficient molecule
In addition to H, the atoms Be, B and Al are exception to the dendency for covalentlybonded atom to complete their octets. These atoms have such low electronegativites that
the best that they can do is to GAIN ONE EXTRA ELECTRON IN A COVALENT BOND
FOR EVERY ELECTRON THEY CAN CONTRIBUTE TO THE BONDS. (They do not
have sufficient electronegativity to pull extra electrons on an adjacent atom into covalent
bonds)
Definition: a molecule in which one or more atoms (other than hydrgoen) does not possess
a full octet of electron is called an ELECTRON-DEFICIENT molecules
For example, BF3
2) Atoms having an expanded octet of valence electrons
Elements in the third and four period of the periodic table frequently attain more than
an octet of valence electrons when they vorm covalent compounds. (The extra electrons
are placed in low-lying d-orbitals.) Other than the fact that the central atom will end up
with more than eight valence electrons, the same rules are used for forming the Lewis
Structure.
For example PCl5
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Exercise