Lewis Dot Structures
• Lewis structures show presence of all valence
electrons in a compound.
• If ionic, metallic or covalently bonded
compounds may all be represented.
• This unit concentrates on covalently bonded
molecular compounds but ionic compound
representation is required.
• Ionic transfer of v e-
• Metallic free v e-
Another ionic compound
One e- from Ca atom may be given to each
fluorine atom to satisfy octet rule
Lewis Structure for CaF2
Your Turn
• Sodium Carbonate
• Magnesium Chloride
• Covalent compounds and polyatomic ions can
be described by
• Molecular formula: kind/types numbers of
atoms but give no information of bonds
connectivity
• Structural formula: depicts arrangement of
atoms in space
But gives no information regarding
arrangement of valence electrons
Lewis Structures Does …
• Shows valence e- arrangement
• Lewis structures written for molecules that
obey Octet Rule!
• (exceptions: deficiency, extended and odd#)
Lewis Dot Diagram Rule 1:
•
•
•
•
Count valence e- of each atom
Add totals for all atoms
Make adjustment for charge if any
(-1 charge add e+2 charge 2 less e-)
Rule 2:
• Arrange atoms in the diagram as follows:
• Choose least EN atom as central atom- usually
atom closest to left side of PT or largest
• Arrange other atoms around central atom (CA)
• Hydrogen can only form one bond- never CA
• Carbon has 4 single bonds except in CO and
CN (Has 3 bonds and unshared v e- pair)
Rule 2 continued…
Arrange other atoms around central atom (CA)
Hydrogen can only have one bond, why?
Carbon has 4 single bonds except CO and CN-1
Your Turn…
Calculate val e- and organize CA and outside
atoms in the following:
Carbon dioxide
Ammonia
Carbon tetrachloride
Methane
Carbon ion
Ammonium ion
Rule 3
• Place vale e- around CA as follow:
• Connect outer atoms to CA with single
covalent bond- each bond takes 2 e• Calculate # of remaining val e• Fill octets of outer atoms
• Place any remaining e- on CA
Rule 4
• Check CA if Octet Rule satisfied
• IF NOT- make multiple bonds double or triple
• (this done by moving unshared e- pair on
outer atom to CA
• ONLY FEW ELEMENTS CAPABLE OF FORMING
MULTIPLE BONDS: C N O P S
• AND RARE-CHLORINE
Your Turn..
Return to these diagrams
Add val eMake Lewis Structures for each
Carbon dioxide
Ammonia
Carbon tetrachloride
Methane
Carbon ion
Ammonium ion
EXCEPTIONS TO OCTET RULE?