Unit 6 Molecular Geometry and
Polarity
Overview of bond types:
ionic
covalent
metallic
Water
Why bent shape?
Why important?
Why does NaCl dissolve in water?
Why polar?
Electrostatic attraction + • Between nuclei and e- clouds
• e- that satisfy octet rule stick around b/c of +• Chemical bond result of e.a. between nuclei
and e• Bond contains energy- E needed to break
• Forces- attractive between atoms
• Repulsion- between e- in bonding atoms and
charged nucleus
Bond formation
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•
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Valence e- only
At highest energy level
Use PT to determine valence
Bonds- result in octet rule satisfaction
All atoms want to be NOBLE
Which periods of atoms want to be which
NOBLES?
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RESULT:
Lowest potential energy
Process of bonding- exothermic
Energy released
IF E released is large then = strong bond
Small E release= weak bond
For valence e- only
• Ionic - transfer of e• Covalent- sharing of e• Metallic- free e-
Valence eTransfer, shared and free
All to get to octet.. All atoms want to be NOBLE
Lewis Structures and Symbols
• Chemical symbol represents nucleus and core
electrons
• Dots are valence e-
Si
Your Turn PT Table orbital tab
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•
•
•
•
Do
Nitrogen
Phosphorus
Arsenic
Antimony
• Notice anything?
Try me…
P
As
Sb
Bi
Lewis Dot Structures in bonding
• 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.
• Metallic Bonds also crystal lattice
• Ions fixed
• Valence e- free
• Ionic bonds
Na
+
Cl
Covalent bonds
• Do Lewis dot structures for HCl
H
+ Cl
Non polar
F F
diagram..
No charge separation
Pull of val e- is =
Polar
• Unequal pull
H Cl
Cl EN is greater than H so Cl pulls more strongly
H b/comes more + as e- pulled more towards Cl
Unequal sharing use lower case delta
ᵟ
Can do EN calculation
ᵟ+ H
Cl ᵟ-
The greater difference in EN, the more likely loss of eand ionic
0-.3
.3to 1.00
1.00 to 1.7
more than 1.7
• 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 Chloride
• Iron II flouride
• Potassium Bromide
• 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:
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•
•
•
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
exception
NH4+
nonmetals form this cation
Only polar atomic ion formed by
two nonmetals
EXCEPTIONS TO OCTET RULE?