CHEMISTRY 1C Professor Watts

1
CHEMISTRY 1CProfessor Watts

• http//www.chem.ucsb.edu/coursepages
• Exam Dates
• Mid-term 1 October 20 (Wed)
• Mid-term 2 November 17 (Wed)
• Final Exam December 10, Friday
• 12 noon 3 pm

2
Chemical Bonding

• Compounds are formed from chemically bound atoms
  or ions.
• Bonding involves only the valence electrons.
• Valence electrons are the ones in the quantum
  shell with largest n value
• Use the periodic chart to guide determination of
  valence electrons

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The Periodic Table(see www.webelements.com)
4
11-1 Lewis Theory An Overview

• Valence e- play a fundamental role in chemical
  bonding.
• e- transfer leads to ionic bonds.
• Sharing of e- leads to covalent bonds.
• e- are transferred or shared to give each atom a
  noble gas configuration
• the octet.

5
Genealogy from G.N. Lewis

• G.N. Lewis (UC Berkeley)
• Michael Kasha (Florida State
  University)
• Stewart Strickler
  (U. Colorado)
• Glenn Crosby
  (Washington State U)
• 
  Richard Watts (UCSB)
  

6
Chemical Bonding

• Ionic Compounds Molecular Compounds
• Ionic Radii Covalent bonds
• Lattice Energy Bond Order
• Bond
  Strength
• Lewis
  Structures

7
Lewis Symbols

• A chemical symbol represents the nucleus and the
  core e-.
• Dots around the symbol represent valence e-.

Si



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Ionic and Molecular Compounds

• Formation of sodium chloride

• Formation of hydrogen chloride

A metal and a nonmetal transfer electrons to form
an ionic compound. Two nonmetals share electrons
to form a molecular compound.
9
Lewis Structures for Ionic Compounds
BaO
MgCl2
10
Ionic Compounds

• Ionic compounds consist of a lattice of positive
  and negative ions.

11
Ionic Bonding in NaCl
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Ionic Bonds

• An ionic bond is simply the electrostatic
  attraction between opposite charges.

Q2
Ions with charges Q1 and Q2
Q1
The potential energy is given by
13
The Octet Rule

• Atoms tend to gain, lose, or share electrons
  until they have eight valence electrons.

Hydrogen is an exception. It shares only one
electron to reach an outer shell of two electrons
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Molecular Compounds

• The simplest molecule is H2

The pair of shared electrons constitutes a
covalent bond.
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Chlorine - Cl2
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Lewis Structures

• Covalent bonding in a molecule is repre-sented
  by a Lewis structure.
• A valid Lewis structure should have an octet for
  each atom except hydrogen.

H2
Cl2
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Skeletal Structure

• Hydrogen atoms are always terminal atoms.
• Central atoms are generally those with the lowest
  electronegativity.
• Carbon atoms are always central atoms.
• Generally structures are compact and symmetrical.

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Skeletal Structure

• Identify central and terminal atoms.

H
H
H
C
C
O
H
H
H
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Lewis Structures

• Draw Lewis structures for

H F
HF



H2O
NH3
CH4
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Double and Triple Bonds

• Atoms can share four electrons to form a double
  bond or six electrons to form a triple bond.

O2
N2

• The number of electron pairs is the
• bond order.

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Multiple Covalent Bonds


N


N




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Multiple Covalent Bonds



O
C

O








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Writing Lewis Structures

• All the valence e- of atoms must appear.
• Usually, the e- are paired.
• Usually, each atom requires an octet.
• H only requires 2 e-.
• Multiple bonds may be needed.
• Readily formed by C, N, O, S, and P.

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Drawing Lewis Structures

• Sum the valence electrons from all atoms. Add
  one for each negative charge and subtract one for
  each positive charge.
• Draw a skeleton structure with atoms attached by
  single bonds.
• Complete the octets of atoms bound to the central
  atom.
• Place extra electrons on the central atom.
• If the central atom doesnt have an octet, try
  forming multiple bonds.

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Molecular formula
For NF3
Atom placement


N s2p3 5e-
F
F





Sum of valence e-
F s2p5 7e-
X 3 21e-
N
Total 26e-
F



Remaining valence e-
Zero NF3 is uncharged
Lewis structure
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SAMPLE PROBLEM 10.1
Writing Lewis Structures for Molecules with One
Central Atom
SOLUTION
Cl
Step 1 Carbon has the lowest EN and is the
central atom. The other atoms are placed
around it.
C
Cl
F
F
Steps 2-4 C has 4 valence e-, Cl and F each
have 7. The sum is 4 4(7) 32 valence e-.
Make bonds and fill in remaining valence
electrons placing 8e- around each atom.
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SAMPLE PROBLEM 10.2
Writing Lewis Structure for Molecules with More
than One Central Atom
SOLUTION
Hydrogen can have only one bond so C and O must
be next to each other with H filling in the
bonds. There are 4(1) 4 6 14 valence
e-. C has 4 bonds and O has 2. O has 2 pair of
nonbonding e-.
H

C
O
H
H

H
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SAMPLE PROBLEM 10.3
Writing Lewis Structures for Molecules with
Multiple Bonds.
PROBLEM
Write Lewis structures for the following (a)
Ethylene (C2H4), the most important reactant in
the manufacture of polymers (b) Nitrogen (N2),
the most abundant atmospheric gas
PLAN
For molecules with multiple bonds, there is a
Step 5 which follows the other steps in Lewis
structure construction. If a central atom does
not have 8e-, an octet, then e- can be moved in
to form a multiple bond.
SOLUTION
(a) There are 2(4) 4(1) 12 valence e-. H
can have only one bond per atom.

(b) N2 has 2(5) 10 valence e-. Therefore a
triple bond is required to make the octet around
each N.
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Drawing Lewis Structures



COCl2
24 ves








14 ves
HOCl



?





26 ves
ClO3?






CH3OH
14 ves

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Strategy for Writing Lewis Structures
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Electronegativity

• Polarity refers to a separation of positive and
  negative charge. In a nonpolar bond, the bonding
  electrons are shared equally

• In a polar bond, electrons are shared unequally
  because of the difference in Zeff.

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Electronegativity

• Electronegativity refers to the ability of an
  atom in a molecule to attract shared electrons.
• The Pauling scale of electro- n
  egativity

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Bond Polarity

• A polar bond can be pictured using partial
  charges

?
??
?? 0.9
2.1
3.0
Electronegativity Difference
Bond Type
0 - 0.5
Nonpolar
0.5 - 2.0
Polar
2.0 ?
Ionic
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Polar Covalent HCl
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The Periodic Table(see www.webelements.com)