Lewis theory
Chapter 10
Electrons (most usually the outermost valence
electrons) play the main role in chemical bonding
2. Electrons can be transferred from one atom to
another, forming a cation and anion that form
strong electrostatic attractions we call ionic
bonds
3. Electrons are transferred or shared until a stable
electron configuration is made for all atoms.
Generally stable configurations involve an octet
(eight) of electrons similar to the combined filled
s and p blocks of the noble gases.
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Chemical Bonding I: Basic Concepts
Dr. Peter Warburton
[email protected]
http://www.chem.mun.ca/zcourses/1050.php
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Chemical bonding
Lewis dot structures
Chemical bonding occurs in a continuum
between two extremes:
Lewis dot structures (or Lewis
structures or formulas) are drawings we
use to keep track of valence electrons in
a chemical species.
Ionic bonding: the electrostatic
interaction between positive and negative
ions.
Covalent bonding: a chemical bond
formed by the sharing of two electrons
between atoms.
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By using chemical symbols and dots to
represent electrons, we can perform
“electron bookkeeping” to show the
valance electron distribution of the
species.
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Problem
Lewis Symbols and Lewis Structures
A chemical symbol represents the nucleus
and the core e-.
Dots around the symbol represent valence
•
e -.
• Si •
•
••
• As •
•
•
••
• Se
•
•
••
• Bi •
•
••
• Sb •
•
••
I
•
••
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••
Ar
••
• Al •
•
••
P•
••
•
••
••
•N•
•
Write the Lewis symbols expected for Mg,
Ge, K, Ne, Sn, Br-, Tl+, and S2-
••
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Problem answer
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Lewis structures
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Problem
Problem answers
Write plausible Lewis structures for the
following ionic compounds:
Na2S, Mg3N2, calcium iodide, barium
sulfide and lithium oxide
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Covalent bonds
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Covalent bonds
Covalent bonds occur between atoms
when the resulting bond will lower the
potential energy compared to the
potential energy of the separate atoms.
In many cases each atom contributes
ONE electron to form the covalent bond,
while gaining the benefit of sharing BOTH
electrons.
In coordinate covalent bonds one atom
contributes BOTH electrons to the
covalent bond. The two atoms then share
BOTH electrons.
This difference in energy is called the
bond dissociation energy
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Bonding pairs of electrons
Covalent bonds are
represented as either a
dash between the two
atom chemical symbols
OR
by the use of a
pair of dots between
the two atom chemical
symbols
Non-bonding (lone) pairs
Non-bonding (lone) pairs of electrons are NOT
FOUND between atoms. They “belong” to
one atom (are NOT shared), and are shown as
a pair of dots beside the chemical symbol.
H-H
Bonding pair
••
H - Cl :
H:H
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••
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Multiple bonds
Sometimes atoms can
share MORE THAN
ONE bonding pair of
electrons. In these
cases, the double
bonds and triple
bonds can be shown
with the correct
number of dashes
between the atoms
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different
lone
pairs
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Octet rule
Many atoms in molecules “prefer” to
have EIGHT (8) valence electrons
around them.
Hydrogen “prefers” TWO (2) valence
electrons.
This preference is called the OCTET
RULE.
However, there are MANY exceptions to
this “rule”, so we must be careful!
H-C = N:
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Problem
Problem answers
Write Lewis structures for Br2, CH4 and
N2H4.
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Polar covalent bonds
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Figure 10.6
Sharing of electrons isn’t always equal!
On average, covalent bonding electrons
will spend more time closer to the atom
with the greater electronegativity.
Such unequally shared covalent bonds are
called polar covalent bonds.
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Electronegativity tends to:
INCREASE from left to right
AND
DECREASE from top to
bottom
in the periodic table.
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Figure 10.6
Bonding continuum
As the absolute electronegativity
difference between two bonded atoms
increases, the bond becomes more
polar.
Eventually the difference is so great that
the more electronegative atom “steals”
electrons from the other atom, forming
negative and positive ions.
This would give us an ionic bond!
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Bonding continuum
Visualizing polar covalent bonds
••
••
H - H H - Cl : Na
+
••
−
: Cl
:
••
Absolute electronegativity differences
AED = |2.1 - 2.1| AED = |2.1 - 3.0|
= 0.0
AED = |0.9 - 3.0|
= 0.9
The more electronegative atom
has a PARTIAL negative charge
(δ
δ-) while the other atom has a
PARTIAL positive charge (δ
δ+)
Sometimes this is symbolized as
an arrow from positive to negative
= 2.1
Bonding description
Nonpolar
Polar
Ionic
covalent
covalent
bond
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••
H - Cl :
••
δ + δ−
••
H - Cl :
••
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