chemical bonding what holds things together? pgcc chm 101 sinex
TRANSCRIPT
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Chemical Bonding
What holds things together?
PGCC CHM 101 Sinex
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Chlorides of Period 2compound LiCl BeCl2 BCl3 CCl4 NCl3 OCl2 Cl2
melting point
610 415 -107 -23 -40 -121 -102
Chlorides of Period 3compound NaCl MgCl2 AlCl3 SiCl4 PCl3 SCl6 Cl2
melting point
801 714 193 -69 -112 -51 -102
Let’s examine the melting point of compounds across two periods.What is the trend?
lowhigh
Conductivity - high Conductivity - low
CRC Handbook of Chemistry and Physics, 1995
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Bonding Can we explain the melting point behavior across a period?
• involves the valence electrons or outermost shell (or highest shell) electrons
• for group A elements - the group number tells how many valence electrons
How many valence electrons on N?
Group 5A – 5 valence electrons
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Bonding…
• Lewis dot structures show the valence electrons around at atom and for most molecules and compounds a complete octet for the elements
• most monatomic ions have an electron configuration of noble gases
N Al
F + e- F1s22s22p5 1s22s22p6 Ne
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Write out the electron configuration for the following atoms and ions:
He
Ne
Ar
Kr
H-
Ca+2
O-2
Br-
Which noble gas is isoelectronic with each ion?
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Na Cl
NaCl
+ -
electron transfer
and the formation of ions
This is the formation of an ionic bondionic bond.
ClCl
Cl2 This is the formation of a covalent bondcovalent bond.
sharing of a pair of electrons
and the formation of molecules
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What about the distance between the atoms in a bond?
NaCl Na+ Cl- d = 281 pm
Cl2 Cl-Cl d = 199 pm
What property can be used to tell when a bond will ionic or covalent?
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ionic
covalentHCl H2O CH4
MgO CaCl2 Na2S
Draw the Lewis dot structures for the following compounds:
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Some exceptions to the Octet Rule
BF3
PCl5
SF6
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Electronegativity
Chlorides of Period 2compound LiCl BeCl2 BCl3 CCl4 NCl3 OCl2 Cl2
EN 2.2 1.6 1.1 0.6 0 0.6 0
Chlorides of Period 3Compound NaCl MgCl2 AlCl3 SiCl4 PCl3 SCl6 Cl2
EN 2.2 1.9 1.6 1.3 1.0 0.6 0
large difference small difference
The electronegativity difference - EN = ENhigher – EN lower
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Using electronegativities to determine bond typeEN > 1.7 ionic bond - transfer
EN < 1.7 covalent bond - sharing
So we have a range of electronegativity difference of 0 to 1.7 for sharing an electron pair.
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Is the sharing of electrons in molecules always equal?
X Y EN = 0
X Y EN = 0.3
X Y EN = 0.6
X Y EN = 0.9
X Y EN = 1.2
ENY > ENX
Which element is more electronegative?
non-polar bond
incr
easi
ng p
ola
rity
of
bond
polar bond
0 < EN < 1.7
Direction of electron migration
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BF3 – a planar molecule
Ball & stick
BF
Space-filled
Electrostatic potential maps
top side
negative
positive
Spartan ‘02
2.0
4.0
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More sharing examples
O2
N2
O O
N N
O O O O
N N N N N N
double bond (2 pairs)
triple bond (3 pairs)
Share until octet is complete.
octet complete
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Bond Energy
F2 single bond BE = 142 kJ/mole
O2 double bond BE = 494
N2 triple bond BE = 942
X2 + energy X + X
incr
easi
ng b
ond s
trength
Is breaking a bond an endothermic or exothermic process?
http://wulfenite.fandm.edu/Data%20/Table_6.html
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NH3
NH
H
H
NH
H
HH+
NH4+ NH3 + H+ NH4
+
coordinate covalent bond
(the pair of electrons from the same atom)
normal covalent bond
(each atom supplies an electron)
Some more sharing examples
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Type of bond? – I, PC, or NC
TiO2 CH4 NaI
CS2 CO2 KCl
AlCl3 CsF HBr
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Using the EN trends to predict bond type
105
Db107
Bh
Increasing EN
Incr
easi
ng
EN
NO RbF FeS H2SModified from http://www.cem.msu.edu/~djm/cem384/ptable.html
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Draw the Lewis dot structures
CO2
H3O+ CO
H2CO
NH2-
HCN
(C in center)
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Show the direction of electron migration ( ) in the
following.C – H
H – F
C = O
C – Cl
Rank the bond polarity (1-most … 3-least)
As-H N-H P-H
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Here is the electrostatic potential map for H2CO.
Show the electron migration on this planar molecule.
C OH
H
How is this molecule different than BF3?
blue – positive red - negative
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ionic covalent
valence electrons
Comparison of Bonding Types
sharing of electrons
transfer of electrons
ions molecules
EN > 1.7 EN < 1.7
high mp
low mp
molten salts conductive
non-conductive
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Bonding spectrum100% covalent 100% ionic
A+ B-A B A B
Increasing EN
Increasing polarity Transfer