atomic bond theory.ppt
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Chapter 8
Covalent
Bonding
Pre-AP ChemistryCharles Page High School
Stephen L. Cotton
Ball-and-stick model
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Section 8.1
Molecular Compounds
OBJECTIVES:
Distinguish between themelting points and
boiling points ofmolecularcompoundsand ioniccompounds.
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Section 8.1
Molecular Compounds
OBJECTIVES:
Describe the informationprovided by a molecular
formula.
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Bonds are
Forces that hold groups ofatoms together and make them
function as a unit. Two types:
1) Ionic bondstransferofelectrons (gained or lost; makes formula unit)
2) Covalent bondssharingofelectrons. The resulting
particle is called a molecule
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Covalent Bonds
The word covalent is acombination of the prefix co-
(from Latin com, meaning with
or together), and the verb
valere, meaning to be strong.
Two electrons shared togetherhave the strength to hold two
atoms together in a bond.
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Molecules
Many elements found in natureare in the form ofmolecules:
a neutral group of atoms joined
together by covalent bonds.
For example, air contains oxygen
molecules, consisting of two
oxygen atoms joined covalently
Called a diatomic molecule (O2
)
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How does H2 form?
The nuclei repel each other,since they both have a positivecharge (like charges repel).
++
(diatomic hydrogen molecule)
+ +
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How does H2 form?
++
But, the nuclei are attracted tothe electrons
They share the electrons, and
this is called a covalent bond,
and involves only NONMETALS!
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Covalent bonds
Nonmetals hold on to their valenceelectrons. They cant give away electrons to bond.
But still want noble gas configuration. Get it by sharing valence electrons with
each other = covalent bonding
By sharing, both atoms get to countthe electrons toward a noble gasconfiguration.
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Covalent bonding
Fluorine has seven valenceelectrons (but would like to have 8)
F
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Covalent bonding
Fluorine has seven valenceelectrons
A second atom also has seven
F F
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Covalent bonding
Fluorine has seven valenceelectrons
A second atom also has seven
By sharing electrons
F F
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Covalent bonding
Fluorine has seven valenceelectrons
A second atom also has seven
By sharing electrons
F F
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Covalent bonding
Fluorine has seven valenceelectrons
A second atom also has seven
By sharing electrons
F F
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Covalent bonding
Fluorine has seven valenceelectrons
A second atom also has seven
By sharing electrons
F F
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Covalent bonding
Fluorine has seven valenceelectrons
A second atom also has seven
By sharing electrons
F F
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Covalent bonding
Fluorine has seven valence electronsA second atom also has seven
By sharing electrons
both end with full orbitals
F F
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Covalent bonding
Fluorine has seven valence electronsA second atom also has seven
By sharing electrons
both end with full orbitals
F F8 Valenceelectrons
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Covalent bonding
Fluorine has seven valence electronsA second atom also has seven
By sharing electrons
both end with full orbitals
F F8 Valenceelectrons
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Molecular Compounds
Compounds that are bondedcovalently (like in water, or carbon
dioxide) are called molecular
compounds
Molecular compounds tend to have
relatively lowermelting and boilingpoints than ionic compounds this is
not as strong a bond as ionic
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Molecular Compounds
Thus, molecular compounds tend to
be gases or liquids at room
temperatureIonic compounds were solids
A molecular compound has a
molecular formula:Shows how many atoms of each
element a molecule contains
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Molecular Compounds
The formula for water is written asH2O
The subscript 2 behind hydrogen
means there are 2 atoms ofhydrogen; if there is only one atom,the subscript 1 is omitted
Molecular formulas do not tell anyinformation about the structure (thearrangement of the various atoms).
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- Page 215
These are some of the
different ways to represent
ammonia:
1. The molecular
formula showshow many atoms
of each element
are present
2. The structuralformula ALSO
shows the
arrangement of
these atoms!
3. The ball and stickmodel is
the BEST, because it showsa 3-dimensional arrangement.
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Section 8.2
The Nature of Covalent Bonding
OBJECTIVES:
Describe howelectronsare shared to formcovalent bonds, andidentify exceptions to theoctet rule.
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Section 8.2
The Nature of Covalent Bonding
OBJECTIVES:
Demonstrate howelectron dot structures
represent sharedelectrons.
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Section 8.2
The Nature of Covalent Bonding
OBJECTIVES:
Describe how atomsform double or triple
covalent bonds.
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Section 8.2
The Nature of Covalent Bonding OBJECTIVES:
Distinguish between acovalent bond and acoordinate covalent bond, and
describe how the strength of acovalent bond is related to itsbond dissociation energy.
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Section 8.2
The Nature of Covalent Bonding
OBJECTIVES:
Describe how oxygenatoms are bonded in
ozone.
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A Single Covalent Bond is...
A sharing of two valence electrons.Only nonmetals and hydrogen.
Different from an ionic bondbecause they actually form
molecules.
Two specific atoms are joined. In an ionic solid, you cant tell which
atom the electrons moved from or to
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Sodium Chloride Crystal Lattice
Ionic compounds
organize in a
characteristic
crystal lattice ofalternating
positive and
negative ions,repeated over and
over.
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How to show the formation
Its like a jigsaw puzzle. You put the pieces together to end up
with the right formula.
Carbon is a special example - can itreally share 4 electrons: 1s22s22p2?
Yes, due to electron promotion!
Another example: lets show how water isformed with covalent bonds, by using an
electron dot diagram
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Water
H
O
Each hydrogen has 1 valenceelectron
- Each hydrogen wants 1
more The oxygen has 6 valence
electrons
- The oxygen wants 2 more They share to make each
other complete
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Water
Put the pieces together The first hydrogen is happy
The oxygen still needs one more
H O
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Water
So, a second hydrogen attachesEvery atom has full energy levels
H OH
Note the two
unshared pairs
of electrons
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Examples:
1. Conceptual Problem8.1 on page 220
2. Do PCl3
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Multiple Bonds
Sometimes atoms share more thanone pairof valence electrons.
A double bond is when atoms share
two pairs of electrons (4 total)A triple bond is when atoms share
three pairs of electrons (6 total)
Table 8.1, p.222 - Knowthese 7elements as diatomic:
Br2 I2 N2 Cl2 H2 O2 F2
Whats the deal
with the oxygen
dot diagram?
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Dot diagram for Carbon dioxide
CO2 - Carbon is centralatom( more metallic )
Carbon has 4 valence
electronsWants 4 more
Oxygen has 6 valence
electrons
Wants 2 moreO
C
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Carbon dioxide
Attaching 1 oxygen leaves theoxygen 1 short, and the carbon 3
short
OC
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Carbon dioxide
Attaching the second oxygenleaves both of the oxygen 1 short,
and the carbon 2 short
OCO
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Carbon dioxide
The only solution is to share more
OCO
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Carbon dioxide
The only solution is to share more
OCO
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Carbon dioxide
The only solution is to share more
OCO
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Carbon dioxide
The only solution is to share more
OCO
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Carbon dioxide
The only solution is to share more
OCO
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Carbon dioxide
The only solution is to share more
OCO
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Carbon dioxide
The only solution is to share moreRequires two double bonds
Each atom can count all the
electrons in the bond
OCO
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Carbon dioxide
The only solution is to share more Requires two double bonds
Each atom can count all the electrons in
the bond
OCO
8 valence
electrons
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Carbon dioxide
The only solution is to share more Requires two double bonds
Each atom can count all the electrons in
the bond
OCO
8 valence
electrons
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Carbon dioxide
The only solution is to share more Requires two double bonds
Each atom can count all the electrons in
the bond
OCO
8 valence
electrons
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How to draw them?
Use the handout guidelines:1) Add up all the valence electrons.
2) Count up the total number of
electrons to make all atoms happy.
3) Subtract; then Divide by 2
4) Tells you how many bonds to draw5) Fill in the rest of the valence
electrons to fill atoms up.
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Example
NH3, which is ammonia N central atom; has 5
valence electrons, wants 8
H - has 1 (x3) valenceelectrons, wants 2 (x3)
NH3 has 5+3 = 8
NH3 wants 8+6 = 14 (14-8)/2= 3 bonds
4 atoms with 3 bonds
N
H
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N HH
H
Examples
Draw in the bonds; start with singlesAll 8 electrons are accounted for
Everything is full done with this one.
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Example: HCN
HCN: C is central atom N - has 5 valence electrons, wants 8
C - has 4 valence electrons, wants 8
H - has 1 valence electron, wants 2 HCNhas 5+4+1 = 10
HCNwants 8+8+2 = 18
(18-10)/2= 4 bonds
3 atoms with 4 bonds this will require
multiple bonds - not to H however
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HCN
Put single bond between each atomNeed to add 2 more bonds
Must go between C and N (Hydrogen is full)
NH C
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HCN
Put in single bonds Needs 2 more bonds
Must go between C and N, not the H
Uses 8 electrons need 2 more toequal the 10 it has
NH C
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HCN
Put in single bonds Need 2 more bonds
Must go between C and N
Uses 8 electrons - 2 more to add Must go on the N to fill its octet
NH C
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Another way of indicating
bondsOften use a line to indicate a bond
Called a structural formula
Each line is 2 valence electrons
H HO = H HO
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Other Structural Examples
H C N
C O
H
H
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A Coordinate Covalent Bond...
When one atom donates bothelectrons in a covalent bond.
Carbon monoxide (CO) is a good
example:
OCBoth the carbon
and oxygen give
another singleelectron to share
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Coordinate Covalent Bond
When one atom donates bothelectrons in a covalent bond.
Carbon monoxide (CO) is a good
example:
OC
Oxygengives both of
these
electrons,
since it hasno more
singles to
share.
This carbon
electron
moves tomake a pair
with the other
single.
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Coordinate Covalent Bond
When one atom donates bothelectrons in a covalent bond.
Carbon monoxide (CO)
OCC O
The
coordinate
covalent bond
is shown withan arrow as:
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Coordinate covalent bond
Mostpolyatomic cations andanions contain covalent and
coordinate covalent bonds
Table 8.2, p.224
Sample Problem 8.2, p.225
The ammonium ion (NH41+)can
be shown as another example
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Bond Dissociation Energies...
The total energy required to breakthe bondbetween 2 covalently
bonded atoms
High dissociation energy usually
means the chemical is relatively
unreactive, because it takesa lot of energy to break it down.
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Resonance is...When more than one valid dot
diagram is possible.
Consider the two ways to draw ozone
(O3)
Which one is it? Does it go back and
forth?
It is a hybrid of both, like a mule; andshown by a double-headed arrow
found in double-bond structures!
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Resonance in Ozone
Neitherstructure is correct, it is
actually a hybrid of the two. To showit, draw all varieties possible, and join
them with a double-headed arrow.
Note the different location of the double bond
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ResonanceOccurs when more than one valid Lewis
structure can be written for a particular
molecule (due to position of double bond)
These are resonance structures of benzene.
The actual structure is an average (or hybrid)
of these structures.
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Resonance
in a
carbonate
ion (CO32-):
Resonance
in anacetate ion
(C2H3O21-):
Polyatomic ions note the different
positions of the double bond.
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The 3 Exceptions to Octet rule
For some molecules, it isimpossible to satisfythe octet rule
#1. usually when there is an odd
numberof valence electronsNO2 has 17 valence electrons,
because the N has 5, and each O
contributes 6. Note N page 228 It is impossible to satisfy octet rule,
yet the stable molecule does exist
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Exceptions to Octet rule Another exception: Boron
Page 228 shows boron trifluoride,and note that one of the fluoridesmight be able to make a coordinate
covalent bond to fulfill the boron
#2 -But fluorine has a highelectronegativity (it is greedy), so this
coordinate bond does not form #3 -Top page 229 examples exist
because they are in period 3 or beyond
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Section 8.3
Bonding TheoriesOBJECTIVES:
Describe the relationshipbetween atomic and
molecular orbitals.
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Section 8.3
Bonding TheoriesOBJECTIVES:
Describe how VSEPRtheory helps predict the
shapes of molecules.
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Molecular Orbitals are...
The model for covalent bondingassumes the orbitals are those ofthe individual atoms = atomic orbital
Orbitals that apply to the overallmolecule, due to atomic orbitaloverlap are the molecular orbitals
A bonding orbital is a molecularorbital that can be occupied bytwo electrons of a covalent bond
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Molecular Orbitals - definitions
Sigma bond- when two atomicorbitals combine to form the
molecular orbital that is
symmetrical along the axisconnecting the nuclei
Pi bond- the bonding electrons are
likely to be found above and belowthe bond axis (weaker than sigma)
Note pictures on the next slide
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- Pages 230 and 231
Sigma bond is symmetrical along theaxis between the two nuclei.
Pi bond is
above and
below thebond axis,
and is
weaker
than sigma
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VSEPR: stands for...
Valence Shell Electron PairRepulsion
Predicts the three dimensional shape ofmolecules.
The name tells you the theory:Valence shell = outside electrons.
Electron Pair repulsion = electron
pairs try to get as far away aspossible from each other.
Can determine the angles of bonds.
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VSEPR
Based on the number of pairs ofvalence electrons, both bondedand
unbonded.
Unbonded pair also called lone pair.CH4 - draw the structural formula
Has 4 + 4(1) = 8
wants 8 + 4(2) = 16
(16-8)/2 = 4 bonds
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VSEPR for methane (a gas): Single bonds fill
all atoms.
There are 4
pairs of
electrons
pushing away.
The furthest theycan get away is
109.5
C HH
H
H
This 2-dimensional drawing does
not show a true representation of
the chemical arrangement.
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4 atoms bonded
Basic shape istetrahedral.
A pyramid with a
triangular base. Same shape for
everything with
4 pairs. CH HH
H 109.5
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Other angles, pages 232 - 233
Ammonia (NH3) = 107oWater (H2O) = 105
o
Carbon dioxide (CO2) = 180o
Note the shapes of these thatare pictured on the next slide
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- Page 232
Methane has
an angle of
109.5o, calledtetrahedral
Ammonia has
an angle of107o, called
pyramidal
Note the unshared pair that is repulsion for other electrons.
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Section 8.4
Polar Bonds and MoleculesOBJECTIVES:
Describe howelectronegativity valuesdetermine thedistribution of charge in apolar molecule.
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Section 8.4
Polar Bonds and MoleculesOBJECTIVES:
Describe what happensto polarmolecules whenthey are placed betweenoppositely charged metalplates.
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Section 8.4
Polar Bonds and MoleculesOBJECTIVES:
Evaluate the strength ofintermolecular attractionscompared with thestrength of ionic andcovalent bonds.
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Section 8.4
Polar Bonds and MoleculesOBJECTIVES:
Identify the reason whynetwork solids have high
melting points.
B d P l it
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Bond Polarity
Covalent bonding means sharedelectrons
but, do they share equally?
Electrons are pulled, as in a tug-of-war, between the atoms nuclei
In equal sharing (such as
diatomic molecules), the bondthat results is called a nonpolarcovalent bond
B d P l it
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Bond Polarity
When two differentatoms bondcovalently, there is an unequalsharing
the more electronegative atom willhave a stronger attraction, and will
acquire a slightly negative charge
called a polar covalent bond, or
simply polar bond.
El t ti it ?
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Electronegativity?
The ability of an
atom in a molecule
to attract sharedelectrons to itself.
Linus Pauling
1901 - 1994
T bl f El t ti iti
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Table of Electronegativities
B d P l it
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Bond Polarity
Refer to Table 6.2, p. 177(or handout)
Consider HCl
H = electronegativity of 2.1
Cl = electronegativity of 3.0
the bond is polar
the chlorine acquires a slightnegative charge, and the
hydrogen a slight positive charge
B d P l it
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Bond Polarity
Only partial charges, much lessthan a true 1+ or 1- as in ionic bond
Written as:
H Cl the positive and minus signs (with
the lower case delta: )denote partial charges.
d+ d-
d+andd-
B d P l it
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Bond Polarity
Can also be shown:
the arrow points to the moreelectronegative atom.
Table 8.3, p.238 shows how the
electronegativity can also indicate
the type of bond that tends to form
H Cl
P l l l
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Polar molecules
Sample Problem 8.3, p.239A polar bond tends to make the
entire moleculepolar
areas of difference
HCl has polar bonds, thus is a polar
molecule.A molecule that has two poles is
called dipole, like HCl
Polar molecules
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Polar molecules The effect of polar bonds on the
polarity of the entire molecule dependson the molecule shape
carbon dioxide has two polar bonds,and is linear = nonpolar molecule!
Polar molecules
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Polar molecules The effect of polar bonds on the
polarity of the entire molecule dependson the molecule shape
water has two polar bonds and a bent
shape; the highly electronegative oxygenpulls the e- away from H = very polar!
Polar molecules
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Polar molecules
When polar molecules areplaced between oppositelycharged plates, they tend to
become oriented with respectto the positive and negative
plates.Figure 8.24, page 239
Attractions between molecules
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Attractions between molecules They are what make solid and liquid
molecular compounds possible.
The weakest are called van derWaalsforces - there are two kinds:
#1. Dispersion forces
weakest of all, caused by motion of e-
increases as # e- increases
halogens start as gases; bromine isliquid; iodine is solid all in Group 7A
#2 Dipole interactions
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#2. Dipole interactions
Occurs when polar molecules areattracted to each other.
2. Dipole interaction happens in
waterFigure 8.25, page 240
positive region of one molecule
attracts the negative region of
another molecule.
#2 Dipole interactions
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#2. Dipole interactions
Occur when polar molecules areattracted to each other.
Slightly stronger than dispersion forces.
Opposites attract, but not completelyhooked like in ionic solids.
H F
d+ d-
H F
d+ d-
#2 Dipole Interactions
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#2. Dipole Interactions
d+ d-
d+ d-
#3 Hydrogen bonding
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#3. Hydrogen bonding
is the attractive force caused byhydrogen bonded to N, O, F, or Cl
N, O, F, and Cl are very
electronegative, so this is a verystrong dipole.
And, the hydrogen shares with the
lone pair in the molecule next to it. This is the strongest of the
intermolecular forces.
Order of Intermolecular attraction strengths
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Order of Intermolecular attraction strengths
1) Dispersion forces are theweakest
2)A little stronger are the dipole
interactions3) The strongest is the hydrogen
bonding
4) All of these are weaker thanionic bonds
#3 Hydrogen bonding defined:
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#3. Hydrogen bonding defined:
When a hydrogen atom is: a) covalentlybonded to a highly electronegative atom,AND b) is also weakly bonded to anunshared electron pairof a nearby
highly electronegative atom.The hydrogen is left very electron
deficient(it only had 1 to start with!)thus it shares with something nearby
Hydrogen is also the ONLY elementwith no shielding for its nucleus wheninvolved in a covalent bond!
Hydrogen Bonding
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Hydrogen Bonding(Shown in water)
H
H
O
d+ d-
d+
This hydrogen is bondedcovalently to: 1) the highly
negative oxygen, and 2) a
nearby unshared pair.
Hydrogen bonding allows H2O to be a
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Hydrogen bonding allows H2O to be a
liquid at room conditions.
H
H
O
Attractions and properties
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Attractions and properties
Why are some chemicals gases,some liquids, some solids?
Depends on the type of
bonding!
Table 8.4, page 244
Network solids solids in whichall the atoms are covalentlybonded to each other
Attractions and properties
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Attractions and properties
Figure 8.28, page 243Network solids melt at very high
temperatures, or not at all (decomposes)
Diamond does not really melt, butvaporizes to a gas at 3500 oC
and beyond
SiC, used in grinding, has a
melting point of about 2700 oC
Covalent Network Compounds
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Covalent Network CompoundsSome covalently bonded substances DO
NOT form discrete molecules.
Diamond, a network of
covalently bonded carbon
atoms
Graphite, a network of
covalently bonded carbon
atoms
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