Covalent bonding Covalent bond: A chemical bond formed when two atoms share electrons.
Chemical Structure: Chemical Bonding. Homonuclear Covalent Bonds
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Transcript of Chemical Structure: Chemical Bonding. Homonuclear Covalent Bonds
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HOMONUCLEAR COVALENT BONDS
University of Lincoln presentation
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Chemical Bonds
A CHEMICAL BOND joins atoms together
There are 4 types of chemical bond:
• COVALENT BONDS• Ionic bonds• Coordinate bonds• Metallic bonds
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Homonuclear Covalent Bonding
What you need to know…• Covalent bond formation• Bond length• Bond energy• Bond order• Relationship between bond length,
bond energy and bond order• Trends in the periodic table
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Definitions…
• A MOLECULE is a discrete neutral species resulting from the formation of a covalent bond or bonds between two or more atoms
• A HOMONUCLEAR BOND is a covalent bond between 2 identical atoms
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Covalent Homonuclear Molecules
Examples of covalent homonuclear molecules
Hydrogen (H2)Oxygen (O2)
Ozone (O3)Iodine (I2)
Phosporous (P4) Sulphur (S6)Sulphur (S8)
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Molecules with Homonuclear Bonds
Molecules with one homonuclear bond
Ethane (C2H6)Hydrazine (N2H4)
Hydrogen peroxide (H2O2)
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Making a Covalent Bond – sharing valence electrons
In order to share valence electrons, 2 atoms have to come into close contact with each other
2 hydrogen atoms 1 hydrogen
molecule, H2
H HH H He
1s1 1s1 1s2
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Bringing 2 atoms together is not easy – there are FOUR forces in
play…
+ +–
ATOM A ATOM B
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The Four Forces
Internuclear separation
+ +
- -
(2)
(3)
(4)
(1)
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How close do the atoms have to be to form a bond?
The VAN DER WAALS RADIUS (rv) of an atom X is measured as half of the distance of closest approach of 2 NON-BONDED atoms of X
The COVALENT RADIUS (rcov) of an atom X is taken as half of the internuclear distance (r) in a HOMONUCLEAR X–X bond.
The internuclear distance (r) in a bonded pair of atoms is called the BOND LENGTH
r
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Non-bonded vs Bonded Radii
Element Van der Waals radius (pm)
NON-BONDED
Covalent X–X radius (pm)
BONDED
Covalent Bond Length (pm)
(2 x rcov)
H 120 37 74
B 208 88 176
C 185 77 154
Si 210 118 236
N 154 75 150
O 140 73 146
S 185 103 206
F 135 71 142
Note: the internuclear distance is SMALLER when atoms are bonded together
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…Hence, atoms must overlap to form a bond
Non-bonded atoms – NO OVERLAP of
atomic orbitals
Bonded atoms – OVERLAP of
atomic orbitals
The bigger the overlap, the SHORTER the bond.
The shorter the bond, the STRONGER it is.
Bond length
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Bond Energy Sometimes called the BOND ENTHALPY
The BOND ENERGY is the amount of energy required to break a bond:
The larger the bond energy, the STRONGER the bondThe larger the bond energy, the STRONGER the bond
H–H 2H
The bond energy is, therefore, a measure of how strong a bond is:
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Breaking Bonds…
Breaking the C-C bond produces two radicals
Breaking the S-S bond opens up the ring structure
C2H6
S6
2CH3·
·S-S-S-S-S-S·
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Bond Order
Type of Bond
Name of Bond
Bond Order
X–X Single 1
X=X Double 2
X≡X Triple 3
The larger the bond order, the STRONGER the bondThe larger the bond order, the STRONGER the bond
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Some Bond EnergiesBond Bond Energy
(kJmol-1)Bond Bond Energy
(kJmol-1)
H–H 436 O–O 146
C–C 346 O=O 498
C=C 598 S–S 266
C≡C 813 S=S 425
N–N 159 F–F 159
N=N 400 Cl–Cl 242
N≡N 945 Br–Br 193
P–P 200 I–I 151
P≡P 490 Group 17
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Bond Energy & Bond Length
Bond Energy (kJmol-1)
Bond Length (pm)
F–F 159 141
Cl–Cl 242 199
Br–Br 193 228
I–I 151 2670
50
100
150
200
250
300
F-F Cl-Cl Br-Br I-I
The shorter the bond, the higher the bond energy
F is anomalous due to its small size. Bond energy would be expected to be ~275 kJmol-1
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Adjacent Lone Pair Effect
verycloseveryclose
Because F is a small atom (look at its position on the Periodic Table – it is the smallest of the 1st row elements) its valence electrons are very close and tend to repel each other. The two atoms are forced apart and the bond is weakened
This anomalous behaviour is common in 1st row elements, particularly, N, O and F
F F
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Group Trends in Homonuclear Single Bond
Energies
0
50
100
150
200
250
300
350B
on
d E
ner
gy
(kJm
ol-1
)
1st row 2nd row 3rd row 4th row
Group14
Group 15
Group 16
Group 17
Note the anomalous behaviour of N–N, O–O and F–F. Group 14 show the expected trend
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Formation of Multiple bonds
Can only make a single bond
Could make a double bond (sharing both of its unpaired electrons with another atom)
Could make a double or a triple bond. A triple bond would be stronger (sharing all three unpaired electrons with another atom)
N
OF
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Bond Energies for X2 Molecules in Group 15 (in their natural state)
N2 has very high bond energy…why?
Bon
d E
nerg
y (
kJm
ol-1
)
0
250
500
750
1000
Bond Energy
(KJmol-1)
N P As Sb Bi
Element
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Formation of the N2
MoleculeN is small enough to overlap with another N atom sufficiently to share all three of its unpaired electrons and make a very strong TRIPLE BOND
LINEAR Molecule
N
N
N
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Other elements in Group 15…
• P, As, Sb and Bi are TOO BIG to form multiple bonds – they can’t get close enough to overlap sufficiently
• These elements form SINGLE BONDS with three other atoms forming TETRAHEDRAL molecules
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…Nitrogen forms a triple bond
X X X2
Other elements in Group 15 can only form single bonds
X = N
P
PP
PX4
X = P, As, Sb or Bi
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Periodic Trends in Bond Length, Bond Energy & Bond
Order
100
120
140
160
180
200
220
240
260
280
Li B C N O F
X -
X b
on
d d
ista
nce
in X
2 m
ole
cule
/pm
0
100
200
300
400
500
600
700
800
900
1000
Li B C N O F
Bo
nd
en
thal
py
kJ m
ol-
1
X-X bond distances X-X bond dissociated enthalpy for X2 molecules containing the first row elements
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Bond Orders of the 1st Row Elements
Homonuclear Diatomic
Bond Order
B–B 1
C=C 2
N≡N 3
O=O 2
F–F 1
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Summary
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Things to Remember…
1. The covalent bond is formed by overlapping atomic orbitals
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2. Bond Order (single, double, triple)
3. Bond Energy (energy to break bond, kJmol-1)
(measure of bond strength)
4. Bond Length (internuclear distance, pm) 5. Trends in the periodic table…
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TRENDS:– Bond energy increases as bond order
increases– Bond length decreases as bond order
increases– Bond energy decreases as bond length
increases
The shorter the bond, the stronger it is
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Definitions
MoleculeHomonuclear bondvan der Waals radiusCovalent radiusBond lengthBond energyBond order
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Acknowledgements
• JISC• HEA• Centre for Educational Research and
Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org