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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