A Case Study on Covalent Bonding

Report
A CASE STUDY ON
COVALENT BONDING
By: Wong Wei Cong (31) 2A1
What is Covalent Bonding
What is Covalent Bonding
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Sharing of electrons so as to achieve stable electronic
configuration of noble gas
Electrostatic attraction between nuclei of the atoms and the
pair(s) of shared electrons
Molecules are formed
Generally, covalent bonds are formed between atoms of nonmetals
Exceptions to This Rule
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Aluminium Chloride (AlCl3)
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Beryllium Chloride (BeCl2)
Electronegativity
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Describes tendency of an atom to attract electrons towards
itself and thus the tendency to form anions
Affected by both the number of protons and the distance that
the valence electrons reside from the nucleus
As you go across a period, electronegativity increases
As you go down a group, electronegativity decreases
Polar Bonds
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If two atoms of equal electronegativity bond together…
 Both have same tendency to attract bonding pair of electrons
 Electron pair found on average half way between the atoms
 Electron pair shared evenly between the atoms
 “Pure” covalent bonds
If B is a lot more electronegative than A…
 Electron pair dragged right over to B's end of the bond
 A has lost control of its electron
 B has complete control over both electrons
 Ionic bonds
Polar Bonds (II)
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If B is slightly more electronegative than A…
 B attracts electron pair more than A does
 B’s end of the bond slightly negative
 A’s end of the bond slightly positive
 Polar bond
Water is a polar
covalent bond!
Electronegativity of Elements
Electronegativity & Bond Type
Electronegativity
Difference
Ionic Character (%) Covalent Character
(%)
Bond Type
0.0
0
100
Covalent
0.5
5
95
Covalent
1.0
20
80
Covalent
1.5
40
60
Polar
2.0
60
40
Polar
2.5
75
25
Ionic
3.0
90
10
Ionic
So… yes! You guessed it…
Electronegativity
difference = 1.4
Aluminium Chloride is a polar covalent bond!
Electronegativity
difference = 1.4
Beryllium Chloride is a polar covalent bond!
Formation of Aluminium
Chloride
Strongly heated aluminium foil burns in chlorine to form aluminium chloride
Aluminium Chloride
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Imagine that aluminum chloride is ionic instead
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Contain Al3+ and Cl- ions
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Aluminium ion: small & packed with 3 positive
charges
High charge density – polarise chlorine ions
Electron pairs dragged back towards
aluminium to such extent that the bonds
become covalent
Chlorine more electronegative than aluminium
– electron pairs will not be pulled half way
between the atoms
Polar covalent bond formed
Factors Affecting Polarising
Ability
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In aluminium chloride, aluminium ions polarise chloride ions
Positive ions can polarise nearby negative ions
The smaller the positive ion and the larger the number of
charges, the greater the polarising ability
The bigger the negative ion, the easier it is to polarise it
Aluminium iodide is covalent as electron pair is
easily dragged away from iodide ion
Aluminium fluoride is ionic as aluminium ion cannot polarise
small fluoride ion sufficiently to form a covalent bond
The Mystery of Aluminium
Chloride
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At room temperature, each aluminium surrounded by 6
chlorines
 Ionic crystal structure with a lot of covalent character
At ordinary atmospheric pressure, it sublimes at about 180°C
If pressure raised to just over 2 atmospheres, it melts at 192°C
 Comparatively weak attractions between molecules
 Each aluminium now surrounded by 4 chlorines rather than 6
 Original lattice converted into Al2Cl6 molecules
As temperature increases further, it increasingly breaks up into
simple AlCl3 molecules
The Mystery of Aluminium
Chloride (II)
Beryllium Chloride
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Imagine that beryllium chloride is
ionic instead
Contain Be2+ and Cl- ions
Beryllium ion: small & packed with 2
positive charges
High charge density – polarise
chlorine ions
Electron pairs dragged back
towards beryllium to such extent
that the bonds become covalent
Chlorine more electronegative than
beryllium – electron pairs will not be
pulled half way between the atoms
Polar covalent bond formed
The Mystery of beryllium
Chloride
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As a gas, Beryllium Chloride is a linear molecule with all three
atoms in a straight line
As a solid, the molecules form long chains (polymers)
 Coordinate bonds
Gas
Solid
Arrows goes from the
atom supplying the pair
of electrons to the atom
with the empty orbital
Why is Beryllium Chloride
not Ionic?
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Beryllium has quite a high electronegativity compared with the
rest of Group II
Attracts bonding pair of electrons towards itself more strongly
In order for an ionic bond to form, the beryllium has to let go of
its electrons, but it is too electronegative to do that
Lewis acid – accepts lone pair of electrons
Boils at 520°C – low boiling point, so cannot contain ions
Reacts vigorously with water, forming acidic, steamy hydrogen
chloride gas – typical of covalent chlorides
Chemistry Music Video –
Enjoy! 
Two atoms met on one fine day
One asked if he could bond
With the other atom there
Of whom he was really fond
The second atom shrugged and said
What’s your pleasure, son?
Are you up for electron transfer
Or electron-sharing fun!
Ionic or covalent?
What kind of bonds are these?
Involve valence electrons
And form compounds with ease!
Metal atoms lose electrons
And become a charge of plus
Non-metals gain them happily
Look negative to all of us.
The opposite charged ions
Attract to make ionic bonds
The E.N.D. one point seven plus
They dissolve real well in ponds!
Ionic or covalent?
What kind of bonds are these?
Involve valence electrons
And form compounds with ease!
If the E.N.D. is point-five or more
And two non-metals had
You have a polar covalent bond
One atoms happy, the others sad!
The atom with less E.N.
Gets a positive charge that’s slight
Chemistry Music Video –
Enjoy! 
And the other, more greedy atom?
Slightly negative to our sight!
Ionic or covalent?
What kind of bonds are these?
Involve valence electrons
And form compounds with ease!
If the E.N.D. is point-four or less
Two non-metal atoms bond
With equal pull on electrons
Each atom is equal fond
No charges will develop
And, because of that, no poles
That’s why the bond is called non-polar
On and on and on we roll!
Ionic or covalent?
Now its easy for you to tell
Just look up the E.N.D.
And you will do real well!
YouTube Link:
http://www.youtube.com/watch?v=o
NBzyM6TcK8
Bibliography
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http://www.chemguide.co.uk/atoms/bonding/electroneg.html
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http://www.chemguide.co.uk/inorganic/period3/chlorides.html
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http://www.chemguide.co.uk/inorganic/group2/beryllium.html
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http://en.wikipedia.org/wiki/Electronegativity
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http://en.wikipedia.org/wiki/Covalent_bond
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http://en.wikipedia.org/wiki/Aluminium_chloride
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http://en.wikipedia.org/wiki/Beryllium_chloride
THANK YOU!
The name’s Bond, Covalent Bond!

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