By: Wong Wei Cong (31) 2A1

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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

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3
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2

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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

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If two atoms of equal electronegativity bond together…
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Both have same tendency to attract bonding pair of electrons
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Electron pair found on average half way between the atoms
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Electron pair shared evenly between the atoms
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“Pure” covalent bonds
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If B is a lot more electronegative than A…
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Electron pair dragged right over to B's end of the bond
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A has lost control of its electron
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B has complete control over both electrons
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Ionic bonds

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If B is slightly more electronegative than A…
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B attracts electron pair more than A does
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B’s end of the bond slightly negative
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A’s end of the bond slightly positive
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Polar bond
Water is a polar covalent bond!

Electronegativity
Difference
0.0
0.5
1.0
1.5
2.0
2.5
3.0
60
75
90
5
20
40
Ionic Character (%) Covalent Character
(%)
0 100
95
80
60
40
25
10
Bond Type
Covalent
Covalent
Covalent
Polar
Polar
Ionic
Ionic

Electronegativity difference = 1.4
Aluminium Chloride is a polar covalent bond!
Electronegativity difference = 1.4
Beryllium Chloride is a polar covalent bond!

Strongly heated aluminium foil burns in chlorine to form aluminium chloride

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Imagine that aluminum chloride is ionic instead
Contain Al 3+ and Cl ions
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

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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

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At room temperature, each aluminium surrounded by 6 chlorines
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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
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Comparatively weak attractions between molecules
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Each aluminium now surrounded by 4 chlorines rather than 6
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Original lattice converted into Al
2
Cl
6 molecules
As temperature increases further, it increasingly breaks up into simple AlCl
3 molecules

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Imagine that beryllium chloride is ionic instead
Contain Be 2+ 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

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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)
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Coordinate bonds
Gas Solid
Arrows goes from the atom supplying the pair of electrons to the atom with the empty orbital

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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

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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

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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

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 http://www.chemguide.co.uk/atoms/bonding/electroneg.html
http://www.chemguide.co.uk/inorganic/period3/chlorides.html
http://www.chemguide.co.uk/inorganic/group2/beryllium.html
http://en.wikipedia.org/wiki/Electronegativity http://en.wikipedia.org/wiki/Covalent_bond http://en.wikipedia.org/wiki/Aluminium_chloride http://en.wikipedia.org/wiki/Beryllium_chloride

The name’s Bond, Covalent Bond!