A CASE STUDY ON COVALENT BONDING

By: Wong Wei Cong (31) 2A1

What is Covalent Bonding

What is Covalent Bonding

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 non-metals

Exceptions to This Rule

Aluminium Chloride (AlCl3)

Beryllium Chloride (BeCl2)

Electronegativity

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

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)

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…

Aluminium Chloride is a polar covalent bond!

Electronegativity difference = 1.4

Electronegativity difference = 1.4Beryllium Chloride is a polar

covalent bond!

Formation of Aluminium Chloride

Strongly heated aluminium foil burns in chlorine to form aluminium chloride

Aluminium Chloride

Imagine that aluminum chloride is ionic instead

Contain Al3+ 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

Factors Affecting Polarising Ability

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

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

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

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

The Mystery of beryllium Chloride

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?

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 dayOne asked if he could bondWith the other atom thereOf whom he was really fondThe second atom shrugged and saidWhat’s your pleasure, son?Are you up for electron transferOr electron-sharing fun!

Ionic or covalent?What kind of bonds are these?Involve valence electronsAnd form compounds with ease!

Metal atoms lose electronsAnd become a charge of plusNon-metals gain them happily

Look negative to all of us.The opposite charged ionsAttract to make ionic bondsThe E.N.D. one point seven plusThey dissolve real well in ponds!

Ionic or covalent?What kind of bonds are these?Involve valence electronsAnd form compounds with ease!

If the E.N.D. is point-five or moreAnd two non-metals hadYou have a polar covalent bondOne atoms happy, the others sad!The atom with less E.N.Gets a positive charge that’s slight

And the other, more greedy atom?Slightly negative to our sight!

Ionic or covalent?What kind of bonds are these?Involve valence electronsAnd form compounds with ease!

If the E.N.D. is point-four or lessTwo non-metal atoms bondWith equal pull on electronsEach atom is equal fondNo charges will developAnd, because of that, no polesThat’s why the bond is called non-polarOn and on and on we roll!

Ionic or covalent?Now its easy for you to tellJust look up the E.N.D.And you will do real well!

Chemistry Music Video – Enjoy!

YouTube Link: http://www.youtube.com/watch?v=oNBzyM6TcK8

Bibliography

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

THANK YOU!

The name’s Bond, Covalent Bond!