SSharing of electrons so as to achieve stable electronic configuration of noble gas EElectrostatic attraction between nuclei of the atoms and the pair(s) of shared electrons MMolecules are formed GGenerally, covalent bonds are formed between atoms of non- metals
Exceptions to This Rule Aluminium Chloride (AlCl 3 ) Beryllium Chloride (BeCl 2 )
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 & Bond Type Electronegativity Difference Ionic Character (%)Covalent Character (%) Bond Type 0.00100Covalent 0.5595Covalent 1.02080Covalent 1.54060Polar 2.06040Polar 2.57525Ionic 3.09010Ionic
So… yes! You guessed it… 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 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
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 Al 2 Cl 6 molecules As temperature increases further, it increasingly breaks up into simple AlCl 3 molecules
Beryllium Chloride 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
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 GasSolid 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 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
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! Chemistry Music Video – Enjoy! YouTube Link: http://www.youtube.com/watch?v=o NBzyM6TcK8 http://www.youtube.com/watch?v=o NBzyM6TcK8