H δ+δ+ δ-δ- AS Chemistry POLAR COVALENT BONDS H Cl H

GAS N2N2N2N2CO MELTING POINT / K Introduction6374 Explain the difference in the melting points above.

Both non-metal + non-metal  expect COVALENT bonding LOW melting points  N 2 and CO have SAME M r (28) WHY NOT? Explained by BOND POLARISATION  expect SIMPLE MOLECULAR structure  expect SAME INTERMOLECULAR FORCES  expect SAME MELTING POINT

POLARISATION OF COVALENT BONDS This depends on different atoms having slightly different ELECTRONEGATIVITY DEFINITION: What type of bonding would you expect if combining atoms’ electronegativities are; (a)Very DIFFERENT eg Na and Cl (b) SIMILAR eg C and H (c) SLIGHTLY DIFFERENT eg O and H The ELECTRONEGATIVITY of an atom is a measure of its tendency to ATTRACT the electron pair(s) from a covalent bond Learn!

What type of bonding would you expect if combining atoms’ electronegativities are; (a) Very DIFFERENT eg Na and Cl (b) SIMILAR eg C and H (c) SLIGHTLY DIFFERENT eg O and H

If combining atoms’ electronegativities are: (a)Very DIFFERENT eg Na and Cl  ionic bonding Na + Cl - (b) SIMILAR eg C and H  covalent bonding CH 4 (b) SLIGHTLY DIFFERENT eg O and H  POLAR covalent bonding H 2 O

Pauling Electronegativity Index used to measure electronegativity. H 2.1 Li 1.0 Be 1.5 TRANSITION B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Na 0.9 Mg 1.2 ELEMENTS Al 1.5 Si 2.1 P 2.1 S 2.5 Cl 3.0 K 0.8 Ca 1.1 ALL SIMILAR Ga 1.6 Ge 1.9 As 2.0 Se 2.4 Br 2.8 Rb 0.7 Sr 1.0 IN RANGE Sn 1.8 I 2.5 Cs 0.6 Ba Pb 1.7 Explain the trend – across a period Explain the trend – down a group

 Increases across periods  Decreases down groups because number of nuclear attracting protons (+) acting on bond e - increases because distance and shielding from attracting protons acting on bond e - increases

For a “normal” covalent bond EQUAL sharing of the electron pair occurs This occurs for atoms with similar electronegativities Such bonds are called NON-POLAR bonds Examples H-HF-FCl-Cl O=O NNNNNNNNC-H Most bonds are POLAR !!!!!

Polar Covalent Bonding Occurs when electron pair shared UNEQUALLY This occurs for atoms of dissimilar electronegativities  Bond electrons NOT centralised between the atoms  e- pair pulled towards more electronegative atom (Y) and away from less electronegative atom (X).  slight negative charge (  -) on Y, and slight positive charge (  +) on X. Represented by X  Y ++ -- NB The molecule is still NEUTRAL overall because  + and  - cancel. H-ClF-BrC-O I - Br O-SC-F ++ -- ++ -- ++ -- -- ++ -- ++ ++ -- Insert the dipole on each of the following: - called a “dipole”

H-Cl Polar bonds  more IONIC character ++ -- ++ --  stronger attractions between neighbouring molecules  higher m pt / b pt than expected N N N N C O C O ++ -- ++ -- N 2 and CO have same M r (28) but mpt CO > mpt N 2 because of polarity of CO.  called DIPOLE-DIPOLE inter- molecular forces

The End