IF YOU ALWAYS DO WHAT YOU’VE ALWAYS DONE, YOU WILL ALWAYS GET WHAT YOU’VE ALWAYS

GOTTEN.

SO FAR, WE HAVE DISCUSSED THREE TYPES OF BONDS.

IONIC BONDS ARE FORMED BETWEEN METALS AND NONMETALS. THESE ARE ATOMS WHICH HAVE VERY LARGE DIFFERENCES IN THEIR ABILITY TO LOSE ELECTRONS (MEASURED BY IONIZASTION POTERTIAL) AND THEIR ABILITY TO GAIN ELECTRONS MEASURED BY ELECTRON (AFFINITY).

COVALENT BONDS ARE FORMED BETWEEN ATOMS WITH SIMILAR ATTRACTIONS FOR ELECTRONS.

METALLIC BONDS ARE FORMED BETWEEN ATOMS WITH VERY LITTLE ATTRACTION FOR ELECTRONS AND THE ELECTRONS IN THE SOLID ARE “DELOCALIZED.”

THERE IS ANOTHER CLASS OF BONDS THAT REPRESENTS A HYBRID OR AVERAGE BETWEEN IONIC BONDS AND COVALENT BONDS.

THESE ARE FORMED BETWEEN ATOMS WHERE THE DIFFERENCES ARE NOT LARGE ENOUGH TO FORM IONIC BONDS BUT NOT SIMILAR ENOUGH TO FORM COVALENT BONDS WHERE THE ELECTRONS ARE EQUALLY SHARED.

POLAR COVALENT BONDS

ATOMS WITH SIMILAR ATTRACTIONS FOR ELECTRONS (ELECTRON AFFINITY) WILL SHARE ELECTRONS TO FORM COVALENT BONDS.

THIS WOULD INCLUDE ELEMENTS CLOSE TOGETHER ON THE PERIODIC TABLE.

LINUS PAULING, A NOBEL PRIZE WINNING CHEMIST, DEFINED A PROPERTY THAT HE CALLED ELECTRONEGATIVITY – THE ABILITY OF AN ATOM IN A BOND TO ATTRACT A SHARED PAIR OF ELECTRONS.

ATOMS WITH THE SAME OR SIMILAR ELECTRONEGATIVITIES WOULD FORM PURE COVALENT BONDS.

THE RULES ARE:

•Non-polar bonds occur when the difference in electronegativity between the two atoms is less than 0.4

•Polar bonds occur when the difference in electronegativity between the two atoms is between 0.4 and 1.7

•Ionic bonds occur when the difference in electronegativity between the two atoms is greater than 1.7

FOR EXAMPLE, IF YOU HAD A BOND BETWEEN CARBON (EN = 2.55) AND HYDROGEN (EN = 2.2), THE DIFFERENCE IS 0.35, SO THE BOND WOULD BE COVALENT.

IF YOU HAD A BOND BETWEEN HYDROGEN (EN=2.2) AND OXYGEN (EN = 3.44), THE DIFFERENCE IS 1.24, SO THE BOND IS POLAR COVALENT.

IF YOU HAD A BOND BETWEEN SODIUM (EN = 0.93) AND CHLORINE (EN = 3.16), THE DIFFERENCE IS 2.23, SO THE BOND IS IONIC.

BONDS IN WATER ARE POLAR COVALENT.

THESE GIVE WATER SOME VERY UNUSUAL PROPERTIES.

SUGARS AND ALCOHOLS REPRESENT OTHER EXAMPLES OF POLAR COVALENT COMPOUNDS.

THE FORMULA FOR GLUCOSE IS C6H12O6 .

THE STRUCTURAL FORMULA IS:

FRUCTOSE HAS THE SAME FORMULA AS GLUCOSE:

C6H12O6

BUT THE STRUCTURAL FORMULA IS DIFFERENT:

ALCOHOLS ARE ALSO CHARACTERIZED BY THE HYDROXYL GROUP (-OH). BELOW ARE THE FORMULAS FOR THREE COMMON ALCOHOLS:

CH3OH METHYL ALCOHOL

CH3CH2OH ETHYL ALCOHOL

CH3CH2CH2OH ISOPROPYL ALCOHOL