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