In Unit #4 (The Periodic Table - Classification of the Elements) we introduced three chemical forces of attraction that hold atoms or ions together in chemical compounds. They are:
In this unit, we will continue to compare characteristics of chemical bonds since the types of bonds that are present in a substance are largely responsible for the physical and chemical properties of the substance.
A arbitrary numerical scale of electronegativity values of most of the elements in the periodic table was developed by Linus Pauling.
Click on the periodic table button in the button bar to obtain electronegativity values for the elements of interest.
Take a look at the general trends in electronegativities.
A low value indicates that an element does not have a strong attraction for bonded electrons.
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Ionic Bonding
We learned in the Unit #4 that the type of bonding occurring between metals and nonmetals is ionic bonding. The bonding requires a transfer of electrons from the metal to the nonmetal, a process that forms ions. Once these ions are formed, they arrange themselves into a crystalline array in the solid state.
Take a look at the arrangement of ions in sodium chloride.
As we learned in Unit #4, covalent bonds are formed by the sharing of electrons between nonmetals. Covalent bonds may either be polar or nonpolar.
(i) Nonpolar covalent Bonding
When the covalent bond is formed between two identical
nonmetals, a nonpolar covalent bond is formed.
Take a look at the kind of molecules that have this kind of bond.
(ii) Polar Covalent Bonding
When electrons in a bond is shared unequally, the
bond is called polar covalent bond or polar bond.
Take a look at the kind of molecules that have this kind of bond.
Do you want to know how polar molecules behave when they are subjected
to an electric field?
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3. Classification of Bond Type
The greater the difference in electronegativity,
the more polar the bond. At one extreme is ionic bonding.
At the other extreme is non-polar covalent bonding.
Do you know how to use electronegativity values to classify the type of bonding?
Although a molecule is made up of one or several polar covalent bonds,
the molecule itself may , or
may not be polar. Remember that bond dipole
moments are vector quantities.
Do you know how to tell whether a molecule is polar or not?
5. Write Chemical Formulas for Ionic compounds
All compounds are electrically neutral.
Remember these three points:
Click here to review the charge of ions that representative elements tend to form.
Follow these simple rules to write chemical formulas for ionic compounds.
4. Determination of Polarity of Molecules
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All contents copyrighted © 1997 Rosamaría Fong, Instructor of Chemistry All Rights reserved