ELECTRONEGATIVITY AND BOND TYPE
Electronegativity, symbol χ, is a chemical property that explains the tendency of an atom or a functional group to attract electrons or electron density in the direction of itself. An atom's electronegativity is influenced by both its atomic number and the distance that its valence electrons exist in from the charged nucleus. The higher the related electronegativity number, the more a compound or element attracts electrons in the direction of it. First proposed by Linus Pauling in year 1932 as a development of valence bond theory, it has been displayed to correlate with a number of other chemical properties. Electronegativity cannot be directly calculated and must be measured from other atomic or molecular properties. Various techniques of calculation have been proposed, and even though there may be small variations in the numerical values of the electronegativity, all techniques show similar periodic trends between elements.
When you examine a periodic table, you will come across that (apart from the noble gases) the electronegativity values tend to increase as you go to the right and up. The opposite statement is that the values tend to decrease going down and to the left. This model will help when you are asked to put various bonds in order from most to least ionic without using the values themselves.
Electronegativity values are helpful in determining if a bond is to be categorized as nonpolar covalent, polar covalent or ionic.
I. Nonpolar Covalent: This type of bond takes place when there is equal sharing (among the two atoms) of the electrons in the bond. Molecules like Cl2, H2 and F2 are the usual examples.
II. Polar Covalent: This type of bond takes place when there is unequal sharing (among the two atoms) of the electrons in the bond. Molecules like NH3 and H2O are the usual instances.
The typical rule is that bonds with an electronegativity variation less than 1.6 are considered polar. Clearly there is a broad range in bond polarity, with the variation in a C-Cl bond being 0.5 -- considered just barely polar -- to the variation the H-O bonds in water being 1.4 and in H-F the variation is 1.9.
III. Ionic: This type of bond takes place when there is complete transfer (between the two atoms) of the electrons in the bond. Substances like NaCl and MgCl2 are the usual instances.