We have a name for the amount of pull that one atom exerts on the electrons that it is sharing with other atoms. It is called electronegativity.
The electronegativity generally increases as you go from left to right across the periodic table. It decreases as you go down the periodic table.
I think you can see that the reason for this is going to depend on those same factors that we used to explain the trends in atomic size, ionization energy, and electron affinity.
First, the horizontal comparison. As you go across a period from left to right, the atoms of each element all have the same number of energy levels and the same number of shielding electrons. Thus the factor that predominates is the increased nuclear charge. When the nuclear charge increases, so will the attraction that the atom has for electrons in its outermost energy level and that means the electronegativity will increase.
Now what about the vertical comparison? As we established previously, when you go from one atom to another down a group, you are adding one more energy level of electrons for each period. The increased shielding nearly balances the increased nuclear charge and the predominant factor is the number of energy levels that are used by the electrons. So as you go from fluorine to chlorine to bromine and so on down the periodic table, the electrons are further away from the nucleus and better shielded from the nuclear charge and thus not as attracted to the nucleus. For that reason the electronegativity decreases as you go down the periodic table.
You might have noticed that there are some high values in the middle of the transition group. Those elements have fairly high electronegativities for metals. The reason for that ties in with the arrangement of electrons and the fact that the atoms are using d orbitals. The way that the d orbitals are shielded is different than the way that s and p orbitals are shielded so there is some variation in the transition metals that is not as easily explained as the general trend that I have just been talking about.
Take a moment to do parts b, c, d, and e of exercise 26 in your workbook.
Electronegativity - Continued
So far, our comparison of electronegativities has been limited to left and right comparisons within a period and vertical comparisons within a group. Sometimes you have to make comparisons of elements that are not both in the same period or both in the same group. One way is simply to look up values for electronegativities of the elements you are comparing. Another way of remembering how the electronegativities of nonmetals compare is by using the word FONClBrISCHP. It has a spelling that corresponds with the symbols of many of the nonmetals in decreasing order of electronegativity. F for fluorine, O for oxygen, N for nitrogen. Those are the 3 most electronegative elements. Chlorine is very close behind nitrogen, perhaps they are tied. Then comes bromine, iodine, followed by sulfur, carbon, hydrogen, and phosphorus. So FONClBrISCHP is a very useful word in helping you remember the order of the electronegativities of the nonmetals.
We can use electronegativity to predict and explain the polarity of bonds between pairs of atoms.
Take some time now to determine the polarity of the bonds shown in exercise 27 in your workbook. The answers follow.
P-F is a polar bond. F-F is a nonpolar bond. H-O is a polar bond. H-C is a slightly polar bond. There is not much difference between the electronegativities of hydrogen and carbon. C-C is a nonpolar covalent bond. If you had trouble with any of those be sure to check with an instructor to figure out why.
Comparing Covalent and Ionic Bonding
Let's push some of these ideas a bit further. By looking at electronegativity we can talk about gradations in metallic and nonmetallic character. Although there are many inconsistencies, we can generalize that metals have low electronegativities (generally below 2) and nonmetals have high electronegativities (generally above 2). We can also generalize about ionic and covalent bonding in this way. Covalent bonding results when there is a small difference in the electronegativities of the two elements. Ionic bonding results when there is a very large difference in electronegativities between the two elements. Some chemists set the dividing line between a small difference and a large difference at about 1.7 to 1.9.
This approach works well if one or both of the elements is a nonmetal. In the next portion of this lesson we will look at what happens when the bonding involves just metal atoms.
E-mail instructor: Eden Francis
Clackamas Community College