Polar Bonds and Electronegativity

A nonpolar covalent bond has a uniform distribution of electron charge between the bonded atoms. The simplest nonpolar covalent bonds exist in "homonuclear diatomic" molecules like H2 and Cl2. Both atoms attract the shared electrons equally. The shared electrons spend equal time on both ends of the bond and molecule. There is no permanent localized electric charge build up.

A polar bond has a unsymmetric electron cloud distribution. The electrons in the bond are not shared equally. This typically happens between two nonmetal atoms that are two or more positions apart in the periodic table. An example is Hl. The electrons in the bond spend more time around the chlorine nucleus. this makes the chlorine end more electron rich than the hydrogen end of the bond. The "arrow" indicates the direction of the electron shift. This is a polar bond.

 

Electronegativity, EN, is an index that tells the relative attraction an element has for electrons in a bond. Electronegativity has a high value of 4.0 for F, fluorine. The lowest electronegativity value is about 0.7 for Cs, cesium. The table below shows the nonmetals have relatively high electronegativities. The metals have relatively low electronegativities. The electronegativities follow the same trends as atomic sizes (radius). Electronegativity gets smaller with increasing distance from fluorine. Atoms that are equidistant from fluorine have similar (not identical) electronegativities. The rare gases generally are not tabulated for EN values. Bond polarity is directly related to electronegativity difference. The greater the difference in electronegativity the more polar the bond. When two elements are next to one another in the periodic table they have similar electronegativities. Chlorine has a value of 3.0 while bromine has a value of 2.8. These two atoms in BrCl would have a nonpolar covalent bond. The bond between carbon with 2.5 and nitrogen with 3.0 would be polar. The bond between chlorine, 3.0, and boron 2.0, would be polar. When electronegativities get too different the bond is not polar, it is ionic. The nonmetal gets the electrons from the metal essentially 100% of the time. The metal atom is stripped of its valence electrons. These atoms don't share the electrons. There is a transfer to the nonmetal. Differences in EN are related to positions in the periodic table. The greater the separation between the elements the greater the electronegativity difference. Look at fluorine and cesium. This is the greatest difference possible, 4.0 - 0.7 = 3.3. This combination is so different that they form ionic bonds. NOTE: hydrogen really fits in best at the bottom of the halogens, group 7A.

The fact that the EN changes so predictably with the element position allows us to make predictions about bonds based on periodic table location and not even have the EN values.

Example: Which of the following pairs of bonds would be predicted to be the most polar? Click for answer a. Cl and B b. F and C c. C and I

Example: Which of the following pairs of bonds would be predicted be the least polar? Click for answer a. Cs and F b. B and At c. H and O

  Revised March 28, 2005 all rights reserved by Dr. Walt Volland,