The degree of polarity and the magnitude of the bond dipole will be proportional to the difference in electronegativity of the bonded atoms. Such a covalent bond is polar, and will have a dipole (one end is positive and the other end negative). When two different atoms are bonded covalently, the shared electrons are attracted to the more electronegative atom of the bond, resulting in a shift of electron density toward the more electronegative atom. It should be noted that carbon is about in the middle of the electronegativity range, and is slightly more electronegative than hydrogen. Fluorine has the greatest electronegativity of all the elements, and the heavier alkali metals such as potassium, rubidium and cesium have the lowest electronegativities. A larger number on this scale signifies a greater affinity for electrons. A rough quantitative scale of electronegativity values was established by Linus Pauling, and some of these are given in the table to the right. The ability of an element to attract or hold onto electrons is called electronegativity. The formal charge on an atom may also be calculated by the following formula:īecause of their differing nuclear charges, and as a result of shielding by inner electron shells, the different atoms of the periodic table have different affinities for nearby electrons. If the number of covalent bonds to an atom is less than its normal valence it will carry a negative charge. In general, for covalently bonded atoms having valence shell electron octets, if the number of covalent bonds to an atom is greater than its normal valence it will carry a positive charge. Finally, azide anion has two negative-charged nitrogens and one positive-charged nitrogen, the total charge being minus one. Similarly, nitromethane has a positive-charged nitrogen and a negative-charged oxygen, the total molecular charge again being zero. The overall charge of the ozone molecule is therefore zero. In the formula for ozone the central oxygen atom has three bonds and a full positive charge while the right hand oxygen has a single bond and is negatively charged. The three Kekulé formulas shown here illustrate this condition. A dipole exists when the centers of positive and negative charge distribution do not coincide.Ī large local charge separation usually results when a shared electron pair is donated unilaterally. Although this is true for diatomic elements such as H 2, N 2 and O 2, most covalent compounds show some degree of local charge separation, resulting in bond and / or molecular dipoles.

If the electron pairs in covalent bonds were donated and shared absolutely evenly there would be no fixed local charges within a molecule.

Izotope insight left polar problem