Recall: Electronegativity Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The Pauling scale is the most commonly used – electronegative values assigned to each atom.

Fluorine (the most electronegative element) is assigned a value of 4.0 Values range down to cesium and francium which are the least electronegative at 0.7.

Molecular Shapes and Molecular Polarity Bond polarity is a measure of how equally the electrons in a bond are shared between two atoms. As the difference in electronegativity between two atoms increases, so does bond polarity.

In molecules containing two atoms, a dipole exists if there is a difference in electronegativity. For a molecule with more than two atoms, the dipole moment depends on both the polarities of the individual bonds and the geometry of the molecule.

Consider linear CO2 Each C=O bond is polar, but since the bonds are identical, the bond dipoles are equal in magnitude. The overall dipole moment = 0.

Bond dipoles and dipole moments are vector quantities, which means they have magnitude and direction. The overall dipole moment of a molecule is the sum of its bond dipoles. Consider the bent molecule, water, with two polar bonds.

Both bonds are identical so the bond dipoles are equal, but since the molecule is bent, the bonds do not directly oppose each other. Therefore, the bond dipoles do not cancel each other out. The water molecule has a non zero dipole moment so it is polar. The oxygen carries a partial negative charge and the hydrogen atoms each carry partial positive charges.

Comparing Models