Covalent bonds are classed as polar if the bond is between two different atoms.
Molecules having more than one polar bond are classed as polar if one side of the molecule is more positive than the other.
The dipole moment is an experimental measure of the molecular polarity.
The term polar is used because the sharing of electrons in these bonds is unequal with the atoms having partial negative and positive charges.
Molecules having more than one polar bond are classed as polar if one side of the molecule is more positive than the other.
The graphic shows that water is a polar molecule. This is because the center of partial positive charges due to the hydrogens (blue) is below the center of negative charge due to the oxygen (red).
The dipole moment is an experimental measure of the molecular polarity.
Water has a dipole moment of 1.85 Debye.
Molecules of type AXn (atom A is bonded to two or more atoms of X) may be nonpolar if atom A is at the center of the group of X atoms. If this is true, there is no dipole moment because the center of partial negative charge and partial positive charge are both at central atom A. This lack of a molecular dipole is also shown by considering the bond dipoles from the central to outer atoms. The blue and black lines add to give zero net dipole because the center of negative charge due to outer atoms is at the center of positive charge.
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| CCl4 C at center of tetrahedron Cl at each corner  | CO2 C between two O's on a line  |
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| BF3 B at center of triangle F at each corner  |
The arrows shown are electric dipole vectors which (by definition) point from negative to positive. In all cases the sum of the vectors is zero, and the molecules have no net dipole.