![]() Each of the bonds is polar, but the molecule as a whole is nonpolar. If the bonds in a molecule are arranged such that their bond moments cancel (vector sum equals zero), then the molecule is nonpolar. When a molecule contains more than one bond, the geometry must be taken into account. For HF, there is a larger dipole moment because there is a larger difference in electronegativity. For heteronuclear molecules such as CO, there is a small dipole moment. Homonuclear diatomic molecules such as Br 2 and N 2 have no difference in electronegativity, so their dipole moment is zero. If we have bond dipoles present in the molecule we have to look at the addition of these and their orientation in 3D space.įor diatomic molecules, there is only one bond, so its bond dipole moment determines the molecular polarity. If no bond dipoles exist the molecule is said to be nonpolar. To determine if a molecule is polar we have to look at all the bond dipoles present within the molecule. (b) The electronegativity difference between B and F is much larger, so the vector representing the bond moment is much longer.Ī polar molecule has a separation of charge within its molecular structure. (a) There is a small difference in electronegativity between C and H, represented as a short vector. ![]() The length of the arrow is proportional to the magnitude of the electronegativity difference between the two atoms. A small plus sign is drawn on the less electronegative end to indicate the partially positive end of the bond. Dipole vectors are shown as arrows pointing along the bond from the less electronegative atom toward the more electronegative atom. This bond moment can be represented as a vector, a quantity having both direction and magnitude ( ). The magnitude of a bond dipole moment is represented by the Greek letter mu ( µ) and is given by the formula shown here, where Q is the magnitude of the partial charges (determined by the electronegativity difference) and r is the distance between the charges: ![]() This separation of charge gives rise to a bond dipole moment. As discussed previously, polar covalent bonds connect two atoms with differing electronegativities, leaving one atom with a partial positive charge (δ+) and the other atom with a partial negative charge (δ–), as the electrons are pulled toward the more electronegative atom.
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