Bond and Molecular Polarity

• Bond polarity arises from the unequal sharing of electrons between two atoms in a chemical bond.

• Molecular polarity depends on both the polarity of individual bonds and the molecular geometry.

Bond Polarity

• Bond polarity is a measure of how equally or unequally the electrons in a chemical bond are shared between the two atoms.

• A bond is polar if one atom is more electronegative than the other.

• Electronegativity is the ability of an atom to attract electrons in a chemical bond.

• The greater the difference in electronegativity between two atoms, the more polar the bond.

• A nonpolar bond occurs when electrons are shared equally; this happens when the electronegativity difference between the bonded atoms is very small (usually less than 0.4).

• A polar bond occurs when electrons are shared unequally; this happens when there is a significant electronegativity difference between the bonded atoms (usually between 0.4 and 1.7).

• An ionic bond is formed when the electronegativity difference is very large (usually greater than 1.7), resulting in essentially a complete transfer of electrons.

• The polarity of a bond is indicated by a dipole moment, which is a vector quantity.

• The dipole moment points from the positive (δ+) end to the negative (δ-) end of the bond.

• The magnitude of the dipole moment is proportional to the size of the partial charges and the distance between the atoms.

Molecular Polarity

• Molecular polarity is a property of a molecule as a whole, based on the arrangement of its atoms and the polarity of its bonds.

• A molecule is polar if it has a net dipole moment.

• If the individual bond dipole moments cancel each other out due to the molecular geometry, the molecule is nonpolar.

• If the individual bond dipole moments do not cancel each other out, the molecule is polar.

• Molecular geometry plays a crucial role in determining molecular polarity.

• Symmetric geometries, such as linear (e.g., CO2), trigonal planar (e.g., BF3), and tetrahedral (e.g., CH4), can result in nonpolar molecules if all the surrounding atoms are the same.

• Even if a molecule contains polar bonds, the overall molecule can be nonpolar if the bond dipoles cancel due to symmetry.

• Asymmetric geometries, such as bent (e.g., H2O) and trigonal pyramidal (e.g., NH3), usually result in polar molecules.

• Lone pairs of electrons on the central atom can significantly affect molecular geometry and polarity.

• Lone pairs exert a greater repulsive force than bonding pairs, which can distort the molecular shape and prevent bond dipoles from canceling.

• To determine if a molecule is polar:

• Draw the Lewis structure of the molecule.

• Determine the molecular geometry using VSEPR theory.

• Determine if the bonds are polar based on electronegativity differences.

• Draw the bond dipoles.

• Determine if the bond dipoles cancel each other out.

• If there is a net dipole moment, the molecule is polar; otherwise, it is nonpolar.

• Polar molecules exhibit intermolecular forces such as dipole-dipole interactions and hydrogen bonding.

• Polar molecules generally have higher boiling points and melting points compared to nonpolar molecules of similar molecular weight.

• Polar molecules are more soluble in polar solvents (e.g., water), while nonpolar molecules are more soluble in nonpolar solvents (e.g., hexane).