結合の極性、双極子モーメント及びイオン特性

The electronegativity difference between bonded atoms determines whether a bond is nonpolar covalent, polar covalent, or ionic. When the difference is zero, the bond is nonpolar covalent. A small difference produces a polar covalent bond, while a large difference results in an ionic bond due to complete electron transfer. The periodic table positions of atoms provide the best guide to predicting bond character.

A dipole moment (µ) quantifies the separation of charge in a polar bond. It is calculated as the product of the charge magnitude (Q) and the distance between charges (r). The formula is µ = Q × r, with results expressed in debye units, where one debye equals 3.34 × 10⁻³⁰ C·m. Dipole moment vectors point from the less electronegative atom toward the more electronegative atom.

Percent ionic character is calculated by dividing a bond's measured dipole moment by the dipole moment assuming complete electron transfer, then multiplying by 100. Bonds with greater than 50% ionic character are classified as ionic. For example, hydrogen fluoride has 41% ionic character, making it polar covalent since it falls below the 50% threshold.

Carbon dioxide is nonpolar because its molecular geometry causes bond dipole moments to cancel. The linear structure places two polar C=O bonds on opposite sides of the carbon atom. Since the bond moments point in opposite directions, their vector sum equals zero, resulting in no net dipole moment for the molecule overall.

Water is polar because its bent molecular geometry prevents bond dipole moments from canceling. The two O-H bonds are polar due to oxygen's higher electronegativity, and the bent shape caused by lone pairs on oxygen means the bond moments do not point in opposite directions. This results in a net dipole moment, making water a polar molecule.

Molecular polarity depends on both individual bond polarity and molecular geometry. For diatomic molecules, bond dipole moment determines polarity. For polyatomic molecules, bond dipoles must be added as vectors in three-dimensional space. If bond moments cancel due to symmetrical arrangement, the molecule is nonpolar; if they do not cancel, the molecule is polar.

Bond length, along with electronegativity difference, helps determine bond type. Shorter bond lengths typically correspond to stronger attractions and greater electron density concentration. In ionic bonds, electron transfer results in different bond characteristics than in covalent bonds. Understanding bond energies and bond lengths helps predict how atoms will interact and what type of bond will form.