Chemical Bonds: Polarity, Metallic & Intermolecular Forces

Posted on Mar 31, 2025 in Chemistry

Polar Covalent Bonds and Polarity

Polarization of Bonds: A polar bond forms when two atoms with different electronegativity come together. This results in partial charges appearing at the ends of the bond. A bond is more polarized the greater the difference in electronegativity between the two atoms. This difference indicates the percentage of ionic character in the covalent bond.

Polar Molecules

A diatomic molecule is polar if its bond is polarized. For a polyatomic molecule to have a molecular dipole moment (μ), two conditions must be met:

  1. It must have polarized bonds.
  2. It must have an appropriate molecular geometry that allows for a net resulting dipole moment.

Keep in mind that even if a molecule has several polar bonds, the overall charge distribution around the center of the molecule can be symmetrical. In such cases, there is no molecular dipole because the center of the positive charge distribution coincides with the center of the negative charge distribution.

The Metallic Bond

According to the electron cloud model, this bond has the following characteristics:

  1. Metal atoms yield their valence electrons to become positive ions. These ions arrange themselves geometrically in a crystal lattice, the specifics of which depend on the metal.
  2. The valence electrons form an ‘electron cloud’ surrounding the positive ions and can move freely within the metal.
  3. The interaction between the positive ions and the electron cloud stabilizes the crystal. Generally, metallic bonding is stronger with a higher number of valence electrons.
  4. The bonding between ions is not rigid, allowing layers of positive ions to slide over each other.

Metals in solid form typically have high-density crystalline lattice arrangements. The most common are: face-centered cubic (FCC) (e.g., Al, Cu, Au), hexagonal close-packed (HCP) (e.g., Mg, Ir, Cd), and body-centered cubic (BCC) (e.g., Fe, V, Ba).

Intermolecular Bonds

These are the forces of attraction between molecules (distinct from intramolecular covalent bonds). There are three main types:

  1. Dispersion Forces (London Forces): These occur between all molecules, even nonpolar ones. At any given instant, the electron cloud in a molecule can shift slightly relative to the nucleus, forming a temporary, instantaneous dipole. This dipole can induce a similar dipole in a neighboring molecule, resulting in a weak attractive force between them. The intensity of these forces increases with the size and surface area of the molecule. They are responsible for the condensed states (liquid and solid) of noble gases and nonpolar molecules.
  2. Dipole-Dipole Attraction: These forces appear between polar molecules. The positive end of one polar molecule is attracted to the negative end of another. These attractive forces increase with the polarity of the molecule and decrease with increasing temperature.
  3. Hydrogen Bond: This is a special, stronger type of dipole-dipole interaction. It occurs when hydrogen atoms are bonded to highly electronegative and small atoms (specifically Fluorine (F), Oxygen (O), or Nitrogen (N)). This bond makes the hydrogen atom very positively polarized. This allows it to form a strong attractive interaction with the lone pair of electrons on the electronegative atom (F, O, or N) of another nearby molecule.