Intramolecular Interactions (Bonds)

Students commonly confuse intramolecular interactions (bonds) with intermolecular interactions (molecular attractions). This post will explain the types of bonds that hold an individual molecule or crystal together, as well as the molecular attractions that bind molecules to their neighbors.

Intramolecular Interactions (Bonds)

Interactions that are inTRAmolecular are within a single molecule or ion pair. We normally call these associations bonds. The bonding interactions within molecules are relatively strong, especially in comparison to the attractive interactions among molecules. Although bonds are often considered to fall into one of two discrete categories, covalent and ionic, it is actually best to consider bonding as a continuum, with perfectly covalent bonds and perfectly ionic bonds as the two extremes.

Nonpolar Covalent Bonds:
Although we often consider C-H bonds to be "nonpolar" in organic chemistry, in reality, perfectly nonpolar covalent bonds with completely equal sharing of bond electrons between both atoms only occur in homonuclear diatomic molecules. Homonuclear means that both of the atoms being bonded are identical (homo = same); diatomic means that there are only two atoms. So examples of perfectly nonpolar covalent bonds would include H2, O2, N2, and the halogens (F2, Cl2, Br2, and I2). For your test, you should memorize that in nature, these seven elements occur in pairs.

Ionic Bonds:
Sometimes two elements have such a large difference in electronegativity  that one atom is able to grab an electron completely away from another. This generally happens when highly electronegative elements from the extreme right side of the periodic table (ex. halogens) are mixed with highly electropositive elements from the extreme left side of the periodic table (ex. alkali metals). Each element forms an ion, with the metal becoming a positively charged cation and the nonmetal becoming a negatively charged anion. The two ions are held together by electrostatic attraction, which is described by Coulomb’s Law.

Polar Covalent Bonds:
Most other bonds between two different elements that have more moderate differences in electronegativity fall somewhere between the above two extremes. Some, such as a C-H bond, lie closer to the nonpolar covalent end of the spectrum. Others, such as a Mg-C bond, lie closer to the ionic end of the spectrum. Polar covalent bonds have covalent character because the bonding electrons are shared between the two atoms, but they also have ionic character because the bonding electrons are not shared equally. Rather, the more electronegative element is able to pull extra electron density to itself, and the more electropositive element is left slightly electron-deficient. This forms a bond dipole, with a partial negative charge on the electronegative atom and a partial positive charge on the electropositive one. Polar covalent bonds can either be protic or aprotic