Intramolecular Forces

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Types of Intramolecular Forces
Intramolecular forces act between atoms within a molecule. The types of intramolecular forces are ionic bonds, covalent bonds, and metallic bonds.

In chemistry, intramolecular forces are that hold atoms together in a molecule. These forces act within different parts of the same molecule and connect atoms via chemical bonds. In contrast, intermolecular forces act between separate molecules. Generally speaking, intramolecular forces bind atoms, but the forces can involve groups of atoms, single atoms, or parts of atoms, including protons and electrons.

How Intramolecular Forces Work

The basis of intramolecular forces is the attraction between unlike charges and repulsion of like charges. The unlike charges are negatively-charged electrons and positively-charged protons. Like charges are either separate atomic nuclei or else electrons. When chemical bonds form, the charge distribution around the atomic nuclei differs from how an atom exists on its own. Even though electrons surround the nucleus of an atom, they don’t entirely shield it from repelling the nuclei of neighboring atoms and attracting neighbor electrons. Similarly, electrons around atoms repel electrons around other atoms and feel attraction to other nuclei. The interplay between these forces determines the type of chemical bonds atoms form and the resulting chemical and physical properties of molecules.

Types of Intramolecular Forces

The types of intramolecular forces are the three main types of chemical bonds: ionic bonds, covalent bonds, and metallic bonds. The distinction between these types of bonds is the degree of charge separation between the atoms forming the bond. However, the electronegativity difference between atoms is a good predictor of charge separation.

  • Ionic Bond: An ionic bond involves the complete transfer of one or more electrons from one atom to the valence shell of another atom. The bond that forms between sodium and chlorine in sodium chloride (NaCl or table salt) is a good example, where the sodium atom donates its valence electron to the chlorine atom. Ionic bonds form between atoms with a large electronegativity difference. Usually, a metal forms an ionic bond with a nonmetal.
  • Covalent Bond: Atoms share electrons in a covalent bond. Most covalent bonds form between nonmetallic atoms that have the same or comparable electronegativities. For example, oxygen atoms form a covalent bond with each other in oxygen gas (O2). Hydrogen forms covalent bonds with oxygen in water (H2O). When atoms have the same electronegativity, the result is a nonpolar covalent bond. When nonmetallic atoms have similar nonidentical electronegativities, the results is a polar covalent bond.
  • Metallic Bond: Metallic bonds form between atoms of metals that have free valence electrons. So, metallic bonding occurs in pure metallic elements (gold, silver, copper) and in alloys (brass, bronze, sterling silver). While electrons participating in ionic and covalent bonds become associated with particular partner atoms, the electrons in metallic bonds are delocalized. That is, they freely flow between multiple atoms. Metallic bonding mostly closely resembles resonance structures formed in certain covalent bonds. As in covalent bonding, there is little or no difference between the electronegativities of atoms forming metallic bonds.

Are Intermolecular Forces Stronger Than Intramolecular Forces?

Intramolecular forces are stronger than intermolecular forces. Ionic bonds tend to be the strongest intermolecular forces, but there are exceptions. For example, the covalent bonds between carbon atoms in a diamond are very strong. Bond strength depends on multiple factors. For example, within a molecule, the strength of any particular bond is affected by the other bonds in the molecule. Molecules containing a mixture of ionic and covalent bonds tend to have lower intermolecular forces (weaker bonds) than purely ionic or purely covalent molecules. Metallic bonds usually are weaker than either ionic or covalent bonds, but there are exceptions. The strongest intermolecular force is hydrogen bonding. So, roughly ranking intramolecular forces from strongest to weakest:

  1. Ionic bonds
  2. Polar covalent bonds
  3. Nonpolar covalent bonds
  4. Metallic bonds
  5. Hydrogen bonds (intermolecular, for comparison)

References

  • Bader, R. F. W.; Henneker, W. H. (1965). “The Ionic Bond.” Journal of the American Chemical Society. 87 (14): 3063–3068. doi:10.1021/ja01092a008
  • IUPAC (2019). “Intramolecular.” Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
  • King, Matcha (1976). “Theory of the Chemical Bond”. JACS. 98 (12): 3415–3420. doi:10.1021/ja00428a004
  • Oxtoby, David W.; Gills, H. P.; Campion, Alan (2012). Principles of Modern Chemistry (7th ed.). Belmont, Calif.: Brooks/Cole Cengage Learning. ISBN 978-0-8400-4931-5.
  • Zumdahl, Steven S.; Zumdahl, Susan A. (2007). Chemistry (7th ed.). Boston: Houghton Mifflin. ISBN 978-0618713707.