As you go over the material in your chemistry course or your SAT/AP test prep, you most certainly spend a lot of time learning about bonds—what they are, how they differ, how strong they are, and so forth. Also, you have probably read about them being classified into intramolecular bonds and intermolecular bonds. During my many one-on-one chemistry tutoring sessions, I have come to realize that students get very confused about the difference between these two groups. While it might seem like just a simple spelling issue, it is much more important than that.
What kind of bonds we are talking about has a lot to do with the kind of chemistry that’s happening. So let’s define these categories:
Think of these bonds as the bonds within. Intramolecular bonds form between atoms in a molecule. These bonds are very strong and take a lot of energy to break. Breaking these bonds results in the loss of identity of the molecule. For example, if you break the bonds between hydrogen and oxygen atoms in the water molecule, you no longer have water. These bonds give a molecule its identity. There are two kinds of intramolecular bonds-covalent and ionic.
Covalent bonds- these bonds occur between atoms with similar electronegativity. This means that the electrons participating in the bond are shared (equally or unequally between the atoms). If the atoms in the bond have almost equal (or equal in the case of the same atoms) electronegativity, then the bond is a covalent nonpolar. These bonds usually occur in diatomic molecules, such as O2 or H2.
If the atoms participating in the bond do not have the same electronegativity, but the difference is not greater than 2, then the bond is polar covalent. In this case, the electrons of the bonds are not equally shared but are pulled more toward the more electronegative atom. Be careful! If a molecule has a polar bond, it does not automatically follow that the molecule is polar. Polarity of the molecule has a lot to do with the geometry of the molecule, but that is a topic for another post.
Ionic bonds- these bonds occur between atoms with an electronegativity difference greater than 2. As you might guess from the name, these bonds occur in the case of ionic compounds. Here, the electrons of the bond are completely sequestered by one atom, which has the negative charge. The other atom (or group) has a positive charge. Ionic bonds are very strong and they require a lot of energy to break. Actually, ionic bonds are in a sort of a gray area between intramolecular and intermolecular bonds. Since the electrons of the bond are completely sequestered by one atom, creating ions, there is no distinction between ions of the same molecule or ions of the nearby molecule. Ionic bonds are the strongest kind of intramolecular bonds as well as the strongest intermolecular bond (covered below).
Think of these bonds as the bonds between. I usually tell my students to think of these as international relations (relations between nations, not within the same nation-nations being the molecules). These bonds occur between molecules and are generally weaker than intramolecular bonds. Breaking these bonds only affects how close or far apart molecules are; it has nothing to do with the identity of the molecule. For instance, when you boil water, as you go from water to vapor you break the intermolecular bonds of water. In liquid molecules are close, but in vapor, these bonds have been broken and thus molecules are further apart. However, as you go from liquid to vapor, the identity of the molecule, water, does not change. In either phase you still have water. These bonds are usually broken as you transition through phase changes. In fact, the strength of these bonds determines how much energy you need to transition from one phase to the other. A liquid with strong intermolecular bonds will require a lot of energy to transition from liquid to vapor and boil at a high temperature. Another liquid with weaker intermolecular bonds requires less energy for the transition and therefore boils at a lower temperature. There are several intermolecular bonds and below I will list them in the order of decreasing strength.
- Ionic bonds- as mentioned before, these are the strongest kinds of bonds and they occur between ionic compounds.
- Ion-dipole-these bonds occur between an ionic compound and a polar molecule.
- Hydrogen bond-these are special kinds of bonds that occur between two polar molecules. One of these molecules has a very electronegative atom, such as O, N or F, and the other molecule has a hydrogen atom bound to a very electronegative atom.
- Dipole-dipole-this occurs between two polar molecules.
- Dipole-induced Dipole-this occurs between a polar molecule and a non-polar molecule.
- Induced Dipole-Induced Dipole-this occurs between two non-polar molecules.
This quick review will help you understand inter- and intra- bonds and the chemistry they affect.