By Angela Huang
Intermolecular Forces (IMF)
Atoms bonds together in a certain way, forming various molecules, which is usually in the size of a few angstroms (Å) to several dozen Å (Note, one angstroms = 10-10 m). (A virus is usually the size of 100 nanometers, which means a virus is 100 to 1,000 times larger than a molecule.) Numerous molecules cluster together to form a substance to a scale that can be seen by our eyes. When the molecules cluster together, there is a weak force mediating the interaction between neighboring molecules so the substance behaves in a specific way, as shown in the image below. Such weak forces between molecules are called intermolecular forces (IMFs). An atom consists of proton(s) and electron(s), which have positive and negative charges, respectively. IMFs are the attractive and repulsive forces, caused by electrostatic interaction. IMFs are responsible for most of the physical and chemical properties of a substance.
Behaviors of solid, liquid and gas substance: Left – solid, center – liquid, right – gas.
Hydrogen Bonds
Due to the structural difference, electrostatic interactions between molecules are different. In general, the IMFs can be divided into three major categories: London dispersion forces (LDF), dipole-dipole forces, and hydrogen bonds. All molecules have LDFs and also may have dipole-dipole forces. Hydrogen bonds are a special dipole-dipole force, as it is much stronger than other LDFs. When two hydrogen atoms bond with one oxygen atom to form a water molecule, the oxygen atom pulls the electron strongly towards it, forming dipoles. In other words, the electrons are not shared equally between the two hydrogen atoms and one oxygen atom within a water molecule. Such an uneven distribution of electrons causes the uneven distribution of the electromagnetic field of a water molecule. Specifically, the oxygen side is partially negative and the hydrogen side is partially positive as shown in the image below. As a result, the oxygen side will attract the hydrogen side of the neighboring water molecule. Such a phenomenon continues among water molecules, making them more strongly bonded than LDFs.
Illustration of hydrogen bonds
Implication and Importance
The existence of hydrogen bonds makes water behave unique. Hydrogen bonds are a strong IMF, so it can hold molecules better than the substance without hydrogen bonds. Compared with other molecules, the boiling point of water is much higher as shown in the graph below. As a result, water is liquid under normal temperature. Without hydrogen bonds, the water would boil at the temperature of -100 oC, as shown by the dash line in Figure 3. If that happens, we would not have water to drink. Researchers have found out that hydrogen bonds also form when hydrogen atoms and nitrogen (N), or fluorine (F) atoms form molecules.
Boiling point comparison
Sources
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Theodore E. Brown, H. Eugene LeMay, Bruce E. Bursten, Catherine Murphy, Patrick Woodward, Matthew E. Stoltzfus (2017). Chemistry: The Central Science, 14th edition, Pearson, 1248p.
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