Question

Hydrogen bonding is a special case of very strong dipoledipole interactions possible among only certain atoms. What atoms in addition to hydrogen are necessary for hydrogen bonding? How does the small size of the hydrogen atom contribute to the unusual strength of the dipole-dipole forces involved in hydrogen bonding?

Short answer

In short, hydrogen bonds involve hydrogen atoms bonded to highly electronegative atoms, such as fluorine (F), oxygen (O), or nitrogen (N). The small size of the hydrogen atom enables the positively charged hydrogen nucleus to be very close to the electronegative atom, leading to a strong electrostatic attraction and exceptionally strong dipole-dipole interactions in hydrogen bonding.

Step-by-step solution

Identify atoms necessary for hydrogen bonding

To form hydrogen bonds, hydrogen must be bonded with a highly electronegative atom, which leads to a strong dipole-dipole interaction. The most electronegative atoms that can form hydrogen bonds are fluorine (F), oxygen (O), and nitrogen (N). These atoms have a high affinity for electrons, creating a significant partial negative charge on them when bonded to hydrogen.

Hydrogen's small size and hydrogen bonding strength

The small size of the hydrogen atom plays a significant role in the strength of hydrogen bonding. Since hydrogen is the smallest and lightest element, its electron cloud is relatively close to the nucleus. When bonded to a highly electronegative atom like fluorine, oxygen, or nitrogen, the electron cloud of hydrogen is drawn towards the electronegative atom, creating a strong dipole. Moreover, the small size of the hydrogen atom allows the positively charged hydrogen nucleus (proton) to come very close to the electronegative atom of a neighboring molecule. This proximity leads to a strong electrostatic attraction, which makes hydrogen bonding significantly stronger than typical dipole-dipole interactions.

Summary

In summary, hydrogen bonds occur when a hydrogen atom is bonded to a highly electronegative atom, such as fluorine, oxygen, or nitrogen. The small size of hydrogen contributes to the strength of hydrogen bonding by allowing the positively charged hydrogen nucleus to come very close to the electronegative atom, resulting in a strong electrostatic attraction and a stronger-than-usual dipole-dipole interaction.

Key concepts

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules with permanent dipoles. These are molecules where there is an uneven distribution of electron density, leading to regions of partial positive and partial negative charges. These partial charges arise from differences in electronegativity between atoms in a molecule.
For example, in hydrogen chloride (HCl), chlorine is more electronegative than hydrogen. This pulls the shared electron cloud towards chlorine, giving chlorine a partial negative charge and hydrogen a partial positive charge. The dipoles align such that opposite charges attract, creating dipole-dipole interactions.
This behavior is the backbone of many molecular interactions but gains exceptional strength in hydrogen bonds, where hydrogen interacts directly with highly electronegative atoms such as fluorine, oxygen, or nitrogen. Here, dipole-dipole interactions become much stronger due to hydrogen's unique characteristics.

Electronegative Atoms

Electronegativity describes an atom's ability to attract and hold onto electrons. The more an atom wants to attract electrons, the higher its electronegativity. This property is crucial for forming hydrogen bonds, as only highly electronegative atoms can sufficiently attract hydrogen's electron cloud to create strong dipoles.
Common highly electronegative atoms in this context are fluorine, oxygen, and nitrogen. These atoms lead to strong polar covalent bonds when bound to hydrogen. The electron density shifts significantly towards the electronegative atom, creating a pronounced partial negative charge on the electronegative atom and a partial positive charge on hydrogen. As a result, these electronegative atoms can form hydrogen bonds due to the enhanced dipole-dipole interaction strength they facilitate.
Atoms with lower electronegativity cannot form extensive hydrogen bonds because they don't attract hydrogen's electrons effectively, leading to weaker dipole attractions.

Fluorine, Oxygen, and Nitrogen

Fluorine, oxygen, and nitrogen are three of the most electronegative elements. Their role in hydrogen bonding is a central topic in understanding molecular interactions. Here's a closer look at each:

• **Fluorine (F):** Fluorine is the most electronegative element. Its powerful urge to attract electrons makes it exceptional in forming hydrogen bonds. Although fluorine forms the strongest hydrogen bonds, its ability to do so is sometimes limited by steric factors and its own small size.
• **Oxygen (O):** Often found in water (H₂O) and alcohols, oxygen is very effective at forming hydrogen bonds. Water's extensive hydrogen bonding leads to its high boiling point and unique properties like surface tension and capillarity.
• **Nitrogen (N):** Nitrogen is another key player in hydrogen bonding, as seen in ammonia (NH₃) and biological molecules like DNA and proteins. Nitrogen's presence in amino groups of proteins allows for the formation of hydrogen bonds, influencing the structure and function of biomolecules.

The combination of these atoms with hydrogen leads to significant molecular stability and affects physical properties such as boiling point, viscosity, and solubility. Hydrogen bonds formed with these atoms are crucial for many biological and chemical processes.