Question

Hydrogen bonding is a special case of very strong dipole-dipole 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 addition to hydrogen, the atoms necessary for hydrogen bonding are nitrogen, oxygen, and fluorine. The small size of the hydrogen atom, having only one electron and occupying minimal space, allows for its positively-charged nucleus to be closer to the electronegative atom it is bound to. This results in a strong electrostatic attraction between the positive and negative ends of the highly polar bond, leading to the increased strength of hydrogen bonding compared to other dipole-dipole interactions.

Step-by-step solution

Identify the atoms necessary for hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs between a hydrogen atom bonded to an electronegative atom (such as nitrogen, oxygen, or fluorine) and a lone pair of electrons on another electronegative atom. Therefore, in addition to hydrogen, the atoms necessary for hydrogen bonding are nitrogen, oxygen, and fluorine.

Understand the reason behind the small size of the hydrogen atom contributing to the strength of hydrogen bonding

The small size of the hydrogen atom is crucial to the strength of hydrogen bonding. As the hydrogen atom has only one electron and occupies a minimal amount of space, the positively-charged nucleus is closer to, and thus more attracted to, the electronegative atom it is bound to. When a hydrogen atom is bonded to an electronegative atom, it experiences a significant difference in electronegativity, leading to the formation of a highly polar bond. Due to this high polarity and the close proximity of the hydrogen atom to the electronegative atom, a strong electrostatic attraction forms between the positive and the negative ends of these polar bonds. This results in a particularly strong dipole-dipole interaction called hydrogen bonding. In conclusion, the atoms necessary for hydrogen bonding are nitrogen, oxygen, and fluorine, in addition to hydrogen. The small size of the hydrogen atom allows it to be in close proximity to the electronegative atom it is bound to. This leads to a strong electrostatic attraction between the polar bonds, causing the increased strength of hydrogen bonding compared to other dipole-dipole interactions.

Key concepts

Dipole-Dipole Interactions

Dipole-dipole interactions are a type of intermolecular force that occurs between polar molecules. These interactions are due to the attraction between the positive end of one polar molecule and the negative end of another. This type of force is considered stronger than London dispersion forces but weaker than hydrogen bonding.

In dipole-dipole interactions:

• Molecules must have a permanent dipole moment.
• The stronger the polarity of the molecules, the stronger the dipole-dipole interactions.
• This force is particularly common in molecules with a significant difference in electronegativity between the atoms involved.

One special case is hydrogen bonding, which is a very strong dipole-dipole interaction that specifically occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. The smaller size of hydrogen allows it to get extremely close to the electronegative atom, making these interactions exceptionally strong compared to typical dipole-dipole forces.

Electronegative Atoms

Electronegative atoms are elements that have a high tendency to attract electrons towards themselves. This property is often observed in nonmetals located on the right side of the periodic table. The most electronegative elements include fluorine, oxygen, and nitrogen.

Key characteristics of electronegative atoms include:

• High ability to attract electrons in a bond, leading to the formation of polar bonds.
• They often participate in hydrogen bonding due to their tendency to attract hydrogen electrons.
• They are critical for the formation of polar molecules.

In hydrogen bonding, electronegative atoms like nitrogen, oxygen, and fluorine are essential because they draw electrons away from the hydrogen atom, creating a highly polar bond. This polarization increases the strength of dipole-dipole interactions, making hydrogen bonds remarkably strong.

Polarity

Polarity is a property of molecules where there is an uneven distribution of electron density, resulting in a dipole moment. A molecule is polar if it has regions of partial positive and negative charge due to differences in electronegativity between its atoms.

For a molecule to be polar:

• It must have polar bonds with a significant electronegativity difference.
• The geometry of the molecule must not cancel out the dipole moments of its bonds.

An example of a polar molecule is water (H₂O), where oxygen is more electronegative than hydrogen, creating a significant dipole moment. In the context of hydrogen bonding, polar molecules enable strong dipole-dipole interactions, where the positive end of one polar molecule is attracted to the negative end of another, amplifying the strength of the interaction. This is why polarity is essential in hydrogen bonding, making these interactions much stronger than typical forces between nonpolar molecules.