Question

What are the requirements for molecules to form hydrogen bonds? (What atoms must be present and involved in such bonds?)

Short answer

For hydrogen bonds to form, a molecule must have hydrogen attached to Fluorine, Oxygen, or Nitrogen, and there must be a lone pair on an electronegative atom nearby.

Step-by-step solution

- Identify the Atoms Involved

Hydrogen bonds are specific types of intermolecular attractions. For a hydrogen bond to occur, a molecule must contain a hydrogen atom covalently bonded to a highly electronegative atom. The key atoms that are involved typically include Fluorine (F), Oxygen (O), or Nitrogen (N).

- Presence of Donor and Acceptor

In addition to having a hydrogen atom bonded to a highly electronegative atom (such as F, O, or N), there must also be an available lone pair on another electronegative atom to accept the hydrogen bond. This lone pair is usually on another molecule or a different part of the same molecule.

- Proximity and Orientation

The molecules must be close enough for the hydrogen bond to form. The hydrogen atom bonded to the electronegative atom should align properly with the lone pair of electrons on the acceptor atom to maximize the bond strength.

Key concepts

Highly Electronegative Atoms

Understanding hydrogen bonds starts with knowing about highly electronegative atoms. These atoms are crucial because they strongly attract electrons. In hydrogen bonds, common highly electronegative atoms include:

• Fluorine (F)
• Oxygen (O)
• Nitrogen (N)

These atoms have high electron affinity and tend to pull bonding electrons closer to themselves. This attraction causes a partial negative charge on the electronegative atom and a partial positive charge on the hydrogen connected to it. This partial charge separation is key for forming hydrogen bonds. Without highly electronegative atoms, hydrogen bonds would not be possible.

Covalent Bonding

Another key element in forming hydrogen bonds is covalent bonding. A covalent bond occurs when two atoms share electrons. In the case of hydrogen bonds, the hydrogen atom must be covalently bonded to a highly electronegative atom like fluorine, oxygen, or nitrogen.
When hydrogen is bonded covalently to these atoms, a significant difference in electronegativity causes the hydrogen to take on a partial positive charge. This positive charge is what allows hydrogen to interact with lone pairs on other electronegative atoms, leading to the formation of a hydrogen bond.

Hydrogen Bond Acceptors

For a hydrogen bond to form, there must be another molecule, or part of the same molecule, that can accept the hydrogen bond. These are known as hydrogen bond acceptors. A hydrogen bond acceptor must have a lone pair of electrons ready to interact with the partially positive hydrogen atom.
The presence of this lone pair allows the formation of the hydrogen bond, which is essentially an attraction between the lone pair and the hydrogen. Without a hydrogen bond acceptor, the hydrogen bond cannot exist. Typical examples of acceptors include oxygen and nitrogen atoms with lone pairs in other molecules.