Question

Hydrogen bonding is exhibited by (1) all the molecules containing H-atom (2) molecules in which tuo H-atoms are present (3) molecules in which H-atom is covalently bonded to \(\mathrm{F}, \mathrm{O}, \mathrm{N}\) (4) molecules in which \(\mathrm{H}\) -atom is bonded to atoms with electronegativity more than \(2.1\)

Short answer

Option (3) is correct.

Step-by-step solution

Understanding Hydrogen Bonding

Hydrogen bonding occurs when a hydrogen atom that is covalently bonded to a highly electronegative atom (like fluorine, oxygen, or nitrogen) interacts with another electronegative atom nearby.

Analyzing Each Option

Examine which options meet the hydrogen bonding criteria: (1) All molecules containing H-atoms: Not true, as not all hydrogen-containing molecules exhibit hydrogen bonding. (2) Molecules with two H-atoms: Incorrect, as the number of H-atoms alone does not dictate hydrogen bonding. (3) Molecules with H-atom covalently bonded to \(\text{F}, \text{O}, \text{N}\): Correct, since these atoms have high electronegativity. (4) Molecules with H-atom bonded to atoms with electronegativity more than 2.1: Incorrect, as this does not specify the particular atoms required for hydrogen bonding.

Concluding the Correct Option

Identify that option 3 is correct because hydrogen bonding specifically involves hydrogen atoms covalently bonded to fluorine, oxygen, or nitrogen.

Key concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a covalent bond. It is a crucial factor in determining how atoms interact with each other in molecules. Atoms with high electronegativity, like fluorine, oxygen, and nitrogen, pull electrons closer to themselves. This causes a partial negative charge on these atoms and a partial positive charge on the hydrogen atoms they are bonded to.

High electronegativity differences between atoms result in polar covalent bonds. In water (H₂O), for instance, oxygen is much more electronegative than hydrogen, creating a polar molecule where oxygen has a slight negative charge and hydrogens have slight positive charges.

• Different elements have different electronegativities:
  
  • Fluorine: 3.98
  • Oxygen: 3.44
  • Nitrogen: 3.04

When hydrogen is bonded to any of these highly electronegative elements, it creates the right conditions for hydrogen bonding.

Covalent Bonds

Covalent bonds form when atoms share pairs of electrons. This type of bond can be either nonpolar or polar, depending on the atoms involved. When atoms of similar electronegativity form a covalent bond, they share electrons equally, resulting in a nonpolar bond.

However, when an atom of high electronegativity forms a covalent bond with an atom of lower electronegativity, like in the case of hydrogen and fluorine, oxygen, or nitrogen, the bond becomes polar. The more electronegative atom pulls the shared electrons closer, creating a dipole with partial positive and negative charges.

> Example of polar covalent bonds:

• Water (H₂O) where oxygen forms polar covalent bonds with hydrogen
• Ammonia (NH₃) where nitrogen forms polar covalent bonds with hydrogen
• Hydrofluoric acid (HF) where fluorine forms a polar covalent bond with hydrogen

These polar covalent bonds are essential in hydrogen bonding.

Fluorine, Oxygen, Nitrogen

Fluorine, oxygen, and nitrogen are three elements known for their high electronegativity and ability to participate in hydrogen bonding.

Fluorine: The most electronegative element, fluorine, readily forms hydrogen bonds due to its strong attraction for electrons, often resulting in very strong hydrogen bonds, as seen in hydrofluoric acid (HF).

Oxygen: Commonly found in water (H₂O) and alcohols, oxygen's high electronegativity allows it to engage in hydrogen bonding both as a hydrogen bond donor and acceptor. This results in the unique properties of water, such as its high boiling point and surface tension.

Nitrogen: Found in compounds like ammonia (NH₃) and amines, nitrogen's electronegativity also makes it an excellent participant in hydrogen bonding. It can form hydrogen bonds that significantly influence the structure and function of biological molecules like proteins and nucleic acids.

High electronegativity of these elements ensures that when they form covalent bonds with hydrogen, the result is polar covalent bonds that facilitate hydrogen bonding.