Hydrogen Bonding

🔍 What is Hydrogen Bonding?

When hydrogen (H) is attached to super-electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F), the shared electrons in the covalent bond get pulled toward the electronegative atom. This leaves the H-atom slightly positive (\( \ce{H^{\delta+}} \)), which then gets attracted to another electronegative atom nearby. This attraction is called a hydrogen bond! 💧

Key facts:

  • Hydrogen bonds are weaker than covalent bonds.
  • They act like a “bridge” 🌉 between molecules.
  • In drawings: Covalent bond = solid line (—), Hydrogen bond = dotted line (•••).

⚡ Why Do Hydrogen Bonds Form?

When H bonds to a very electronegative atom “X” (like F, O, N), the shared electrons shift toward X. This makes:

  • H slightly positive (\( \ce{H^{\delta+}} \)),
  • X slightly negative (\( \ce{X^{\delta-}} \)).

The \( \ce{H^{\delta+}} \) from one molecule then gets attracted to the \( \ce{X^{\delta-}} \) of another molecule, forming a chain:
\( \ce{…H^{\delta+} – X^{\delta-} ••• H^{\delta+} – X^{\delta-} •••} \)

📏 Strength Depends on State!

Hydrogen bonds are strongest in solids 🧊, weaker in liquids 💦, and weakest in gases 💨. They hugely affect a compound’s properties!

🔁 Two Types of Hydrogen Bonds

TypeDescriptionExample
Intermolecular
(between molecules)		Forms between H of one molecule and F/O/N of a different molecule.HF molecules, water (\( \ce{H2O} \)), or alcohol groups sticking together.
Intramolecular
(within a molecule)		H is sandwiched between two electronegative atoms (F/O/N) in the same molecule.In o-nitrophenol, H bonds between two oxygen atoms:
\( \ce{O} \)
\( \ce{||} \)
\( \ce{N} \)
\( \ce{|} \)
\( \ce{O – H•••O} \)

🎯 NEET Must-Knows!

Here are high-yield concepts for exams:

  1. Definition & Cause: H-bond forms when H (attached to F/O/N) attracts another F/O/N atom. It’s due to δ⁺/δ⁻ charges!
  2. Types: Intermolecular (between molecules) vs. Intramolecular (within a molecule).
  3. Strength Trend: Solid > Liquid > Gas.
  4. Impact: H-bonding affects physical properties (melting/boiling points, solubility).
  5. Examples: HF chains, water clusters, o-nitrophenol.

💡 Why This Matters?

Hydrogen bonding explains why water expands when it freezes (ice floats! 🧊), why DNA has its shape 🧬, and how proteins fold! ✨