Intermolecular Forces: The Invisible Glue

You know about ionic and covalent bonds — the strong forces that hold atoms together within molecules. But there's another category of forces that's equally important: intermolecular forces — the weaker attractions between molecules that determine whether a substance is a gas, liquid, or solid at room temperature.

Intramolecular vs Intermolecular

This distinction trips up a lot of students:

• Intramolecular: Forces within a molecule (ionic, covalent bonds). Breaking these = chemical reaction.
• Intermolecular: Forces between molecules. Breaking these = phase change (boiling, melting). No chemical reaction — the molecules stay intact.

When water boils, the O-H covalent bonds don't break. Only the hydrogen bonds between water molecules break. The H₂O molecules fly off as steam, perfectly intact.

The Three Forces (Weakest to Strongest)

1. London Dispersion Forces

Present in: ALL molecules (including nonpolar ones)

At any instant, the electrons in a molecule might happen to be slightly more on one side than the other, creating a temporary dipole. This temporary dipole induces a dipole in a neighboring molecule. The result: a weak, fleeting attraction.

The key factor is polarizability — how easily the electron cloud can be distorted. More electrons = bigger, more polarizable cloud = stronger London forces.

That's why noble gas boiling points increase down the group:

| Gas | Electrons | Boiling Point | |-----|-----------|---------------| | He | 2 | -269°C | | Ne | 10 | -246°C | | Ar | 18 | -186°C | | Kr | 36 | -153°C | | Xe | 54 | -108°C |

See this trend interactively

2. Dipole-Dipole Forces

Present in: Polar molecules only

Permanent partial charges (δ+ and δ-) on polar molecules attract each other. The δ+ end of one molecule is attracted to the δ- end of a neighbor.

These are stronger than London forces (for molecules of similar size) because the dipole is permanent, not temporary. But they only exist in polar molecules — nonpolar molecules like CH₄ or CO₂ don't have them.

3. Hydrogen Bonds

Present in: Molecules where H is bonded to F, O, or N

The strongest intermolecular force. When hydrogen is bonded to a very electronegative atom (F, O, or N), the H becomes very δ+. This strongly positive H is attracted to a lone pair on a nearby F, O, or N.

Hydrogen bonds are about 10x weaker than covalent bonds but 5-10x stronger than typical London forces. They're responsible for water's extraordinary properties, DNA's double helix, and protein folding.

Important: Despite the name, hydrogen bonds are NOT real bonds. They're intermolecular forces — attractions between molecules.

How Intermolecular Forces Determine Properties

Boiling Point

Stronger intermolecular forces = higher boiling point (more energy needed to separate molecules). This is why:

• H₂O (H-bonding) boils at 100°C
• H₂S (dipole-dipole + London) boils at -60°C
• CH₄ (London only) boils at -161°C

All three are small molecules. The massive difference in boiling points is entirely due to intermolecular forces.

Viscosity

Stronger intermolecular forces = higher viscosity. Honey (lots of H-bonding from sugar molecules) is viscous. Acetone (weak dipole-dipole) flows easily.

Surface Tension

Strong intermolecular forces at the surface pull molecules inward, creating a "skin." Water has high surface tension (H-bonding) — high enough for insects to walk on it and paperclips to float.

Solubility

"Like dissolves like." Polar solvents (water) dissolve polar and ionic compounds. Nonpolar solvents (hexane) dissolve nonpolar compounds. The intermolecular forces in the solute must be compatible with those in the solvent.

The Water Anomaly

Water's hydrogen bonding network creates properties that break the trends seen in every other compound:

1. Anomalously high boiling point: 100°C instead of the expected -80°C
2. Ice is less dense than liquid water: H-bond network spaces molecules apart
3. High specific heat: Energy breaks H-bonds before raising temperature
4. High surface tension: Strong H-bond network at the surface

Without hydrogen bonding, there would be no liquid water on Earth, and no life as we know it.

Summary Table

| Force | Strength | Present In | Key Factor | |-------|----------|-----------|------------| | London dispersion | Weakest | All molecules | Number of electrons | | Dipole-dipole | Medium | Polar molecules | Electronegativity difference | | Hydrogen bond | Strongest (IMF) | H bonded to F, O, N | Very δ+ hydrogen |

Key Takeaway

Intermolecular forces are weaker than chemical bonds, but they determine the physical properties of every substance: boiling point, melting point, viscosity, surface tension, and solubility. Understanding these forces explains why water is liquid at room temperature, why oil and water don't mix, and why geckos can walk on ceilings (London forces between their toe pads and the surface).

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This post supports the interactive explainer: How Chemical Bonds Actually Work