Atlas MCAT Course
Chemical Foundations → Intermolecular Forces
Intermolecular Forces
How molecules interact without bonding.
01The idea
Chemical bonds explain how atoms stay together.
Intermolecular forces explain how molecules interact with each other.
These interactions determine:
- Physical state (solid, liquid, gas)
- Boiling and melting points
- Solubility and miscibility
- Protein folding and molecular recognition
THREAD · Structure → function
They are weaker than covalent or ionic bonds, but collectively powerful.
02Build the model
AThe origin of intermolecular forces
All intermolecular forces arise from the same source: uneven distribution of charge.
Whether permanent or temporary, attractions form whenever regions of partial positive and negative charge interact.
BLondon dispersion forces: universal and inevitable
London dispersion forces arise from:
- Temporary fluctuations in electron density
- Instantaneous dipoles inducing dipoles in neighboring molecules
Key properties: present in all molecules. They increase with:
- Molecular size
- Electron count
- Polarizability
CDipole–dipole interactions: permanent polarity
Dipole–dipole forces occur between molecules with permanent dipoles.
Requirements:
- Polar bonds
- Asymmetric geometry
Key idea: opposite partial charges attract across molecules.
These forces are:
- Directional
- Stronger than dispersion (for similar-sized molecules)
DHydrogen bonding: a special case
Hydrogen bonding occurs when:
- Hydrogen is covalently bonded to N, O, or F
- Hydrogen interacts with a lone pair on N, O, or F of another molecule
Why it’s strong:
- Hydrogen is very small
- Bond polarity is extreme
- Charge density is high
EIon–dipole interactions: bridging chemistry and biology
Ion–dipole forces occur between:
- Charged ions
- Polar molecules
These interactions explain:
- Salt solubility in water
- Hydration shells around ions
- Electrolyte behavior in biology
FBoiling and melting points
Stronger intermolecular forces require:
- More energy to separate molecules
- Higher boiling and melting points
GSolubility: “like dissolves like” (properly interpreted)
Solubility depends on whether solute–solvent interactions compensate for:
- Solute–solute attractions
- Solvent–solvent attractions
HBiological relevance
Intermolecular forces govern:
- Protein folding
- DNA base pairing
- Ligand–receptor binding
- Membrane assembly
Biology exploits many weak interactions acting together.
03See the connections
The lesson names these links itself. Read the map left to right.
One source
- Uneven distribution of charge
- Permanent or temporary
Four forces
- London dispersion
- Dipole–dipole
- Hydrogen bonding
- Ion–dipole
They set
- Boiling and melting points
- Solubility
- Protein folding, base pairing, binding, membranes
Next: states of matter, phase changes, and energy and entropy.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
Two nonpolar molecules: one large, one small. Which most likely has the higher boiling point?
Think first.
What is a hydrogen bond?
Think first.
Why does table salt dissolve in water?
Think first.
A small polar molecule boils at a lower temperature than a much larger nonpolar molecule. Is that possible?
05MCAT lens
The MCAT emphasizes
Relative strength comparisons
Structural causes of interactions
Consequences for phase and solubility
You are rarely asked to calculate energies. You are often asked to rank or predict outcomes.
06Common traps
Trap 1: Hydrogen bonds are covalent bonds
Misconception
They are intermolecular attractions.
Correct model
Strong, but noncovalent.
Trap 2: Nonpolar molecules have no intermolecular forces
Misconception
Dispersion forces are universal.
Correct model
Temporary dipoles always exist.
Trap 3: Polarity guarantees high boiling point
Misconception
Molecular size and shape also matter.
Correct model
Net intermolecular force strength matters.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why larger molecules often boil at higher temperatures
Check against 02 · B and F: dispersion forces grow with size, and stronger forces raise boiling points.
Why water has unusually strong intermolecular cohesion
Check against 02 · D: water’s O–H hydrogens form hydrogen bonds with oxygen lone pairs on neighboring molecules.
Why salts dissolve in polar solvents
Check against 02 · E and G: ion–dipole attractions replace the attractions they break.
Why weak interactions dominate biological structure
Check against 02 · H: biology exploits many weak interactions acting together.
If something feels unclear, the missing idea is usually thisIntermolecular forces are electrostatics expressed between molecules.
08Remember
REMEMBER · if you keep only a few things
- Intermolecular forces are electrostatic attractions between molecules, not bonds.
- All come from uneven charge distribution, permanent or temporary.
- Dispersion: in every molecule, stronger with size and polarizability.
- Hydrogen bonding: H on N, O, or F meeting a lone pair on N, O, or F. Strong, but noncovalent.
- Ion–dipole is often stronger than hydrogen bonding, and it explains salts dissolving in water.
- Stronger forces raise boiling and melting points. Like dissolves like because similar forces can replace each other.
09Forward hook
Intermolecular forces explain how molecules attract each other. The next question is:
How do these interactions scale to determine bulk phases and transitions?
That leads directly to states of matter, phase changes, and energy and entropy.
Next lessonStates of Matter & Phase Transitions→